UNIT I: Benzene and its derivatives

a. Analytical, synthetic and other evidences in the derivation of structure of benzene, Orbital
picture, resonance in benzene, aromatic characters, Huckel’s rule
b. Reactions of benzene - nitration, sulphonation, halogenationreactivity, Friedelcrafts alkylation-
reactivity, limitations, Friedelcrafts acylation.
c. Substituents, effect of substituents on reactivity and orientation of mono substituted benzene
compounds towards electrophilic substitution reaction
d. Structure and uses of DDT, Saccharin, BHC and Chloramine

UNIT II: Phenols, Aromatic Amines and Aromatic Acids

a. Phenols* - Acidity of phenols, effect of substituents on acidity, qualitative tests, Structure and
uses of phenol, cresols, resorcinol, naphthols
b. Aromatic Amines* - Basicity of amines, effect of substituents on basicity, and synthetic uses of
aryl diazonium salts
c. Aromatic Acids* –Acidity, effect of substituents on acidity and important reactions of benzoic
acid.

UNIT III: Fats and Oils

Fats and Oils

a. Fatty acids – reactions.

b. Hydrolysis, Hydrogenation, Saponification and Rancidity of oils, Drying oils.

c. Analytical constants – Acid value, Saponification value, Ester value, Iodine value, Acetyl value,
Reichert Meissl (RM) value – significance and principle involved in their determination.

UNIT IV: Polynuclear hydrocarbons

a. Synthesis, reactions
b. Structure and medicinal uses of Naphthalene, Phenanthrene, Anthracene, Diphenylmethane,
Triphenylmethane and their derivatives

UNIT V: Cyclo alkanes

Stabilities – Baeyer’s strain theory, limitation of Baeyer’s strain theory, Coulson and Moffitt’s
modification, Sachse Mohr’s theory (Theory of strainless rings), reactions of cyclopropane and
cyclobutane only
Aliphatic and aromatic compounds:
Chemists have found it useful to divide all organic compounds into two broad classes: aliphatic
compounds and aromatic compounds. The original meanings of the words "aliphatic" (fatty) and
"aromatic" (fragrant) no longer have any significance.

Aliphatic compounds are open-chain compounds and those cyclic compounds that resemble the open-
chain compounds. The families we have studied so far alkanes, alkenes, alkynes, and their cyclic
analogs are all members of the aliphatic class.

Aromatic compounds are benzene and compounds that resemble benzene in chemical behavior.
Aromatic properties are those properties of benzene that distinguish it from aliphatic hydrocarbons.
Some compounds that possess aromatic properties have structures that seem to differ considerably from
the structure of benzene: actually, however, there is a basic similarity in electronic configuration.

Aliphatic hydrocarbons, as we have seen, undergo chiefly addition and free radical substitution;
addition occurs at multiple bonds, and free-radical substitution occurs at other points along the aliphatic
chain.

In contrast, we shall find that aromatic hydrocarbons are characterized by a tendency to undergo ionic
substitution.
Structure of benzene:
It is obvious from our definition of aromatic Compounds that any study of their chemistry must
begin with a study of benzene. Benzene has been known since 1825; its chemical and physical
properties are perhaps better known than those of any other single organic compound. In spite of this,
no satisfactory structure for benzene had been advanced until about 1931, and it was ten to fifteen years
before this structure was generally used by organic chemists. The difficulty was not the complexity of
the benzene molecule, but rather the limitations of the structural theory as it had so far developed. Since
an understanding of the structure of benzene is important both in our study of aromatic compounds and
in extending our knowledge of the structural theory, we shall examine in some detail the facts upon
which this structure of benzene is built.

Analytical, synthetic and other evidences in the derivation of structure of benzene:

The following are the analytical and synthetic evidences for benzene structure, (a), (f) & (g) are
analytical evidences and (b),(c), (d) & (e) are synthetic evidences.
(a) Benzene has the molecular formula C6H6
(b) Benzene yields only one monosubstitution product.
(c) Benzene yields three isomeric disubstitution products.
(d) Catalytic hydrogenation
(e) Benzene undergoes substitution rather than addition.
(f) Heats of hydrogenation
(g) All carbon- carbon bond lengths are equal

(a) Benzene has the molecular formula C6H6: From its elemental composition and molecular weight,
benzene was known to contain six carbon atoms and six hydrogen atoms.
The question was: how are these atoms arranged?

In 1858, August Kekule (of the University of Bonn) had proposed that carbon atoms can join to
one another to form chains. Then, in 1865, he offered an answer to the question of benzene: these
carbon chains can sometimes be closed, to form rings.
"I was sitting writing at my textbook, but the work did not progress; my thoughts were elsewhere. I
turned my chair to the fire, and dozed. Again the atoms were gamboling before my eyes. This time the
smaller groups kept modestly in the background. My mental eye, rendered more acute by repeated
visions of this kind, could now distinguish larger structures of manifold conformations; long rows,
sometimes more closely fitted together; all twisting and turning in snake-like motion. But look! What
was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my
eyes. As if by a flash of lightning I woke; . . . I spent the rest of the night working out the consequences
of the hypothesis. Let us learn to dream, gentlemen, and then perhaps we shall learn the truth."
---August Kekule, 1865.

Kekule's structure of benzene was one that we would represent today as I.

Other structures are, of course, consistent with the formula C6H6 : for example, II- V. Of all
these, Kekule's structure was accepted as the most nearly satisfactory;

(b) Benzene yields only one mono substitution product C6H5Y. Only one bromobenzene, C6H5Br, is
obtained when one hydrogen atom is replaced by bromine; similarly, only one chlorobenzene, C 6H5C1,
or one nitrobenzene, C6H5NO2, etc., has ever been made. This fact places a severe limitation on the
structure of benzene: each hydrogen must be exactly equivalent to every other hydrogen, since the
replacement of any one of them yields the same product.

Structure V, for example, must now be rejected, since it would yield two isomeric monobromo
derivatives, the 1-bromo and the 2-bromo compounds; all hydrogens are not equivalent in V. Similar
reasoning shows us that II and III are likewise unsatisfactory. (How many monosubstitution products
would each of these yield?)

I and IV, among others, are still possibilities, however.

(c) Benzene yields three isomeric disubstitution products: C6H4Y2 or C6H4YZ. Three and only
three isomeric dibromobenzenes, C6H4Br2 , three chloronitrobenzenes, C6H4C1NO2, etc., have ever
been made. This fact further limits our choice of a structure; for example, IV must now be rejected.
(How many disubstitution products would IV yield?)

At first glance, structure I seems to be consistent with this new fact; that is, we can expect three
isomeric dibromo derivatives, the 1,2- the 1,3-, and the 1,4- dibromo compounds shown:

Closer examination of structure I shows, however, that two 1,2-dibromo isomers (VI and VII),
'differing in the positions of bromine relative to the double bonds, should be possible:
But Kekule visualized the benzene molecule as a dynamic thing: ". . . the form whirled mockingly
before my eyes . .V He described it in terms of two structures, VIII and IX, between which the benzene
molecule alternates. As a consequence, the two 1,2-dibromobenzenes (VI and VID would be in rapid
equilibrium and hence could not be separated.

Later, when the idea of tautomerism became defined, it was assumed that Kekule's "alternation"
essentially amounted to tautomerism. On the other hand, it is believed by some that Kekule had
intuitively anticipated by some 75 years our present concept of delocalized electrons, and drew two
pictures (VIlI and IX) as we shall do, too- as a crude representation of something that neither picture
alone satisfactorily represents. Rightly or wrongly, the term "Kekule structure" has come to mean a
(hypothetical) molecule with alternating single and double bonds just as the term "Dewar benzene" has
come to mean a structure (II) that James Dewar devised in 1867 as an example of what benzene was
not.

Kekule's structure, then, accounts satisfactorily for facts (a), (b), and (c). But there are a number of facts
that are still not accounted for by this structure; most of these unexplained facts seem related to unusual
stability of the benzene ring. The most striking evidence of this stability is found in the chemical
reactions of benzene.
(d) Catalytic hydrogenation of benzene yielded cyclohexane
(e) Benzene undergoes substitution rather than addition: Kekule's structure of benzene is one that
we would call "cyclohexatriene." We would expect this cyclohexatriene, like the very similar
compounds, cyclohexadiene and cyclohexene, to undergo readily the addition reactions characteristic of
the alkene structure. As the examples in Table 10.1 show, this is not the case; under conditions that
cause an alkene to undergo rapid addition, benzene reacts either not at all or very slowly.

(f) Heats of hydrogenation and combustion of benzene are lower than expected, We recall that heat of
hydrogenation is the quantity of heat evolved when one mole of an unsaturated compound is
hydrogenated. In most cases the value is about 28-30 kcal for each double bond the compound contains.
It is not surprising, then, that cyclohexene has a heat of hydrogenation of 28.6 kcal and cyclohexadiene
has one about twice that (55.4 kcal.)

We might reasonably expect cyclohexatriene to have a heat of hydrogenation about three times
as large as cyclohexene, that is, about 85.8 kcal. Actually, the value for benzene (49.8 kcal) is 36 kcal
less than this expected amount. This can be more easily visualized, perhaps, by means of an energy
diagram (Fig. 10.1), in which the height of a horizontal line represents the potential energy content of a
molecule. The broken lines represent the expected values, based upon three equal steps of 28.6 kcal.
The final product, cyclohexane, is the same in all three cases.
 The fact that benzene evolves 36 kcal less energy than predicted can only mean that benzene
contains 36 kcal less energy than predicted; in other words, benzene is more stable by 36 kcal than we
would have expected cyclohexatriene to be. The heat of combustion of benzene is also lower than that
expected, and by about the same amount.

(g) All carbon - carbon bonds in benzene are equal and are intermediate length between single and
double bonds. Carbon-carbon double bonds in a wide variety of compounds are found to be about 1.34
A long. Carbon-carbon single bonds, in which the nuclei are held together by only one pair of electrons,
are considerably longer: 1.53 A in ethane, for example, 1.50 A in propylene, i.48 A in 1,3-butadiene. If
benzene actually possessed three single and three double bonds, as in a Kekule structure, we would
expect to find three short bonds (1.34 A) and three long bonds (1.48 A, probably, as in 1,3-butadiene).
Actually, x-ray diffraction studies show that the six carbon-carbon bonds in benzene are equal and have
a length of 1.39 A, and are thus intermediate between single and double bonds.

Orbital picture of benzene:

A more detailed picture of the benzene molecule is obtained from a consideration of the bond
orbitals in this molecule. Since each carbon is bonded to three other atoms, it uses SP 2 orbitals (as in
ethylene). These lie in the same plane, as that of the carbon nucleus, and are directed toward the corners
of an equilateral triangle. If we arrange the six carbons and six hydrogens of benzene to permit
maximum overlap of these orbitals, we obtain the structure shown in Fig. 2
 Benzene is a flat molecule, with every carbon and every hydrogen lying in the same plane. It is
a very symmetrical molecule, too, with each carbon atom lying at the angle of a regular hexagon; every
bond angle is 120֠. Each bond orbital is cylindrically symmetrical about the line joining the atomic
nuclei and hence, as before, these bonds are designated as σ bonds.
The molecule is not yet complete, however. There are still six electrons to be accounted for. In
addition to the three orbitals already used, each carbon atom has a fourth orbital, a p orbital. As we
know, this p orbital consists of two equal lobes, one lying above and the other lying below the plane of
the other three orbitals, that is, above and below the plane of the ring; it is occupied by a single
electron.
As in the case of ethylene; the orbital of one carbon can overlap the p orbital of an adjacent
carbon atom, permitting the electrons to pair and an additional п bond to be formed. But the overlap
here is not limited to a pair of p orbitals as it was in ethylene; the p orbital of any one carbon atom
overlaps equally well the p orbitals of both carbon atoms to which it is bonded. The result (see Fig. 3) is
two continuous dough nut-shaped electron clouds, one lying above and the other below the plane of the
atoms.

As with the allyl radical, it is the overlap of the p orbitals in both directions, and the resulting
participation of each electron in several bonds that corresponds to our description of the molecule as a
resonance hybrid of two structures. Again it is the delocalization of the п electrons their participation in
several bonds that makes the molecule more stable. To accommodate six п electrons, there must be
three orbitals. Their sum is, however, the symmetrical TT clouds we have described. The orbital
approach reveals the importance of the planarity of the benzene ring. The ring is flat because the
trigonal (sp2) bond angles of carbon just fit the 120 angles of a regular hexagon; it is this flatness that
permits ihe overlap of the p orbitals in both directions, with the resulting delocalization and
stabilization. The facts are consistent with the orbital picture of the benzene molecule. X-ray and
electron diffraction show benzene (Fig. 4) to be a completely flat, symmetrical molecule with all
carbon-carbon bonds equal, and all bond angles 120֠.

As we shall see, the chemical properties of benzene are just what we would expect of this
structure. Despite delocalization, the п electrons are nevertheless more loosely held than the σ
electrons. The п electrons are thus particularly available to a reagent that is seeking electrons: the
typical reactions of the benzene ring are those in which it serves as a source of electrons for
electrophiiic (acidic) reagents. Because of the resonance stabilization of the benzene ring, these
reactions lead to substitution, in which the aromatic character of the benzene ring is preserved.
Resonance in benzene ring:

For convenience we shall represent the benzene ring by a regular hexagon containing a circle
(I); it is understood that a hydrogen atom is attached to each angle of the hexagon unless another atom
or group is indicated.

I represents a resonance hybrid of the Kekule structures II and III. The straight lines stand for
the a bonds joining carbon atoms. The circle stands for the cloud of six delocalized n electrons. (From
another viewpoint, the straight lines stand for single bonds, and the circle stands for the extra half-
bonds.) I is a particularly useful representation of the benzene ring, since it emphasizes the equivalence
of the various carbon-carbon bonds. The presence of the circle distinguishes the benzene ring from the
cyclohexane ring, which is often represented today by a plain hexagon.
Aromatic character, The Huckel 4n + 2 rule:

We have defined aromatic compounds as those that resemble benzene. But just which properties
of benzene must a compound possess before we speak of it as being aromatic? Besides the compounds
that contain benzene rings, there are many other substances that are called aromatic; yet some of these
superficially bear little resemblance to benzene.
What properties do all aromatic compounds have in common?

From a theoretical standpoint, to be aromatic a compound must have a molecule that contains;
Rules of aromaticity:
 It should contain a cyclic ring structure.
 All the atoms of ring should be in SP2 hybridization or the atoms of ring should contain a p
orbital that doesn’t participate in hybridization.
 All the atoms of the ring should be in the same plane. The molecule should be planer.
 There must be a particular number of п electrons: 2, or 6, or 10, etc. This requirement, called
the 4n + 2 rule or Huckel rule.
From the experimental standpoint, aromatic compounds are compounds whose molecular
formulas would lead us to expect a high degree of unsaturation, and yet which are resistant to the
addition reactions generally characteristic of unsaturated compounds. Instead of addition reactions, we
often find that these aromatic compounds undergo electrophilic substitution reactions like those of
benzene. Along with this resistance toward addition and presumably the cause of it we find evidence of
unusual stability: low heats of hydrogenation and low heats of combustion. Aromatic compounds are
cyclic generally containing five-, six-, or seven-membered rings and when examined by physical
methods, they are found to have flat (or nearly fiat) molecules.

Let us look at some of the evidence supporting the Hiickel rule. Benzene has six п electrons, the
aromatic sextet; six is, of course, a Huckel number, corresponding to n = 1. Besides benzene and its
relatives (naphthalene, anthracene, phenanthrene), we shall encounter a number of heterocyclic
compounds that are clearly aromatic; these aromatic heterocycles, we shall see, are just the ones that
can provide an aromatic sextet. or, as further examples, consider these six compounds, for each of
which just one contributing structure is shown:

Each molecule is a hybrid of either five or seven equivalent structures, with the charge or odd
electron on each carbon. Yet, of the six compounds, only two give evidence of unusually high stability:
the cyclopentadienyl anion and the cycloheptatrienyl cation.

Cyclopentadienyl anion. (a) Two electrons in p orbital of one carbon ; one electron in p orbital
of each of the other carbons, (b) Overlap ofp orbitals to form -n bonds, (c) ir clouds above and below
plane of ring; total of six IT electrons, the aromatic sextet.
Reactivity and reactions of benzene:
We have already seen that the characteristic reactions of benzene involve substitution, in which
the resonance-stabilized ring system is preserved. What kind of reagents bring about this substitution?
What is the mechanism by which these reactions take place?
Above and below the plane of the benzene ring there is a cloud of п electrons. Because of
resonance, these п electrons are more involved in holding together carbon nuclei than are the п
electrons of a carbon-carbon double bond. Still, in comparison with a electrons, these n electrons are
loosely held and are available to a reagent that is seeking electrons.

Reactivity and reactions of Benzene:

It is not surprising that in its typical reactions the benzene ring serves as a source of electrons, that is, as
a base. The compounds with which it reacts are deficient in elections, that is, are electrophilic reagents
or acids. The characteristic reactions of the benzene ring are electrophilic substitution reactions.
These reactions are characteristic not only of benzene itself, but of the benzene ring wherever it is
found and, indeed, of many aromatic rings, benzenoid and non-benzenoid.

Electrophilic aromatic substitution includes a wide variety of reactions: nitration, halogenation,

sulfonation, and Friedel-Crafts reactions, undergone by nearly all aromatic rings; reactions like
nitrosation and diazo coupling, undergone only by rings of high reactivity

General mechanism of electrophilic substitution:

The benzene ring with its п electron behaves as an electron-rich system. The electrons in the п
clouds are readily available to form new bonds with electron-deficient spieces, the electrophile (E +).
The various electrophillic substitution reactions follow the same mechanistic
pathway.
Step I. Generation of electrophiles either by spontaneous dissociation of the reagent (E-Nu) or by acid-
catalysed dissociation.
Step 2. Formation of п-complex due to a loose association of the electrophile (E +) with the aromatic
ring. In this п-complex, the electrophile is not attached to any specific position of the ring, but later
arranges to give the σ-complex.

Step 3. A proton (H+) is then eliminated from the σ-complex by base (:B -) to yield the final
substitution product.

Nitration Reaction:
benzene reacts with nitric acid in presence of sulphuric acid to form nitrobenzene

Mechanism:
The commonly accepted mechanism for nitration with a mixture of nitric and sulfuric acids (the
widely used "mixed acid" of the organic chemist) involves the following sequence of reactions:
Sulfonation reaction:
Benzene may be suiphonated by treating it with concentrated sulphuric acid or fuming sulphuric
acid

Mechanism:
Sulfonation of many aromatic compounds involves the following steps
Unlike most other electrophilic substitution reactions, sulfonation is reversible, and this fact gives us
our clue. Reversibility means that carbonium ion II can lose SO3 to form the hydrocarbon. Evidently
here reaction (2) is not much faster than the reverse of reaction (1). In sulfonation, the energy barriers
on either side of the carbonium ion II must be roughly the same height; some ions go one way, some go
the other. Thus, the particular shape of potential energy curve that makes sulfonation reversible also
permits an isotope effect to be observed.

Halogenation Reaction:
Benzene reacts with chlorine or bromine in the presence of Lewis acid catalyst such as AlCl 3,
FeCI3 or FeBr3 when substitution in the ring takes place, a proton being lost as HCl or HBr.

Mechanism:
Aromatic halogenation, illustrated for chlorination, involves the following steps.
Friedel-Crafts alkylation:
This reaction involves the introduction of an alkyl group in the benzene ring for the synthesis of
alkyl benzenes which are not ordinarily available. For illustration, when benzene reacts with an alkyl
halide (RCI or RBr) in the presence of anhydrous AICl 3 as catalyst, one of the hydrogen atoms of the
ring is substituted by the alkyl group.

Mechanism:
In Friedel-Crafts alkylation, the electrophile is typically a carbonium ion. It, too, is formed in an acid-
base equilibrium, this time in the Lewis sense:

In certain cases, there is no free carbonium ion involved. Instead, the alkyl group is transferred without
a pair of electrons directly to the aromatic ring from the polar complex I, between AlCl 3 and the alkyl
halide:

The electrophile is thus either (a) R + or (b) a molecule like I that can readily transfer R + to the
aromatic ring. This duality of mechanism is common In electrophilic aromatic substitution. In either
case, the Lewis acid R+ is displaced from RC1 by the other Lewis acid, A1C13.
Limitations of Friedel-Crafts alkylation:
Although, the Friedel-Crafts alkylation reaction is very advantageous for attaching an alkyl group to an
aromatic ring, it suffers from the following limitations.
(i) Rearrangement of the Alkyl group. It is difficult to introduce an alkyl group higher than CH 3CH2--
group as it tends to undergo skeletal rearrangement. For example, alkylation of benzene with n-propyl
chloride gives isopropylbenzene, and not n-propylbenzene.

This is due to the fact that n-propylcarbonium that results from interaction with AlCl 3 undergoes
rearrangement to give more stable isopropyl carbonium ion, which electrophile then attacks benzene as
usual to form isopropylbenzene.

(ii) Polyalkylation. The introduction of an alkyl group in benzene activates the ring for further
electrophilic substitution. Thus more than one alkyl groups get attached to the aromatic ring.

(iii) Hindrance due to meta orienting groups. The presence of a meta-orienting group in the aromatic
ring hinders the Friedei-Crafts alkylation as such a group lowers the electron density in the ring. Thus
nitrobenzene does not respond to Friedel-Crafts reaction.

Friedel-Crafts Acylation:
This reaction involves the use of acid chlorides rather than alkyl halides. An acyl group, RCO +,
becomes attached to the aromatic ring, thus forming a ketone; the process is called acylation. As usual
for the Friedel-Crafts reaction the aromatic ring undergoing substitution must be at least as reactive as
that of a halobenzene; catalysis by aluminum chloride or another Lewis acid is required.
Mechanism:
The most likely mechanism for Friedel-Crafts acylation is analogous to the carbonium ion
mechanism for Friedel-Crafts alkylation, and involves the following steps:

This fits the pattern of electrophilic aromatic substitution; the attacking reagent this time being
the acylium ion. The acylium ion is considerably more stable than ordinary carbonium ions since in it
every atom has an octet of electrons. Alternatively, it may be that the electrophile is a complex between
acid chloride and Lewis acid.

Reactivity and orientation:

We have seen that certain groups activate the benzene ring and direct substitution to ortho and
para positions, and that other groups deactivate the ring and (except halogens) direct substitution to
meta positions. Let us see if we can account for these effects on the basis of principles we have already
learned.
First of all, we must remember that reactivity and orientation are both matters of relative rates of
reaction. Methyl is said to activate the ring because it makes the ring read faster than benzene; it causes
ortho,para orientation because it makes the ortho and para positions react faster than the nieta positions.
Now, we know that, whatever the specific reagent involved, the rate of electrophilic aromatic
substitution is determined by the same slow step ie., attack of the electrophile on the ring to form a
carbonium ion:

Any differences in rate of substitution must therefore be due to differences in the rate of this step.
For closely related reactions, a difference in rate of formation of carbonium ions is largely
determined by a difference in Eact that is, by a difference in stability of transition states. As with other
carbonium ion reactions we have studied, factors that stabilize the ion by dispersing the positive charge
should for the same reason stabilize the incipient carbonium ion of the transition state. Here again we
expect the more stable carbonium ion to be formed more rapidly. We shall therefore concentrate on the
relative stabilities of the carbonium ions. In electrophilic aromatic substitution the intermediate
carbonium ion is a hybrid of structures I, II, and III, in which the positive charge is distributed about the
ring, being strongest as the positions ortho and para to the carbon atom being attacked.

A group already attached to the benzene ring should affect the stability of the carbonium ion by
dispersing or intensifying the positive charge, depending upon its electron-releasing or electron-
withdrawing nature. It is evident from the structure of the ion (I-III) that this stabilizing or destabilizing
effect should be especially important when the group is attached ortho or para to the carbon being
attacked.

Theory of reactivity:
To compare rates of substitution in benzene, toluene, and nitrobenzene, we compare the structures of
the carbonium ions formed from the three compounds:

By releasing electrons, the methyl group (II) tends to neutralize the positive charge of the ring
and so become more positive itself; this dispersal of the charge stabilizes the carbonium ion. In the
same way the inductive effect stabilizes the developing positive charge in the transition state and thus
leads to a faster reaction. The NO2 group, on the other hand, has an electron-withdrawing inductive
effect (III); this tends to intensify .the positive charge, destabilizes the carbonium ion, and thus causes a
slower reaction.
Reactivity in electrophilic aromatic substitution depends, then, upon the tendency of a
substituent group to release or withdraw electrons. A group that releases electrons activates the ring; a
group that withdraws electrons deactivates the ring.
 We might expect replacement of hydrogen in CH3 by halogen to decrease the electron-releasing
tendency of the group, and perhaps to convert it into an electron-withdrawing group.

Theory of orientation:
Before we try to account for orientation in electrophilic substitution, let us look more closely at
the facts. An activating group activates all positions of the benzene ring; even the positions meta to it
are more reactive than any single position in benzene itself. It directs ortho and para simply because it
activates the ortho and para positions much more than it does the meta.
A deactivating group deactivates all positions in the ring, even the positions meta to it. It directs
meta simply because it deactivates the ortho and para positions even more than it does the meta. Thus
both ortho, para orientation and meta orientation arise in the same way: the effect of any group whether
activating or deactivating is strongest at the ortho and para positions.

In Electron donating group substituted benzenes:

To see if this is what we would expect, let us compare, for example, the carbonium ions formed
by attack at the para and meta positions of toluene, a compound that contains an activating group. Each
of these is a hybrid of three structures, I-III for para, IV-VI for meta. In one of these six structures, II,
the positive charge is located on the carbon atom to which CH 3 is attached. Although CH3 releases
electrons to all positions of the ring, it does so most strongly to the carbon atom nearest it;
consequently, structure II is a particularly stable one. Because of contribution from structure II, the
hybrid carbonium ion resulting from attack at the para position is more stable than the carbonium ion
resulting from attack at a meta position. Para substitution, therefore, occurs faster than meta
substitution.

In the same way, it can be seen that attack at an ortho position (VII-IX) also yields a more stable
carbonium ion, through contribution from IX, than attack at a meta position.

In toluene, ortho, para substitution is thus faster than meta substitution because electron release
by CH3 is more effective during attack at the positions ortho and para to it.

In Electron donating group substituted benzenes:

Let us compare the carbonium ions formed by attack at the para and meta positions of
nitrobenzene, a compound that contains a deactivating group. Each of these is a hybrid of three
structures, X-XII for para attack, XI II-XV for meta attack.
 In one of the six structures, XI, the positive charge is located on the carbon atom to which NO 2
is attached. Although NO2 withdraws electrons from all positions, it does so most from the carbon atom
nearest it, and hence this carbon atom, already positive, has little tendency to accommodatefc the
positive charge of the carbonium ion. Structure XI is thus a particularly unstable one and does little to
help stabilize the ion resulting from attack at the para position. The ion for para attack is virtually a
hybrid of only two structures, X and XII; the positive charge is mainly restricted to only two carbon
atoms. It is less stable than the ion resulting from attack at a meta position, which is a hybrid of three
structures, and in which the positive charge is accommodated by three carbon atoms. Para substitution,
therefore, occurs more slowly than meta substitution. In the same way it can be seen that attack at an
ortho position (XVI-XVIII) yields a less stable carbonium ion, because of the instability of XVIII, than
attack at a meta position.

In nitrobenzene, ortho, para substitution is thus slower than meta substitution because electron
withdrawal by NO2 is more effective during attack at the positions ortho and para to it.

Thus we see that both ortho, para orientation by activating groups and meta orientation by deactivating
groups follow logically from the structure of the intermediate carbonium ion. The charge of the
carbonium ion is strongest at the positions ortho and para to the point of attack, and hence a group
attached to one of these positions can exert the strongest effect, whether activating or deactivating.
Effect of halogen on electrophilic aromatic substitution:
The unusual behavior of the halogens, which direct ortho and para although deactivating, results from a
combination of two opposing factors. Halogen withdraws electrons through its inductive effect, and
releases electrons through its resonance effect. So, presumably, can the NH 2 and OH groups, but there
the much stronger resonance effect greatly outweighs the other. For halogen, the two effects are more
evenly balanced, and we observe the operation of both. Let us first consider reactivity. Electrophilic
attack on benzene yields carbonium ion I, attack on chlorobenzene yields carbonium ion II.

The electron withdrawing inductive effect of chlorine intensifies the positive charge in
carbonium ion II, makes the ion less stable, and causes a slower reaction. Next, to understand
orientation, let us compare the structures of the carbonium ions formed by attack at the para and meta
positions of chlorobenzene. Each of these is a hybrid of three structures, III-V for para, VI-VIII for
meta.

In one of these six structures, IV, the positive charge is located on the carbon atom to which
chlorine is attached. Through its inductive effect chlorine withdraws electrons most from the carbon to
which it is joined, and thus makes structure IV especially unstable. As before, we expect IV to make
little contribution to the hybrid, which should therefore be less stable than the hybrid ion resulting from
attack at the meta positions. If only the inductive effect were involved, then, we would expect not only
deactivation but also meta orientation. But the existence of halonium ions has shown us that halogen
can share more than a pair of electrons and can accommodate a positive charge. If we apply that idea to
the present problem, what do we find? The ion resulting from para attack is a hybrid not only of
structures III-V, but also of structure IX, in which chlorine bears a positive charge and is joined to the
ring by a double bond. This structure should be comparatively stable, since in it every atom (except
hydrogen, of course) has a complete octet of electrons. (Structure IX is exactly analogous to those
proposed to account for activation and ortho, para direction by NH 2 and OH.) No such structure is
possible for the ion resulting from meta attack. To the extent that structure IX contributes to the hybrid,
it makes the ion resulting from para attack more stable than the ion resulting from meta attack.
Although we could not have predicted the relative importance of the two factors the instability of IV
and the stabilization by IX the result indicates that the contribution from IX is the more important.

In the same way it can be seen that attack at an ortho position also yields an ion (X-X1II) that
can be stabilized by accommodation of the positive charge by chlorine.

Through its inductive effect halogen tends to withdraw electrons and thus to destabilize the
intermediate carbonium ion. This effect is felt for attack at all positions, but particularly for attack at the
positions ortho and para to the halogen. Through its resonance effect halogen tends to release electrons
and thus to stabilize the intermediate carbonium ion. This electron release is effective only for attack at
the positions ortho and para to the halogen.
The inductive effect is stronger than the resonance effect and causes net electron withdrawal and hence
deactivation for attack at all positions. The resonance effect tends to oppose the inductive effect for
attack at the ortho and para positions, and hence makes the deactivation less for ortho, para attack than
for meta. Reactivity is thus controlled by the stronger inductive eflect, and orientation is controlled by
the resonance effect, which, although weaker, seems to be more selective.

Electron release via resonance:

Certain groups (-NH2 and -OH, and their derivatives) act as powerful activators toward
electrophilic aromatic substitution, even though they contain electronegative atoms and can be shown
in other ways to have electron withdrawing inductive effects. If our approach to the problem is correct,
these groups must release electrons in some other way than through their inductive effects; they are the
carbonium ion formed by attack para to the NH2 group of aniline, for example, is considered to be a
hybrid not only of structures I, II, and III, with positive charges located on carbons of the ring, but also
of structure IV in which the positive charge is carried by nitrogen.

Structure IV is especially stable, since in it every atom (except hydrogen, of course) has a
complete octet of electrons. This carbonium ion is much more stable than the one obtained by attack on
benzene itself, or the one obtained (V-VI1) from attack rneta to the -NH2 group of aniline; in neither of
these cases is a structure like IV possible. (Compare, for example, the stabilities of the ions NH 4+ and
CH3+. Here it is not a matter of which atom, nitrogen or carbon, can better accommodate a positive
charge; it is a matter of which atom has a complete octet of electrons.)

Examination of the corresponding structures (VIII-XI) shows that ortho attack is much like para
attack: Thus substitution in aniline occurs faster than substitution in benzene, and occurs predominantly
at the positions ortho and para to NH2.

In the same way activation and ortho, para orientation by the OH group is accounted for by
contribution of structures like XII and XIII, in which every atom has a complete octet of electrons: The
similar effects of the derivatives of NH2 and OH are accounted for by similar structures (shown only for
para attack):
Dicophane(DDT): 1,1,1-trichloro-2,2-di-(4-chloro-phenyl)ethane
It is a crystalline powder, insoluble in water,
Cl
Cl Cl

Cl Cl
Uses:
 In form of solutions or aerial sprays against malaria mosquito
 As dusting powder for eradication of typhus bearing body lice

Saccharin:
It is white crystalline solid, sparingly soluble in water having an intensely sweet taste.
O

NH
S
O
O
Uses:
 It is used as artificial sweetening agent

Gamma benzenehexachloride: lindane, Gammexane, Benzenehexachloride(BHC)

Is a white crystalline powder, insoluble in water
Cl H
H H
Cl Cl

Cl H
H Cl
Cl H
Uses:
 In treatment of head lice
 Treatment of scabies
 Destruction of various objectionable insects.
Chloramine: Sodium toluene p-sulphonchloramide
Colorless crystalline solid of characteristic odour
O Na
H3C S N
O Cl

Uses:
 Used as Disinfectant
Unit-II:
Phenols are compounds of the general formula ArOH, where Ar is phenyl, substituted phenyl, or
one of the otfer aryl groups we shall study later (e.g., naphthyl). Phenols differ from alcohols in having
the OH group attached directly to an aromatic ring. Phenols are generally named as derivatives of the
simplest member of the family, phenol. The methyl phenols are given the special name of cresols.
Occasionally phenols are named as hydroxyl-compounds.

Both phenols and alcohols contain the OH group, and as a result the two families resemble each
other to a limited extent. The two kinds of compounds differ so greatly that they well deserve to be
classified as different families.
The simplest phenols are liquids or low-melting solids; because of hydrogen bonding, they have
quite high boiling points. Phenol itself is somewhat soluble. in water (9 g per 100 g of water),
presumably because of hydrogen bonding with the water; most other phenols are essentially insoluble
in water.

Preparation of phenols:
Diazonium salts react with water to yield phenols. This reaction takes place slowly in the ice-cold
solutions of diazonium salts, and is the reason diazonium salts are used immediately upon preparation;
at elevated temperatures it can be made the chief reaction of diazonium salts.

Hydrolysis of diazonium salts is a highly versatile method of making phenols. It is the last step
in a synthetic route that generally begins with nitration Much simpler and more direct is a recently
developed route via thallation. An arylthallium compound is oxidized by lead tetraacetate (in the
presence of triphenylphosphine, Ph3P) to the phenolic ester of trifluoroacetic acid, which on hydrolysis
yields the phenol. The entire sequence, including thallation, can be carried out without isolation of
intermediates. Although the full scope of the method has not yet been reported, it has two advantages
over the diazonium route: (a) the speed and high yield made possible by the fewer steps; and (b)
orientation control in the thallation step.

Like the phenols we have already studied, naphthols can be prepared from the corresponding
sulfonic acids by fusion with alkali. Naphthols can also be made from the naphthylamines by direct
hydrolysis under acidic conditions.

4. Hydrolysis of Aryl halides:

 Aryl halides yield phenol when hydrolyzed with aqueous sodium hydroxide. For example,
chlorobenzene is hydrolyzed to phenol at high temperature and pressure by NaOH (Dow’s Process)

.
The above hydrolysis can also be effected by water in the presence of copper as catalyst.

5. Decarboxylationof Phenolic acids.

When phenolic acids are distilled with sodalime, they decarboxylate to form sodium phenoxide.
This on acid hydrolysis gives phenol, Thus,

6. Oxidation of Grignard Reagent followed by hydrolyses.

An aryl Grignard reagent on treatment with oxygen in the presence of light, and subsequent
hydrolysis of the product with mineral acid yields phenol in fair quantities.

Reactions of phenols:
a) REACTIONS INVOLVING –OH BOND
1. Acid Character; Salt Formation.
The acidity of a compound is defined as Its proton releasing ability in the presence of
water (Lewis base). Phenols behave as weak acids because they ionize when dissolved in water, to form
phenoxide anions. Thus,
2. COLOURED COMPLEXES WITH FeCl3:
Phenols form coloured iron complexes when treated with neutral ferric chloride solution. These
complexes have characteristic colours and this reaction is often used as a test for phenols. Phenol gives
violet colouration resorcinol, violet ; catechol, green pyrogallol, red phloroglucinol, dark violet and so
on.
The formation of the iron complexis is attributed to the existence of keto-enol tautomerism in
phenols as already mentioned. Since a phenol is mostly in the enol form, it forms
coloured complexes with ferric chloride - a character associated with compounds containing enolic
structure,

3. REPLACEMENT OF -OH BY H:
When a phenol is distilled with zinc dust, the –OH function is replaced by a hydrogen atom. Zinc
removes the oxygen of the phenolic group as zinc oxide, to yield the parent hydrocarbon.

4. REPLACEMENT OF -OR BY HALOGENS:

Unlike alcohols, phenolic OR cannot be replaced by halogen by reaction with hydrogen halides.
The yield is very poor owing to the main reaction of phenol with PCI 3 to form esters of phosphoric
acid. Phosphorus trichloride reacts with phenol to form only the ester of phosphorus acid, and no aryl
chloride is obtained.
5. REPLACEMENT OF -OR BY –NH2 FUNCTION:
The replacement of phenolic OR by amine function can be brought about by heating a phenol with
ammonia in presence of zinc chloride.

6. ETHER FORMATION:
Phenols form ethers easily by reaction of sodium phenoxide with aryl halides (Williamson
Synthesis). The phenoxide ion is nucleophilic and will displace the halogen from an alkyl halide, giving
a mixed alkyl aryl ether.

7. ESTERIFICATION:
Phenols react with acyl halides or anhydrides to form esters, in presence of a base (NaOH or
pyridine)

b) Reactions involving aromatic ring:

1) HALOGENATION.
The —OH group on the benzene ring is a very powerful activator and special precautions are
necessary to check polysubstitution in the ring. Phenol when treated with excess of aqueous bromine
solution yields 2,4,6tribromophenol straightway.
2) SULPHONATION:
Phenols are readily sulphonated with conc. H2SO4, Here SO3H+ is the electrophile.

3) NITRATION.
Benzene ring in phenols being highly activated, phenols undergo nitration with dilute nitric
acid, NO2+ (nitronium ion) is the electrophile.

4) NITROSATION.
Here the electrophile is nitrosonium ion, NO+

5) ALKYLATION:
Phenols are alkylated in the aromatic ring when treated with alkyl halides, alkali solution, and
acidified.
6) ACYLATION:
Phenols react with aryl chlorides in the presence of FeCI3 to form ortho and para. acylphenols
(Friedel-Craft Reaction).

7) MERCURATION:
When a phenol is refluxed with aqueous mercuric acetate, o-acetoxymercuriphenol is obtained.

8) COUPLING REACTION:
Phenol reacts with aryldiazonium salt in the presence of alkali to form azo compounds, thus

9) KOLBE'S REACTION:
On passing carbon dioxide over sodium phenoxide at 125֠, COOH is directly introduced into the
aromatic ring in ortho position to OH. Thus:

10) GATTERMAAU REACTION; FORMYLATION.

 When a phenol is treated with a mixture of hydrogen cyanide, hydrogen chloride and zinc
chloride catalyst, an aldehyde (or formyl,-CHO) group is introduced in the ortho position to OH. Thus,
phenol yields o-hydroxybenzaldehyde.

11) REIMER TIEMAR REACTION:

The treatment of a phenol with chloroform (CHCI3) and aqueous NaOH at 60°C, followed by
acidification, introduces a -CHO group in the aromatic ring mainly in the ortho position to OH group.
For example

(i) Formation of dichloromethylene (dichiorocarbene) from chloroform

(ii) Electrophilic substitution in the phenol ring

(iii) Hydrolysis and acidification

12) CONDENSATION WITH ALDEHYDES:

Phenols undergo condensation with aldehydes by electrophilic substitution in ortho and para positions
in presence of acids or alkalis as catalyst.

13) CONDENSATION WITH PHTHALLIC ANHYDRIDE:

Many phenols undergo condensation with phthallic anhydride to give useful products. Thus
phenol when heated with H2SO4 gives phenolphthalein, a colourless compound which produces
beautiful pink colour with alkali solutions due to formation of coloured sodium salt.

14) OXIDATION:
Phenol is easily oxidized without disruption of its carbon skeleton to form p-benzoquinone.

Acidity of phenols:
 Phenols are converted into their salts by aqueous hydroxides, but not by aqueous bicarbonates.
The salts are converted into the free phenols by aqueous mineral acids, carboxylic acids, or carbonic
acid. Phenols must therefore be considerably stronger acids than water, but considerably weaker acids
than the carboxylic acids. Most phenols have K a's of about 10-10, whereas carboxylic acids have Ka's of
about 10-5.

Although weaker than carboxylic acids, phenols are tremendously more acidic than alcohols,
which have Ka's in the neighborhood of 10-16 to 10-18. How does it happen that an OH attached to an
aromatic ring is so much more acidic than an -OH attached to an alkyl group? The answer is to be
found in an examination of the structures involved. As usual we shall assume that differences in acidity
are due to differences in stabilities of reactants and products.
Let us examine the structures of reactants and products in the ionization of an alcohol and of
phenol. We see that the alcohol and the alkoxide ion are each represented satisfactorily by a single
structure. Phenol and the phenoxide ion contain a benzene ring and therefore must be hybrids of the
Kekule structures I and II, and III and IV.

This resonance presumably stabilizes both molecule and ion to the same extent. It lowers the
energy content of each by the same number of kcal/mole, and hence does not affect the difference in
their energy contents. If there were no other factors involved, then, we might expect the acidity of a
phenol to be about the same as the acidity of an alcohol.
However, there are additional structures to be considered. Being basic, oxygen can share more
than a pair of electrons with the ring; this is indicated by contribution from structures V-VII for phenol,
and VIII-X for the phenoxide ion. Now, are these two sets of structures equally important? Structures
V-VII for phenol carry both positive and negative charges; structures VIII-X for phenoxide ion carry
only a negative charge. Since energy must be supplied to separate opposite charges, the structures for
the phenol should contain more energy and hence be less stable than the structures for phenoxide ion.
The net effect of resonance is therefore to stabilize the phenoxide ion to a greater extent than the
phenol, and thus to shift the equilibrium toward ionization and make Ka larger than for an alcohol.

Effect of substituent on acidity of phenols:

Phenols being stronger acids than their aliphatic counterparts, the alcohols, because resonance
stabilizes the ion to a greater extent than it do the free phenol. We see that electron-attracting
substituents (B) like X or NO2 increase the acidity of phenols, and electron-releasing substituents(A)
like CH3 decrease acidity. Thus substituents affect acidity of phenols in the same way that they affect
acidity of carboxylic acids; it is, of course, opposite to the way these groups affect basicity of amines.
Electron-attracting substituents tend to disperse the negative charge of the phenoxide ion, whereas
electron-releasing substituents tend to intensify the charge.

Qualitative tests for phenols:

(1) Solubility Test:
If the substance is more soluble in NaOH solution than in water, but insoluble in sodium
bicarbonate, it’s probably a phenol.
(2) Ferric Chloride Test:
To the aqueous or ethanolic solution of the substance, add a few drops of neutral FeCl3 solution.
The appearance of an intense colour—red, green or black confirms the presence of a phenol.
(3) Bromine Water Test:
Bromine water is added to aqueous solution of the substance until the yellow colour of bromine
persists. The Formation of a precipitate (substitution product) confirms the presence of a phenol.
(4) Libermann’s Test:
 The substance is warmed with a little concentrated sulphuric acid and sodium nitrate. The dark-
coloured product so obtained is treated with dilute NaOH solution. The production of intense blue or
green colour confirms the presence of a phenol.

Phenol:
It is a white crystalline solid that is volatile. It is mildly acidic and requires careful handling due
to its propensity to cause chemical burns.

Uses:
 Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin but also
many herbicides and pharmaceutical drugs.
 Phenol once was widely used as an antiseptic.
 it was used as a soap, known as carbolic soap.
 Phenol is also used as a preservative in some vaccines.
 Phenol spray is used medically to help sore throat.
 It is the active ingredient in some oral analgesics such as Chloraseptic spray and Carmex,
commonly used to temporarily treat pharyngitis.

Cresol:
Cresols (also hydroxytoluene) are organic compounds which are methylphenols. Depending on
the temperature, cresols can be solid or liquid because they have melting points not far from room
temperature. There are three forms (isomers) of cresol: ortho-cresol (o-cresol), meta-cresol (m-cresol),
and para-cresol (p-cresol).

Uses:
Cresols are precursors or synthetic intermediates to other compounds and materials, including plastics,
pesticides, pharmaceuticals, and dyes.

Derivatives of p-cresol include:

  Bupranolol, a non-selective beta blocker

 Butylated hydroxytoluene, a common antioxidant

 Indo-1, a popular calcium indicator

Derivatives of o-cresol include:

 MCPA, (4-chloro-2-methylphenoxy)acetic acid

 MCPB, 4-(4-chloro-2-methylphenoxy)butanoic acid

 Mecoprop, (RS)-2-(4-chloro-2-methylphenoxy)propanoic acid

 atomoxetine, (3R)-N-methyl-3-(2-methylphenoxy)-3-phenylpropan-1-amine

 the diol mephenesin, 3-(2-methylphenoxy)propane-1,2-diol

Derivatives of m-cresol include:

 Amylmetacresol, an antiseptic

 Bevantolol,(RS)-[2-(3,4-dimethoxyphenyl)ethyl][2-hydroxy-3-(3-methylphenoxy)-
propyl]amine

 Bromocresol green

 Chloro-m-cresol which is used as a household disinfectant

 Tolimidone, 5-(3-methylphenoxy)pyrimidin-2(1H)-one
Resorcinol:
It is the 1,3-isomer (or meta-isomer) of benzenediol with the formula C6H4(OH)2. Resorcinol
crystallizes from benzene as colorless needles that are readily soluble in water, alcohol, and ether, but
insoluble in chloroform and carbon disulfide.

Uses:
  It is used as an antiseptic and disinfectant in topical pharmaceutical products in the treatment of
skin disorders and infections such as acne, seborrheic dermatitis, eczema, psoriasis, corns,
calluses, and warts.
 It exerts a keratolytic activity.
 Resorcinol works by helping to remove hard, scaly, or roughened skin.
 It has also been employed in the treatment of gastric ulcers.
 It is also worked up in certain medicated soaps.

Naphthols:
The naphthols are naphthalene homologues of phenol, with the hydroxyl group being more
reactive than in the phenols. Both isomers are soluble in simple alcohols, ethers, and chloroform. They
are precursors to a variety of useful compounds.

Uses:
 1-naphthol dissolved in ethanol, known as Molisch's reagent, is used as reagent for detecting the
presence of carbohydrates.
 1-Naphthol is a precursor to a variety of insecticides including carbaryl and pharmaceuticals
including nadolol.
 2-Naphthol is a widely used intermediate for the production of dyes and other compounds.

AROMATIC AMINES:
The amino derivatives of the aromatic hydrocarbons are of two types
(a) Aryl amines or Aromatic amines in which the NH2 group (or substituted –NHR group) is
attached directly to a carbon of the benzene ring.
(b) Aryl-alkyl or Aralkyl amines in which the –NH2 group is attached to a carbon of the side-
chain.
Both with respect to the methods of preparation and reactions of –NH 2 group, aralkyl amines
are similar to the aliphatic amines. The aromatic or aryl amines are generally prepared by reduction of
the aromatic nitro compounds, which are readily obtained by direct nitration of aromatic hydrocarbons.
They differ considerably in a number of respects from aliphatic amines.
 Like the aliphatic amines, the aromatic amines may also be divided into primary, secondary and
tertiary amities according as one, two or three hydrocarbon groups are attached to the amino N atom.
Thus:

Preparation of Aromatic amines:

1) Reduction of Nitro Compounds:
Monoamines are nearly always prepared by reduction of the corresponding nitro compounds.
This reduction may be carried with metal and mineral acid e.g tin, zinc or iron, and HCI or H,SO4.

The reduction of a nitro compound can also be effected with hydrogen in the presence of
platinum or Raney nickel at room temperature.

2) Ammonolysis of Aryl halides:

Aryl halides react with ammonia in the presence a catalyst (copper salts) at high temperature
and under high pressure to form the corresponding amino compounds.

3) Ammonolysis of Phenols:
Phenols react with ammonia in the presence of zinc chloride at about 300֠C to form the
corresponding amines.
4) Degradation of amides (Hoffmann rearrangement).
Like primary aliphatic amines, primary aromatic amine can be obtained by the degradation of
aryl amides with bromine or chlorine in alkaline solution.

5) Reduction of Azo Compounds:

The reduction of azo compounds by the way of hydrazo compounds yields primary aromatic
amines.

6) Action of Hydroxylamine with Hydrocarbons:

Aromatic hydrocarbons react directly with hydroxylamine in the presence of a catalyst (FeCI3,
AICI3) to give monoamines.

Methods of preparation of 2° And 3° Aromatic Amines:

1) Alkylation of 1° and 2° Aromatic amines:
Primary and secondary aromatic amines on reaction with alkyl halide, give secondary and
tertiary aromatic amines. Thus
2) Reduction of appropriate Anilides:
When anilides are reduced with LiAlH4 secondary and tertiary aromatic amines are formed.

3) Reductive ammination:
Aldehydes and ketones react with primary or secondary aromatic amines in the presence of a
reducing agent to form secondary or tertiary aromatic amines.

Reactions of aromatic amines:

1) Salt Formation:
Although aromatic amines are weaker bases than ammonia or aliphatic amines, they form well-
defined crystalline salts on reaction with strong mineral acids as HCI or H2SO4. For example,
2) Acylation:
The primary and secondary aromatic amines react with aryl chlorides or anhydrides, when the
hydrogen atom attached to N-atom is replaced by aryl group (Acylation).

3) Sulphonylation:
Here aromatic secondary and tertiary amines are allowed to react with an aryl suiphonyl
chloride in pyridine. The hydrogen atom attached to N atom is replaced by the suiphonyl group to form
N-arylsulphonamide. Thus aniline reacts with benzene sulphonyl chloride to form
N-benzenesulphonamide.

The preparation of N-arylsuiphonamides may also be used for the identification of aromatic 2°
and 3° amines. As described under aliphatic amines, the formation of sulphonamides provides a method
(Hinsberg method) for the separation of 1°, 2° and 3° amines.

4) Alkylation and Arylation:

 Like the aliphatic amines, the primary aromatic amines react with alkyl halides to form mono-,
di-, and finally, trialkylammonium salts. These mono and di-alkylammonium salts on treatment with
sodium hydroxide, are decomposed to yield the free N-alkyl and N, N.dialkyl-arylamines. For example,

Alkylaion of aromatic primary amines may also be done by heating under pressure with an
alcohol in the presence of a strong mineral acid (HCI or H2SO4).

Primary arylamines react with simple aryl halides only with difficulty; Some diphenylamine is
produced by heating aniline at 200°C (under pressure) with chiorobenzene in the presence of cuprous
chloride.

5) Reaction with Nitrous acid (Diazotization Reaction):

Aromatic amines react with nitrous acid (NaNO2 + HCI) like the aliphatic amines, and the
nature of the reaction depends on whether the amine is primary, secondary or tertiary. However, there is
close correspondence in the behavior of aliphatic and aromatic amines in the case of secondary amines
only.
6) Oxidation:
Aromatic amines are readily oxidized. In fact they are so sensitive in this respect that they
undergo slow aerial oxidation on storage and become dark in colour. Thus freshly distilled aniline is
colourless but soon turns yellow and then dark red on exposure to the air. The product of oxidation of
an aromatic amine depends on the conditions of oxidation and the reagent used. Vigorous oxidation of
primary aromatic amities with potassium dichromate and sulphuric acid results in the formation of
quinones.

7) Reaction with Aldehydes:

Primary aromatic amines react with aldehydes in a manner similar to that of the primary
aliphatic amines, giving condensation products Thus when aniline reacts with an aldehyde on warming
to give Anils or Schiff’s bases. These products are most stable when the aldehyde is an aromatic one
e.g., benzaldehyde.
8) Carbylamine Reaction:
Like primary aliphatic amines, primary aromatic amines react with chloroform and ethanolic
potash, to form Carbylamines (isocyanides or isonitriles). These have very disagreeable odour. The
reaction is, therefore; used as a test for the detection of amines and for chloroform.

9) Reaction with Carbon Disulphide:

Unlike the aliphatic primary amines, aniline does not react with carbon disulphide at room
temperature. When aniline and carbon disulphide are heated together, thiocarbanilide
(s-diphenylthiourea) and H2S are obtained.

10) Reaction with Grignard Reagents:

Primary and secondary aromatic amines react with Grignard reagents to form hydrocarbons.
Thus,

REACTIONS INVOLVING THE BENZENE RING:

1) Bromination:
When an aromatic amine is treated with chlorine, or bromine-water, halogenation occurs
readily. Thus the benzene ring in aniline is so greatly activated by -NH 2 group that treatment with
aqueous bromine at once forms 2, 4, 6-tribromoanhline.
2) Nitration:
Direct nitration of an aromatic ring which contains unprotected primary or secondary amino
group (-NH2 or -NHR) is unsatisfactory because of the susceptibility of amines toward oxidation.
However, such nitration of aniline, for example, by use of HNO 3 + H2SO4 mixture yields m-
nitroaniline.

3) Suiphonatlon:
Primary aromatic amines can be suiphonated without prior protection of –NH 2 group,
presumably because sulphuric acid is a weaker oxidising agent than nitric acid. Thus aniline can be
sulphonated to give p-aminobenzenesulphonic acid or sulphanilic acid.
Aniline reacts with sulphuric acid to form aniline hydrogen sulphate. It is assumed that heating
at 180 -200˚C forms phenylsulphanicacid. This undergo rearrangement to yield p-aminobenzene
sulphonicacid

4) Hofmann-Martius Rearrangement:
When N-alkyl. or N, N-dialkylanilines are heated in strong acid media at 300°C, intermolecular
migration of alkyl groups occurs. Thus when N,N-dimethylaniline hydrochloride is strongly heated, one
methyl group migrates preferentially to the para position of the ring. The N-methylaniline
hydrochloride so produced then undergoes migration of the remaining methyl group to the ortho
position, since the para position has been blocked. This reaction is known as Hofmann-Martius
Rearrangement and may be used for the preparation of homologues of aniline.
BASICITY OF AMINES AND ANILINES:
Like ammonia, amines are converted into their salts by aqueous mineral acids and are liberated
from their salts by aqueous hydroxides. Like ammonia, therefore, amines are more basic than water and
less basic than hydroxide ion:

We found it convenient to compare acidities of carboxylic acids by measuring the extent to

which they give up hydrogen ion to water; the equilibrium constant for this reaction was called the
acidity constant, Ka . In the same way, it is convenient to compare basicities of amines by measuring the
extent to which they accept hydrogen ion from water; the equilibrium constant for this reaction is called
a basicity constant, Kb.

Each amine has its characteristic Kb ; the larger the Kb , the stronger the base. We must not lose
sight of the fact that the principal base in an aqueous solution of an amine (or of ammonia, for that
matter) is the amine itself, not hydroxide ion. Measurement of [OH -] is simply a convenient way to
compare basicities. That aliphatic amines of all three classes have Kb's of about 10-3 to 10-4 (0.001 to
0.0001); they are thus somewhat stronger bases than ammonia (K b = 1.8 x 10-5). Aromatic amines, on
the other hand, are considerably weaker bases than ammonia, having K b's of 10-9 or less. Substituents
on the ring have a marked effect on the basicity of aromatic amines, p-nitroaniline, for example, being
only 1/4000 as basic as aniline.
Structure and basicity:
Let us see how basicity of amines is related to structure. We shall handle basicity just as we
handled acidity: we shall compare the stabilities of amines with the stabilities of their ions; the more
stable the ion relative to the amine from which it is formed, the more basic the amine. First of all,
amines are more basic than alcohols, ethers, esters, etc:, for the same reason that ammonia is more basic
than water: nitrogen is less electronegative than oxygen, and can better accommodate the positive
charge of the ion. An aliphatic amine is more basic than ammonia because the electron-releasing alkyl
groups tend to disperse the positive charge of the substituted ammonium ion and therefore stabilize it in
a way that is not possible for the unsubstituted ammonium ion. Thus an ammonium ion is stabilized by
electron release in the same way as a carbonium ion. From another point of view, we can consider that
an alkyl group pushes electrons toward nitrogen, and thus makes the fourth pair more available for
sharing with an acid. (The differences in basicity among primary, secondary, and tertiary aliphatic
amines are due to a combination of solvation and electronic factors.)

How can we account for the fact that aromatic amines are weaker bases than ammonia? Let us
compare the structures of aniline and the anilinium ion with the structures of ammonia and the
ammonium ion. We see that ammonia and the ammonium ion are each represented satisfactorily by a
single structure:
 Aniline and anilinum ion contain the benzene ring and therefore are hybrids of the Kekule
structures I and II, and III and IV. This resonance presumably stabilizes both amine and ion to the same
extent.

It lowers the energy content of each by the same number of kcal/mole, and hence doesn’t affect
indifference in their energy contents. If there were no other factors involved, then, we might expect the
basicity of aniline to be about the same as the basicity of ammonia. However, there are additional
structures to be considered. To account for the powerful activating effect of the NH 2 group on
electrophilic aromatic substitution, we considered that the intermediate carbonium ion is stabilized by
structures in which there is a double bond between nitrogen and the ring; contribution from these
structures is simply a way of indicating the tendency for nitrogen to share its fourth pair of electrons
and to accept a positive charge. It is generally believed that the -NH 2 group tends to share electrons
with the ring, not only in the carbonium ion which is the intermediate in electrophilic aromatic
substitution, but also in the aniline molecule itself.
Thus aniline is a hybrid not only of structures I and II but also of structures V, VI, and VII. We
cannot draw comparable structures for the anilinium ion. Contribution from the three structures V, VI,
and VII stabilizes the amine in a way that is not possible for the ammonium ion; resonance thus lowers
the energy content of aniline more than it lowers the energy content of the anilinium ion. The net effect
is to shift the equilibrium in the direction of less ionization, that is, to make Kb smaller.
 The low basicity of aromatic amines is thus due to the fact that the amine is stabilized by
resonance to a greater extent than is the ion.

From another point of view, we can say that aniline is a weaker base than ammonia because the
fourth pair of electrons is partly shared with the ring and is thus less available for sharing with a
hydrogen ion. The tendency (through resonance) for the NH 2 group to release electrons to the aromatic
ring makes the ring more reactive toward electrophilie attack; at the same time its tendency necessarily
makes the amine less basic. Similar considerations apply to other aromatic amines.

Effect of substituents on basicity of aromatic amines:

How is the basicity of an aromatic amine affected by substituents on the ring? In Table we see
that an electron-releasing substituent like CH3 increases the basicity of aniline, and an electron-
withdrawing substituent like X or NO2 decreases the basicity. These effects are understandable.

Electron release tends to disperse the positive charge of the anilinium ion, and thus stabilizes the
ion relative to the amine. Electron withdrawal tends to intensify the positive charge of the, anilinium
ion, and thus destabilizes the ion relative to the amine. We notice that the base-strengthening
substituents are the ones that activate an aromatic ring toward electrophilic substitution; the base-
weakening substituents are the ones that deactivate an aromatic ring toward electrophilic substitution.
Basicity depends upon position of equilibrium, and hence on relative stabilities of reactants and
products. Reactivity in electrophilic aromatic substitution depends upon rate, and hence on relative
stabilities of reactants and transition state. The effect of a particular substituent is the same in both
cases, however, since the controlling factor is accommodation of a positive charge. A given substituent
affects the basicity of an amine and the acidity of a carboxylic acid in opposite ways. This is to be
expected, since basicity depends upon ability to accommodate a positive charge, and acidity depends
upon ability to accommodate a negative charge. Once again we see the operation of the ortho effect.
Even electron releasing substituents weaken basicity then they are ortho to the amino group, and
electron-withdrawing substituents do so to a much greater extent from the ortho position than from the
meta or para position.
From another point of view, we can consider that an electron-releasing group pushes electrons
toward nitrogen and makes the fourth pair more available for sharing with an acid, whereas an electron-
withdrawing group helps pull electrons away from nitrogen and thus makes the fourth pair less
available for sharing.

ARYLDIAZONIUM SALTS:
Aromatic amines when treated with nitrous acid in cold mineral acid solution, yield a very
important class of compounds known as Aryldiazonium Salts, For example, aniline reacts with nitrous
acid in hydrochloric acid solution at 0-5°C to form a solution of benzene diazonium chloride.

Such aryldiazonium salts were discovered by Johan Peter Griess in 1858, and the reaction producing
them is referred to as Diazotization.
Diazonium salts have the general formula Ar-N2+X-, where X is any of a large number of anions,
such as Cl-, Br-, NO2-, HSO4- BF4- etc. They are named by adding diazonium to the name of the
hydrocarbon followed by the name of the anion. Thus,
 The relative stability of the diazonium cation is also ascribed to the fact that its structure is a
resonance hybrid of the following canonical forms involving the participation of the benzene ring.

Synthetic uses of aryldiazonium salts:

Aryldiazonium salts give two types of reactions:
 Those in which the -N2X is replaced by another univalent atom or group, with the liberation
of N2 gas.
 Those in which the two N atoms are retained.
The first type of reactions again can be subdivided into two classes:
 When the diazonium group is replaced by a free radical mechanism in the presence of a catalyst
(A+),

 When N2 is abstracted to form benzene cation which is then attacked by the nucleophile to give
the corresponding substituted benzene,
 A large number of benzene derivatives can be thus synthesized via diazonium salts. This will be
illustrated by taking example of benzenediazonium chloride

1) Replacement by Hydrogen:
When an aryldiazonium salt is reduced by hypophosphrous acid(H3PO3) in presence of cuprous
chloride (Cu+Cl-), the product is the corresponding aromatic hydrocarbon.

2) Replacement by Chlorine or Bromine:

The diazonium group can be replaced readily by any of the halogens, although different
conditions may be required. For replacàment by chlorine (-Cl) or bromine (-Br), the aqueous solution
of the diazonium salt is heated with cuprous chloride (Cu+CI-) or cuprous bromide (Cu+Br-).

3) Replacement by Iodine:
The diazonium group may be replaced by iodine (-I) by simply heating an aqueous solution of
the diazonium salt and potassium iodide.
4) Replacement by Fluorine:
Aryl fluorides are conveniently prepared by adding the diazonium salt solution to fluoroboric
acid, HBF4 . The fluoroborate precipitates, and is washed and dried When heated it decomposes into
nitrogen, boron trifluoride and aryl fluoride (Balz-Schiemann Reaction).

5) Replacement by Cyano group:

The diazonium group may be replaced by a cyano group (-CN) by using cuprous cyanide
(instead of cuprous halide) in the Sandmeyer Reaction,

6) Replacement by Hydroxyl group:

When an aqueous solution of aryldiazonium Iodide is heated, the diazonium group is replaced
by -OH group. Thus a phenol results.

7) Replacement by Alkoxy group:

On heating an aqueous solution of aryldiazonium chloride in presence of an excess of an alcohol
(R-OH), the diazonium group is replaced by the alkoxy group -OR. Thus alkyl aryl ether is formed.

8) Replacement by Nitro group:

When diazonium fluoroborates are heated with aqueous sodium nitrite, the diazonium group is
replaced by –NO2 group.
9) Replacement by arsono group:
The arsono group -AsO3H3 may be introduced into the aromatic ring by heating
benzenediazonium chloride solution with sodium arsenite in the presence of cupric salts. In this way the
sodium salt of phenylarsonic acid is obtained. (Bart Reaction)

10) Replacement by Aryl group:

When an aryldiazonium chloride is treated with an aromatic hydrocarbon in the presence of
sodium hydroxide, diaryl results.

REACTIONS NOT INVOLVING LOSS OF NITROGEN

1) Reduction to Hydrazine:
Diazonium salts when reduced with sodium sulphite are reduced to give phenylhydrazine. The
process is carried in three steps :
(i) addition of diazonium salt solution to a warm solution of sodium sulphite heated to 100°C
(ii) this is followed by acidification with HCI when phenylhydrazonium chloride is produced
(iii) the resulting solution when treated with alkali yields phenylhydrazine.
2) Coupling Reactions:
Aryldiazonium salts react with aromatic rings of phenols and tertiary amines (Ar-NR 2), to form
highly coloured azo compounds, in which the two atoms are retained. Thus.

3) Amine Coupling:
The amine coupling is also A similar electrophilic substitution reaction. Here the diazonium
cation adds to the tertiary amine in the pare position and a proton is eliminated,

4) Reaction with 1˚ mod 2˚ Amines:

With primary and secondary aromatic amines, the coupling reaction first forms diamino
compounds (N-azo compounds). Thus benzenediazonium chloride reacts with aniline to yield
diazoaminobenzene.

5) Reaction with Alkalis:

When an aryldiazonium salt is made alkaline, the diazonium hydroxide liberated rapidly
rearranges to the diazo hydroxide. This reacts with alkali to form salts which are called Diazotates.
AROMATIC ACIDS:
Aromatic acids contain one or more carboxyl groups (COOH) attached directly to the aromatic
nucleus. The acids in which the COOH group is attached to the side-chain may be regarded as aryl-
substituted aliphatic acids. However, there are no characteristic differences in the behavior of the
nuclear and side-chain acids and the term 'aromatic acids' is rather loosely extended to include both
classes of compounds.
These are called by their common names or after the name of the parent hydrocarbon
(ILJPAC). Thus

METHODS OF PREPARATION:
Aromatic acids can be prepared by the same general methods which are available for aliphatic
acids. In addition they may be obtained by oxidation of aromatic hydrocarbons having a side chain. It
will suffice to list below the reactions by means of which aromatic acids can be prepared.
Reactions:
1. Acidity.
Aromatic carboxylic acids with unsubstituted benzene ring are slightly stronger acids than the
aliphatic acids. Thus benzoic acid is somewhat stronger acid than acetic acid.
2. Esterification:
Aromatic acids having no substituent in the ortho position to COOH group, are readily
converted into esters by direct reaction with alcohols in the presence of minetal acids (HCl or H 2SO4) as
catalyst.

If, however, the ortho substituent is present, the rate of esterification is considerably reduced.

When both ortho positions are occupied, esterification does not occur at all.

3. Anhydride Formation:
The reaction of an aromatic acid chloride with sodium salt of the acid yields the anhydride. An
aromatic dicarboxylic acid in which the two -COOH groups are close together, give anhydride simply
on heating. Thus, o-phthalic acid when heated forms o-phthalic anhydride, while p-phthalic acid is not
decomposed.
4. Acid Halide formation:
Aromatic acids, like the aliphatic acids, are converted to the corresponding acid halides by
healing with phosphorus pentachioride or with thionyl chloride.

The aromatic acid chlorides (aroyl chlorides), are insoluble in water and react with it very
slowly. Hence alcohols, phenols and amines can be acylated in aqueous solution. When the acylation
reaction is carried in the presence of alkali (NaOH), it is catalysed and the HCI formed in the reaction
mixture is removed. This is called Schotten Bowmann Reaction. Thus,

5. Acid amide Formation:

Aromatic acids form ammonium carboxylates which upon heating yield acid amides.

In the presence of phosphorus pentoxide, amides further dehydrate to form nitrites.

6. Decarboxylatlon:
Aromatic acids are readily decarboxylated by heating their salts with sodalime.
7. Reduction:
Aromatic acids on reduction with lithium aluminium hydride give benzyl alcohols in 80 to 99
percent yield.

8. Electrophilic substitution in benzene ring:

Aromatic acids undergo electrophilic substitution reactions in the benzene ring. The COOH
group is meta director and the substitution takes place less readily than in the parent hydrocarbon ring.
For example, benzoic acid undergoes halogenation, nitration and sulphonation giving,
m-chlorobenzoic acid, m-nitrobenzoic acid and m-benzene sulphonic acid.

The overall result is a pi electron drain from the benzene ring which is rendered electron-deficient. This
means that the benzene ring has become deactivated towards attack by electrophilic reagents. Although
the benzene ring is deactivated, the electrophilic attack will still take place, but less readily. Owing to
resonance effect the carbon atoms 2, 4, and 6 become positively charged with respect to carbon atoms 3
and 5. Thus the electrophilic reagents will attack at carbon atoms 3 and 5.

Effect of substituent on acidity:

Next, let us see how changes in the structure of the group bearing the COOH affect the acidity.
Any factor that stabilizes the anion more than it stabilizes the acid should increase the acidity; any
factor that makes the anion less stable should decrease acidity. From what we have learned about
carbonium ions, we know what we might reasonably expect. Electron-withdrawing substituents should
disperse the negative charge, stabilize the anion, and thus increase acidity. Electron-releasing
substituents should intensify the negative charge, destabilize the anion, and thus decrease acidit

The aromatic acids are similarly affected by substituents: -CH 3, -OH, and -NH2 make benzoic
acid weaker, and -Cl and -NO2 make benzoic acid stronger. We recognize the acid-weakening groups as
the ones that activate the ring toward electrophilic substitution (and deactivate toward nucleophilic
substitution). The acid-strengthening groups are the ones that deactivate toward electrophilic
substitution (and activate toward nucleophilic substitution). Furthermore, the groups that have the
largest effects on reactivity whether activating or deactivating have the largest effects on acidity. The
-OH and -OCH3 groups display both kinds of effect we have attributed to them: from the meta position,
an electron-withdrawing acid-strengthening inductive effect; and from the para position, an electron-
releasing acid-weakening resonance effect (which at this position outweighs the inductive effect).
Compare the two effects exerted by halogen.

ortho Substituted aromatic acids do not fit into the pattern set by their meta and para isomers,
and by aliphatic acids. Nearly all ortho substituents exert an effect of the same kind acid-strengthening
whether they are electron-withdrawing or electron releasing, and the effect is unusually large.
(Compare, for example, the effects of o-NO2 and o-CH3, of o-NO2 and m- or p-NO2) This ortho effect
undoubtedly has to do with the nearness of the groups involved, but is more than just steric hindrance
arising from their bulk.

Important reactions of Benzoic Acid:

The reactions of benzoic acid are those of the carboxyl group and the benzene ring.
 The benzene ring of benzoic acid gives the usual electrophilic substitution reactions. The COOR
group is meta director and substitution, therefore, takes place less readily than with benzene. Thus:

Similarly with sulphuric acid it gives m-sulphonic acid and with chlorine m-chlorobenzoic acid.
Beozoic acid is used:
(I) In medicine as urinary antiseptic and in vapour, form for disinfecting bronchial tubes;
(2) In the dye industry for making aniline blue; and
(3) As a preservathe. Sodium benzoate being less toxic, is used for preserving food products such as
tomato ketchup and fruit juices.

Unit-III
Fats, oils and waxes belong to the group of naturally occurring compounds called Lipids
(Greek, lipos= fat). Lipids are those constituents of animals and plants which are soluble in organic
solvents such as ether, chloroform, carbon tetrachloride, benzene, hexane etc., but insoluble in water.
The lipids which yield fatty acids and alcohols on hydrolysis with aqueous base (saponified) are
referred to as Simple Lipids. These can be further divided into two classes
(a) Fats and Oils, which yield long-chain fatty acids and glycerol upon hydrolysis and
(b) Waxes, which yield long-chain fatty acids and long-chain alcohols upon hydroloysis.
 We will first proceed to study the former class of compounds i.e., Fats and Oils. Fats and oils
are the most important lipids found in nature. They are one of the three major 'food factors' needed for
human body, the other two being proteins and carbohydrates. Fats and oils are widely distributed in
Foods and are of great nutritional value. They provide concentrated reserve of energy in animal body
for maintaining optimum body temperature. One gram of metabolised fat or oil yields 9 kcal, while the
corresponding values for carbohydrate and protein are 4 kcal and 5.5 kcal respectively. Not only the
edible fats and oils occupy a place of pride in human diet but they also find use as raw material for the
manufacture of soaps and synthetic detergents, paints and varnishes, polishes, glycerol, lubricants,
drying oils cosmetics, printing inks, linoleum oil cloth and pharmaceuticals. At the present time the
human race uses an estimated 43 million tonnes a year of fats and oils which reflects both their
nutritional and industrial importance.

STRUCTURE AND COMPOSITION OF FATS AND OILS

Animal and vegetable fats and oils have similar chemical structures. They are trimesters formed
from glycerol and long-chain carboxylic acids (often called fatty acids). A triester of glycerol is called a
triglyceride or glyceride.

If all the R groups in the above general formula are identical, the triester is designated as a
Simple glyceride, and if they are not a Mixed glyceride, Most natural fats and oils are mixed
triglycerides having two or three different fatty acid groups.
The carboxylic acids or fatty acids that go to form the fat or oil molecules (glyceride:) have carbon
chains with only even number of carbon atoms. The most common fatty acids have unbranched carbon
chains of 14, 16 or IS carbons. The chains may be saturated or may include one or more double bonds.
The glycerides are referred to as saturated or unsaturated depending on whether the fatty acid
component chains are saturated or contain double bonds.
The most common saturated fatty acids found in fats and oils are myristic acid, C13H27COOH,
palmicic acid, C15H31CO0H, and stearic acid, C17H35CO0H. Amongst the unsaturated fatty acids, oleic
acid, C17H33CO0H and linoleic acid, C17H31CO0H, are widely distributed in almost all fats and oils.
Oleic acid chain contains one double bond and linoleic acid chain two double bonds. We also
know that the presence of a double bond in a fatty acid can cause cis-trans isomerism, depending on the
configuration of the H atoms attached to the doubly-bonded carbon atoms. Thus oleic acid is the cis-
isomer, while its trans-isomer is elaidic acid. Linoleic acid has two double bonds and both possess cis
configuration. Generally speaking, the cis-isomers are found naturally occurring in the unsaturated fatty
acid components of fats and oils.
Composition of Fats and Oils:
As already mentioned, fats and oils are invariably composed of a number of mixed glycerides,
e.g., In the mixed glycerides present in fats and oils, a single molecule of glyceride may contain two or
three different fatty acids linked by ester bonds to the glycerol. While it is difficult to know exactly as
to which triglycerides are present in a particular fat or oil, the overall percentage composition of fatty
acids which make up the fat or oil can be determined by analysis.
Reactions of fatty acids:
The reactions of oils and fats are the reactions of triglycerides or triesters of glycerol. Thus they
can undergo hydrolysis at alt the three ester groups. Also, we know that the chains of the acid
components of glycerides may contain one or more double bonds. Therefore the unsaturated glycerides
give the addition and oxidation reactions characteristic of alkenes at the seats of these double bonds.
1. Hydrolysis:
Triglycerides are easily hydrolyzed by enzymes called lipases (catalysts) in the digestive tracts
of human beings and animals to give fatty acids and glycerol. The fatty acids so produced play an
important role in the metabolic process in the animal body.
2. Saponification;
When triglycerides are hydrolyzed by alkalis, glycerol plus the salts of fatty acids are produced.
Generally the sodium or potassium salts are obtained which are termed as soaps.

3. Hydrogenation or Hardening of Oils:

Unsaturated glycerides react with hydrogen in the presence of a metal catalyst (usually nickel)
to give saturated glycerides. This reaction in similar to the catalytic hydrgenation of alkenes. Here, the
hydrogenation process saturates the double bonds present in the fatty acid components of the glyceride;
thereby converting them to saturated acid components. The result is the transformation of a liquid
glyceride (an oil) into a semi-solid glyceride (a fat). For example, glyceryl trioleate (mp -5˚C) upon
hydrogenation yields glyceryl tristearate (mp 71˚C).
 The process of hydrogenation which results in hardening of an oil owing to the formation of fat,
is often referred to as Hardening. This reaction is used commercially to harden vegetable oils for the
production of cooking fat (vegetable ghee or margarine). Hardened oils are also extensively used for
making soaps and candles.
4. Hydrogenolysis (Reduction to Alcohols):
Upon treatment with hydrogen at high pressure and temperature in the presence of copper
chromite (CuCr2O4) as catalyst, glycerides are split up like other esters. The products are glycerol and
the reduction products of the fatty acid, along chain alcohols. Thus glyceryl tristearate forms glycerol
and octadecyl alcohol.

This reaction which causes the cleavage of the fat by hydrogenation to yield glycerol and a
higher aliphatic alcohol, is termed Hydrogenolysis. The long-chain alcohols produced by the
hydrogereolysis of glycerides are used in the manufacture of synthetic detergents.
5. Addition of Halogens (Halogenation):
Just as simple alkenes react with halogens by addition at the double bond, unsaturated
glycerides add halogens to give the corresponding dihalides. Thus unsaturated glycerides on treatment
with iodine in the presence of mercuric chloride as catalyst, give diiodides by addition at the double
bonds in the acid component chains.
For example,
 Evidently the amount of iodine consumed by a glyceride will be proportional to the number of
double bonds in the fatty acid components. This reaction is, therefore, used to determine the extent of
unsaturation in a given fat or oil.

6. "Drying" of Oils (Oxidation-Polymerization):

The methylene groups flanked by doubly bonded carbon atoms present in highly uasaturatcd
glycerides are very reactive. Linseed oil, the most widely used drying oil, contains about 63 per cent of
its fatty acids as linoleates. The CH3 group present in linoleic acid component of the unsaturated
glyceride is readily attacked by oxygen of the air to form hydroperoxy group (-O-O-H) at these sites.
These hydroperoxy groups then react with unchanged CH, groups in other glyceride molecules to form
peroxide bridges. The hydroperoxide formation accompanied by polymerisation as illustrated below,
converts polyunsaturated glycerides into a vast network of interlinked units. Such a network forms a
dry, tough and durable film when exposed to air. For this reason drying oils are used as the medium of
paints and varnishes. In order to make oils 'faster drying' catalysis (lead and cobalt salts) are added.
Other ingredients of paints are pigments and volatile thinners such as turpentine.

Oil Cloth is made by coating woven canvas with several layers of linseed oil paint. To make
linolium, rosin and ground cork are mixed with thickened linseed oil, and the mixture is allowed to
harden.
7. Rancidity (Hydrolysis-Oxidation):
The term rancid is applied to any fat or oil that develops a disagreeable odour when left exposed
to warm, moist air for any length of time. Rancidity is chiefly caused by hydrolysis of the ester links
and oxidation of double bonds of the triglycerides. The lower molecular weight acids that are produced
are volatile and impart an offensive odour to fat or oil.
(a) Hydrolytic Rancidity.
This type of rancidity is due to the liberation of lower fatty acids by hydrolysis of ester links of
triglycerides. Hydrolytic rancidity is particularly applicable to butter. Under moist and warm
conditions, hydrolysis of the glycerides in butter liberates the odorous butyric acid, caproic acid,
caprylic acid, and capric acid.

Micro-organisms present in the air provide the enzymes (lipases) that catalyse the hydrolytic process.
Rancidity so caused can be prevented by keeping butter covered in a refrigerator.
(b) Oxidative rancidity:
It occur in triglycerides containing unsaturated acid components. It is believed that first the ester
linkages are hydrolysed to yield unsaturated acids. The acids so produced are subjected to oxidative
cleavage at the site of the double bonds forming short chain offensive aldehydes and acids. For
example,

Oxidation leading to rancidity in lass and oils is catalyzed by the presence of certain metallic salts. The
addition of antioxidants will preserve edible fats for long periods of storage. Two antioxidants
occurring in natural fats are vitamin E and ascorbic acid.
ANALYSIS OF FATS AND OILS:
Determination of Acid Value
Definition: The acid value is defined as the number of milligrams of potassium hydroxide required to
neutralize the free fatty acids present in one gram of fat. It is a relative measure of rancidity as free fatty
acids are normally formed during decomposition of oil glycerides. The value is also expressed as per
cent of free fatty acids calculated as oleic acid.

Principle: The acid value is determined by directly titrating the oil/fat in an alcoholic medium against
standard potassium hydroxide/sodium hydroxide solution.

Analytical Importance: The value is a measure of the amount of fatty acids which have been liberated
by hydrolysis from the glycerides due to the action of moisture, temperature and/or lypolytic enzyme
lipase.

Apparatus: 250 ml conical flasks.

Reagents:
a) Ethyl alcohol: - Ninety-five per cent alcohol or rectified spirit neutral to phenolphthalein indicator.
b) Phenolphthalein indicator solution: - Dissolve one gram of phenolphthalein in 100 ml of ethyl
alcohol. When testing rice bran oil based blended oils or oils or fats which give dark colored soap
solution, the observation of the end point of the titration may be facilitated, by using Alkali blue 6B in
place of phenolphthalein.
c) Standard aqueous potassium hydroxide or sodium hydroxide solution 0.1 or 0.5 N. The solution
should be colourless and stored in a brown glass bottle. For refined oils, the strength of the alkali
should be fixed to 0.1 N.

Procedure:
Weigh accurately appropriate amount of the cooled oil sample in a 250 ml conical flask and add 50 ml
to 100 ml of freshly neutralised hot ethyl alcohol and about one ml of phenolphthalein indicator
solution. Boil the mixture for about five minutes and titrate while hot against standard alkali solution
shaking vigorously during the titration. The weight of the oil/fat taken for the estimation and the
strength of the alkali used for titration shall be such that the volume of alkali required for the titration
does not exceed 10 ml.
Calculation:
Acid value = 56.1VN /W
Where,
V = Volume in ml of standard potassium hydroxide or sodium hydroxide used.
N = Normality of the potassium hydroxide solution or Sodium hydroxide solution; and
W = Weight in g of the sample The acidity is frequently expressed as free fatty acid for which
calculation shall be;

Determination of Saponification Value

Definition: The saponification value is the number of mg of potassium hydroxide required to
saponify 1 gram of oil/fat.
Principle: The oil sample is saponified by refluxing with a known excess of alcoholic potassium
hydroxide solution. The alkali required for saponification is determined by titration of the excess
potassium hydroxide with standard hydrochloric acid.

Analytical importance:
The saponification value is an index of mean molecular weight of the fatty acids of glycerides
comprising a fat. Lower the saponification value, larger the molecular weight of fatty acids in the
glycerides and vice-versa.
Apparatus:
a. 250 ml capacity conical flask with ground glass joints.
b. 1 m long air condenser, or reflux condenser (65 cm minimum in length) to fit the flask (a).
c. Hot water bath or electric hot plate fitted with thermostat.

Reagents:
(i) Alcoholic potassium hydroxide solution - Reflux 1.2 litre alcohol 30 minutes with 10 gm KOH and 6
gm granulated Aluminium or Al foil. Distill and collect 1 litre after discarding first 50 ml. Dissolve 40 g
of potassium hydroxide in this 1 litre alcohol keeping temperature below 15 0 C while dissolving alkali.
Allow to stand overnight, decant the clear liquid and keep in a bottle closed tightly with a cork or
rubber stopper.
ii) Phenolphthalein indicator solution - Dissolve 1.0 g of phenolphthalein in 100 ml rectified spirit.
iii) Standard hydrochloric acid: approximately 0.5N
Procedure:
Melt the sample if it is not already liquid and filter through a filter paper to remove any
impurities and the last traces of moisture. Make sure that the sample is completely dry. Mix the sample
thoroughly and weigh about 1.5 to 2.0 g of dry sample into a 250 ml Erlenmeyer flask. Pipette 25 ml of
the alcoholic potassium hydroxide solution into the flask. Conduct a blank determination along with the
sample. Connect the sample flasks and the blank flask with air condensers, keep on the water bath, boil
gently but steadily until saponification is complete, as indicated by absence of any oily matter and
appearance of clear solution. Clarity may be achieved within one hour of boiling. After the flask and
condenser have cooled somewhat wash down the inside of the condenser with about 10 ml of hot ethyl
alcohol neutral to phenolphthalein. Titrate the excess potassium hydroxide with 0.5N hydrochloric acid,
using about 1.0 ml phenolphthalein indicator.

Calculation:
Saponification Value = 56.1 (B-S)N /W
Where,
B = Volume in ml of standard hydrochloric acid required for the blank.
S = Volume in ml of standard hydrochloric acid required for the sample
N = Normality of the standard hydrochloric acid and
W = Weight in gm of the oil/fat taken for the test.

Note:- When titrating oils and fats which give dark coloured soap solution the observation of the end
point of titration may be facilitated either (a) by using thymolpthalein or alkali blue 6B in place of
phenolphthalein or (b) by shaking 1 ml of 0.1 % (w/v) solution of methylene blue in water to each 100
ml of phenolphthalein indicator solution before the titration.

Determination of ester value:

Definition: ester value is defined as the mg of KOH required to react with glycerin (glycerol /
or glycerin) after saponify one gram of fat. It is calculated from the saponification value (SV) and the
acid value (AV):

ESTER VALUE (EV) = SAPONIFICATION VALUE (SV) – ACID VALUE (AV)

Determination of Iodine Value

Definition: The iodine value of an oil/fat is the number of grams of iodine absorbed by 100g of
the oil/fat, when determined by using Wijs solution.

Principle: The oil/fat sample taken in carbon-tetrachloride is treated with a known excess of iodine
monochloride solution in glacial acetic (Wijs solution). The excess of iodine monochloride is treated
with potassium iodide and the liberated iodine estimated by titration with sodium thiosulfate solution.

Analytical importance:

The iodine value is a measure of the amount of unsaturation (number of double bonds) in a fat.
Apparatus:
500 ml Erlenmeyer flask.
Reagents:
i) Potassium dichromate AR
ii) Concentrated hydrochloric acid AR
iii) Glacial acetic acid, free from ethanol
iv) Carbon tetrachloride, analytical reagent grade
v) Iodine mono-chloride (ICl)
vi) Potassium iodide (free from potassium iodate) - 10% solution prepared fresh
vii) Starch solution - Mix 5 g of starch and 0.01 g of the mercuric iodide with 30 ml of cold water and
slowly pour it with stirring into one litre of boiling water. Boil for three minutes. Allow to cool and
decant off the supernatant clear liquid.
viii) Wij’s Iodine monochloride solution - Dissolve 10 ml of iodine monochloride in about 1800 ml of
glacial acetic acid and shake vigorously. Pipette 5 ml of Wij's solution, add 10 ml of potassium iodide
solution and titrate with 0.1N standard sodium thiosulphate solution using starch as indicator. Adjust
the volume of the solution till it is approximately 0.2 N or prepare Wij’s iodine solution by dissolving
13 gm resublimed Iodine in 1 litre acetic acid and pass in dried chlorine ( dried through H2SO4.) until
original Sod thiosulphate titre of the solution is not quite doubled (characteristic colour change at the
end point indicates proper amount of Chlorine. Convenient method is to reserve some amount of
original I solution, add slight excess of Cl to bulk of solution and bring to desired titre by re additions
of reserved portion). Store in an amber bottle sealed with paraffin until ready for use. Wij’s solutions
are sensitive to temp, moisture and light. Store in dark at less than 30°C. Determine I / Cl ratio as
follows Iodine Content – Pipette 5 ml Wij Solution into 500 ml Erlenmeyer flask containing 150 ml
saturated Cl – water and some glass beads. Shake , heat to boiling point and boil briskly 10 minutes.
Cool, add 30 ml

H2SO4 (1 + 49) and 15 ml 15 % KI solution and titrate immediately with 0.1 N Na 2S2O3. Total Halogen
content – Pipette 20 ml Wij’s solution into 500 erlenmeyer flask containing 150 ml recently boiled and
cooled water and 15 ml 15 % KI solution. Titrate immediately with 0.1 N Na2S2O3.

I / Cl = 2 X / (3B – 2 X)

where X = ml of 0.1 Na2 S2O3 required for I content and

B = ml required for total halogen content. I / Cl ratio must be 1.10±0.1 ix)

Standard sodium thiosulphate solution (0.1N)- Dissolve approximately 24.8 g of sodium thiosulphate
crystals (Na2S2O3.5H2O) in distilled water and make up to 1000 ml. Standardise this solution by the
following procedure: Weigh accurately about 5.0 g of finely powdered potassium dichromate which has
been previously dried at 105ºC±2ºC for one hour, dissolve it in distilled water and make up to 1 L. For
standardisation of sodium thiosulphate, pipette 25 ml of this solution into a 250 ml conical flask. Add 5
ml of concentrated hydrochloric acid and 15 ml of a 10 percent potassium iodide solution. Allow to
stand in dark for 5 min and titrate the content with sodium thiosulphate solution using starch as
indicator at the end. End point is change of blue colour to green.

N = 25W 49.03 V

Where,

N = Normality of the sodium thiosulphate

W = Weight in g of the potassium dichromate, and

V = Volume in ml of sodium thiosulphate solution required for titration.

Procedure:

Weigh accurately an appropriate quantity of the dry oil/fat as indicated in the Table above, into a
500 ml conical flask with glass stopper, to which 25 ml of carbon tetrachloride have been added. Mix
the content well. The weight of the sample shall be such that there is an excess of 50 to 60 percent of
Wij’s solution over that actually needed. Pipette 25 ml of Wij's solution and replace the glass stopper
after wetting with potassium iodine solution. Swirl for proper mixing and keep the flasks in dark for
half an hour for non-drying and semi-drying oils and one hour for drying oils. Carry out a blank
simultaneously. After standing, add 15 ml of potassium iodide solution, followed by 100 ml of recently
boiled and cooled water, rinsing in the stopper also. Titrate liberated iodine with standardized sodium
thiosulphate solution, using starch as indicator at the end until the blue colour formed disappears after
thorough shaking with the stopper on. Conduct blank determinations in the same manner as test sample
but without oil/fat. Slight variations in temperature appreciably affect titre of I 2 solution as chloroform
has a high coefficient of expansion. It is thus necessary that blanks and determinations are made at the
same time.

Calculation:

Iodine value = 12.69 (B – S) N /W

Where, B = volume in ml of standard sodium thiosulphate solution required for the blank.

S = volume in ml of standard sodium thiosulphate solution required for the sample.

N = normality of the standard sodium thiosulphate solution.

W = weight in g of the sample.

DETERMINATION OF ACETYL VALUE AND HYDROXYL VALUE:

Defenition:
The number of free hydroxyl groups in a fat or oil. The acetyl value is determined by the
milligrams of potassium hydroxide require to neutralize the acetic acid produced when 1 gram
of fat or oil is acetylated with acetic anhydride.

Principle:
A sample of oil or fat is acetylated by reftuxing with acetic anhydride and the excess anhydride
is decomposed with water and sodium bicarbonate solution. The saponification value of the washed and
dried acetylated oil is determined as per the procedure given in saponification value.
Analytical importance:

The iodine value is a measure of the number of Hydroxyl groups in a fat.

Apparatus:
Beaker-800 ml capacity.
Separating funnel- 500 ml capacity.
Conical flasks - 250 to 300 ml capacity.
Reflux condenser - any efficient reflux condenser, at least 65 mm long.
Sand-bath or electric hot-plate with rheostat control

Reagents:
Acetic anhydride - containing 95 to 100 percent ( by weight) or the actual acetic anhydride. Determine
the acetic anhydride content as follows:
a) Weigh accurately about 2 g of the acetic anhydride into a 200-ml glass-stoppered conical
flask, cool in ice and add 5 ml of freshly distilled aniline. Insert the glass stopper immediately, shake
vigorously and allow to stand at room temperature for 30 minutes. Wash down the sides of the flask
with 50 ml of ice-cold water. Mix well and titrate with previously standardized I N sodium hydroxide
solution, using phenolphthalein as indicator, until the pink colour persists for 10 minutes,
A = Volume in ml of I N sodium hydroxide solutionrequired/ Weight in g of acetic anhydride
taken for the test
b) Weigh accurately about 2 g of the acetic anhydride into another flask, add 50 ml of water,
allow to stand for 30 minutes and titrate with the standard sodium hydroxide solution to the same end
point as above using phenolphthalein as indicator.
B =Volume in ml of I N sodium hydroxide solution required/ Weight in g of acetic anhydride
taken for the test
Acetic anhydride, percent by weight = 10.209 (B-A)
Sodium bicarbonate solution - freshly prepared 0·5 percent (wlv) and neutral to litmus.
Anhydrous sodium sulphate
Alcoholi' potassium hydroxide solution – 0.5 N.
phenolphthalein indicator solution - Dissolve 0'1 g in 100 ml of 60 percent rectified spirit.
Standard hydrochloric acid - approximately 0.5 N.

Procedure:
Weigh accurately about 10 g of the material in a conical ftask t add 20 ml of acetic anhydride
and boil the mixture under a reflux air condenser for about 2 hours. Pour the mixture into a beaker
containing 500 ml of water and boil for 15 minutes. Bubble a stream of carbon dioxide or nitrogen
through the mixture during boiling to prevent bumping. Discontinue boiling, cool slightly, and remove
the water with a siphon. Add another 500 ml of water and boil again. Discontinue boiling, cool and
transfer the contents of the beaker to a separating funnel and reject the lower layer. Wash the acetylated
sample successively (a) three times with 50 ml of water, (b) twice with 50 ml of sodium bicarbonate
solution, and (c) twice with 50 ml of warm water (60 to 70·C). Drain and remove as much of tlte water
as possible and then transfer the acetylated sample to a beaker and add approximately 5 g of anhydrous
sodium sulphate. Allow to stand for about one hour with occasional stirring, Filter through a dry filter
paper, preferably in an oven at 100 to 110°C, remove the filter paper and) eep the sample in the oven
until it is thoroughly dry. Determine the saponification values of the original material and the acetylated
product by the procedure described under saponification value.

Determination of Reichert-Meissl Value

Butter is distinguished from other fats by the presence of glyceryl esters of relatively low
molecular weight fatty acids, especially butyric but also caproic, capric, caprylic, lauric and myristic
acids. These acids are wholly or partially steam volatile and water soluble. The Reichert value reflects
the amount of butyric and caproic acids present and Polenske chiefly capryilic, capric and lauric acid
with some contribution from myristic and even palmitic acid.

Definition:
The Reichert-Meissl value is the number of millilitres of 0.1N aqueous sodium hydroxide
solution required to neutralise steam volatile water soluble fatty acids distilled from 5g of an oil/fat
under the prescribed conditions. It is a measure of water soluble steam volatile fatty acids chiefly
butyric and caproic acids present in oil or fat.
 The Polenske value is the number of mililiters of 0.1N aqueous alkali solution required to
neutralise steam volatile water insoluble fatty acids distilled from 5g of the oil/fat under the prescribed
conditions. It is a measure of the steam volatile and water insoluble fatty acids, chiefly caprylic, capric
and lauric acids present in oil or fat.
Principle:
The material is saponified by heating with glycerol sodium hydroxide solution and then split by
treatment with dilute sulfuric acid. The volatile acids are immediately steam distilled. The soluble
volatile acid in the distillate are filtered out and estimated by titration with standard sodium hydroxide
solution.

Analytical Importance:
These determinations have been used principally for analysis of butter and margarines. Butter
fat contains mainly butyric acid glycerides. Butyric acid is volatile and soluble in water. No other fat
contains butyric acid glycerides, and therefore, the Reichert-Meissl value of the butter fat is higher than
that for any other fat. Coconut oil and palm kernel oil contain appreciable quantities of caprylic, capric
and lauric acid glycerides. These fatty acids are steam volatile but not soluble in water, and hence give
high Polenske value.

Apparatus:
a. An all-glass distillation assembly conforming to specifications given in AOCS Official Methods Cd
5-40 or Methods of Analysis, AOAC- 17th Edn.,2000 or distillation apparatus as shown in the
diagram below
b. 25 ml beaker
c. 100 ml graduated cylinder
d. 100 ml pipette
e. Graduated burette
f. Asbestos board with a hole about 65 mm dia for supporting the flask over the burner. During
distillation the flask shall fit snugly into the hole of the board to prevent the flame from impinging on
the surface of the flask above the hole. g. Bunsen burner sufficiently large to allow completion of
distillation in the prescribed time.
Reagents:
a) Glycerine:
b) Concentrated sodium hydroxide solution: 50 % (w /w) Dissolve Sodium Hydroxide in equal wt of
water and store solution in a bottle. Use clear solution free from deposit.
c) Pumice stone grains
d) Dilute sulfuric acid solution: Approximately 1.0 N
e) Sodium hydroxide solution: 0.1N solution in water, accurately standardised
f) Phenolpthalein indicator: Dissolve 0.1 g of phenolpthalein in 100 ml of ethyl alcohol.
g) Ethyl alcohol: 90% by volume and neutral to phenolphthalein
Procedure:
Weigh accurately 5 ±0.1g of filtered oil or fat sample into a clean, dry, 300ml distilling flask.
Add 20 ml of glycerine and 2 ml of concentrated sodium hydroxide solution, and heat with swirling
over a flame until completely saponified, as shown by the mixture becoming perfectly clear. Cool the
contents slightly and add 90 ml of boiling distilled water, which has been vigorously boiled for about
15 min. After thorough mixing the solution should remain clear. If the solution is not clear (indicating
incomplete saponification) or is darker than light yellow (indicating over-heating), repeat the
saponification with a fresh sample of the oil or fat. If the sample is old, the solution may sometimes be
dark and not clear. Add about 0.6 - 0.7 gm of pumice stone grains, and 50 ml of dilute sulfuric acid
solution. Immediately connect the flask to the distillation apparatus. Place the flask on asbestos board
so that it fits snugly into the aperture. This will prevent the flame from impinging on the surface of the
flask above the level of the liquid and avoid super heating. Heat very gently until the liberated fatty
acids melt and separate. Then set the flame so that 110 ml of distillate shall be collected within 19 to 21
min. The beginning of the distillation is to be taken as the moment when the first drop of the distillate
falls from the condenser in the receiving flask. Keep the water in the condenser flowing at a sufficient
speed to maintain the temperature of the outgoing water from the condenser between 15 and 20°C.
Collect the distillate in a graduated flask. When the distillate exactly reaches the 110 ml mark on the
flask, remove the flame and quickly replace the flask by a 25 ml measuring cylinder. Stopper the
graduated flask and without mixing place d it in a water bath maintained at 15ºC for 10 min so that the
110 ml graduation mark is 1 cm below the water level in the bath. Swirl round the contents of the flask
from time to time. Remove the graduated flask from the cold water bath, dry the outside and mix the
content gently by inverting the flask 4 to 5 times without shaking. Avoid wetting the stopper with the
insoluble acids. Filter the liquid through a dry, 9 cm Whatman No. 4 filter paper. Reject the first 2-3 ml
of the filterate and collect the rest in a dry flask. The filtrate should be clear. Pipette 100 ml of the
filtrate and add 5 drops of the phenolphthalein solution, and titrate against standard 0.1N sodium
hydroxide solution. Run a Blank Test without the fat, but using the same quantities of the reagents.
Calculation
Reichert-Meissl Value = (A – B) x N x 11
where,
A = Volume in ml of standard sodium hydroxide solution required for the the test;
B = Volume in ml in standard sodium hydroxide solution required for the blank; and
N = Normality of standard sodium hydroxide solution.
Unit-IV
Two aromatic rings that share a pair of carbon atoms are said to be fused. In this Unit we shall
study the chemistry of the simplest and most important of the fused-ring hydrocarbons, naphthalene,
C10H8, and look briefly at two others of formula C14H10, Anthracene and Phenanthrene.

All three of these hydrocarbons are obtained from coal tar, naphthalene being the most abundant
(5%) of all constituents of coal tar.

NAPHTHALENE:
Nomenclature of naphthalene derivatives:
Positions in the naphthalene ring system are designated as in I.

Two isomeric monosubstituted naphthalenes are differentiated by the prefixes 1- and 2-, or α-
and β-. The arrangement of groups in more highly substituted naphthalenes is indicated by numbers.
For example:

Structure of naphthalene:
Naphthalene is classified as aromatic because its properties resemble those of benzene. Its
molecular formula C10H8, might lead one to expect a high degree of unsaturation; yet naphthalene is
resistant (although less* so than benzene) to the addition reactions characteristic of unsaturated
compounds. Instead, the typical reactions of naphthalene are electrophilic substitution reactions, in
which hydrogen is displaced as hydrogen ion and the naphthalene ring system is preserved. Like
benzene, naphthalene is unusually stable: its heat of combustion is 61 kcal lower than that calculated on
the assumption that it is aliphatic.
 From the experimental standpoint, then, naphthalene is classified as an aromatic the basis of its
properties. From a theoretical standpoint, naphthalene has the structure required of an aromatic
compound: it contains flat six-membered rings, and consideration of atomic orbitals shows that the
structure can provide п clouds containing six electrons the aromatic sextet. Ten carbons lie at the
corners of two fused hexagons. Each carbon is attached to three other atoms by σ bonds; since these a
bonds result from the overlap of trigonal sp2 orbitals, all carbon and hydrogen atoms lie in a single
plane. Above and below this plane there is a cloud of п electrons formed by the overlap of p orbitals.
We can consider this cloud as two partially overlapping sextets that have a pair of п electrons in common.

In terms of valence bonds, naphthalene is considered to be a resonance hybrid of the three structures I,
II, and III. Its resonance energy, as shown by the heat of combustion, is 61 kcal/mole.

X-ray analysis shows that, in contrast to benzene, all carbon-carbon bonds in naphthalene are not the
same; in particular, the C1-C2 bond is considerably shorter (1.365 A) than the C2-C3 bond (1.404 A).
 Examination of structures I, 11, and III shows us that this difference in bond lengths is to be
expected. The C1- C2 bond is double in two structures and single in only one; the C 2 -C3 bond is single
in two structures and double in only one. We would therefore expect the C 1-C2 bond to have more
double bond character than single, and the C2 -C3 bond to have more single-bond character than double.
For convenience, we shall represent naphthalene as the single structure IV,

SYNTHESIS OF NAPHTHALENE:
Naphthalene may be obtained
(1) From Petroleum:
When petroleum fractions are passed over copper catalyst at 190°C, naphthalene and
methylnaphthalenes are formed. Methylnaphthalenes are separated and converted into naphthalene by
heating with hydrogen under pressure (dealkylatian).

Prior to 1960 all naphthalene came from coal-tar, but now almost half of it is produced from petroleum
by the above method.

(2) From 4-Phenyl-1-butene.

When 4-phenyl-1-butene is passed over red hot calcium oxide, naphthalene is obtained

(3) From 4-Pheuyl-3-butenoic Acid:

When 4-phenyl-3-butenoic acid is heated with concentrated sulphuric acid, 1-naphthol is
formed. This on distillation with zinc dust gives naphthalene.
(4) By Haworth Synthesis:
This involves the following five steps
Step 1. Benzene and succinic anhydride are heated in the presence of aluminum chloride to form
β-benzoylpropionic acid. (Friedel-Crafts Acylation)

Step 2. β- Benzoylpropionic acid is treated with amalgamated zinc in the presence of hydrochloric acid
to give γ-phenylbutyric acid. (Clemmensen Reduction)

Step 3. γ-Phenylbutyric acid is heated with concentrated sulphuric acid or polyphosphoric acid to form
α-tetralone. (Ring-closure Reaction)
Step 4. α-Tetralone is heated with amalgamated zinc and hydrochloric acid to give tetralin.
(CIemmensen Reduction)

Step 5. Tetralin is heated with selenium or palladium to yield naphthalene (Aromatization Reaction).

PROPERTIES OF NAPHTHALENE

Physical Properties:
Naphthalene is a colourless crystalline solid. It melts at 82°C and boils at 218°C. Naphthalene is
insoluble in water, but dissolves in ether, benzene, and hot ethanol. It sublimes readily when warmed
and is volatile with steam. Naphthalene has a characteristic 'moth ball' odour.

Chemical Reactions:
Some of the important chemical reactions of naphthalene are described below.

A. Electrophilic Substitution Reactions:

Naphthalene, like benzene, undergoes electrophilic substitution reactions. Substitution occurs
primarily at C-1 (α-position). This can be understood if we examine the intermediate carbonium ion
Two resonance froms can be written for the intermediate carbonium ion obtained from the attack at C-1
(without involving the other ring), whereas only one such form is possible for substitution at C-2.
E in the above equations represents an electrophile. Consequently the former intermediate is
more stable and the product with a substittient at C-1 predominates. Substitution at C-2 (β-position)
occurs only when the reactions are carried at higher temperatures or when bulkier solvents are used.
(1) Nitration:
Naphthalene undergoes nitration with concentrated nitric acid in the presence of sulphuric acid
at 60°C to produce l-nitronaphthalene.

(2) Sulphonation:
Naphthalene undergoes sulphonation with concentrated sulphuric acid at 60°C to form
1-naphthalenesulphonic acid while the reaction is carried at 165°C, 2-naphthalenesuiphonic acid is
obtained.

(3) Halogenation:
Naphthalene undergoes chlorination or bromination in boiling carbon tetrachloride to give 1-
chloronaphthalene. Unlike benzene, no Lewis acid catalyst is required:
When the above reaction is carried at room temperature, naphthalene dichloride (1,4- addition product)
is obtained. On heating, naphthalene dichloride loses HCI to yield 1-chloronaphthalene.

(4) Friedel-Crafts Acylation:

Naphthalene undergoes acylation with acetyl chloride and aluminium chloride in carbon
disulphide to give l-acetylnaphthalene. When nitrobenzene is used as a solvent, 2-acetylnaphthalene is
obtained.

(5) Friedel-Crafts Alkylation:

Naphthalene undergoes alkylation with alkyl halides (methyl halides do not react) in the
presence of aluminium chloride to give 2-alkyl naphthalenes.

(6) Chloromethylation:
Naphthalene reacts with formaldehyde and HCI in the presence of zinc chloride to form
l-chloromethyl naphthalene.
(7) Reduction:
Naphthalene undergoes reduction more readily than benzene. With sodium and ethyl alcohol (bp 78°C)
it gives 1,4-dialin or 1,4-dihydronaphthalene. With sodium and isopentanol (bp 130°C) it gives tetralin
or l,2,3,4-tetrahydronaphthalene

Catalytic reduction completely hydrogenates both rings and produces decalin or

decahydronaphthalene.

Decalin consists of two cyclohexane molecules (in chair forms) fused together along a common
side. It exist in cis and trans isomeric forms which differ in the space relationship of hydrogen atoms at
C- 9 and C-10 positions. When nickel is used as a catalyst, the main product is trans-decalin. When
plainum is used as a catalyst, the main product is cis-decalin.

Decalin sold commercially is the mixture of both these forms, and is used as a solvent in the varnish
and lacquer trade
(8) Oxidation:
Naphthalene is much more easily oxidised than benzene. With chromiurn trioxide in acetic acid
at room temperature, it gives 1,4-naphthaquinone.
 Oxidation of naphthalene with vanadium pentoxide at 145˚C yields phthalic anhydride. This
method is used industrially.

Important derivatives of Naphthalene:

Uses:
 Naphthalene as moth balls has been used to protect woolen goods from moths for many rears.
 Recently, p-dichlorobenzene has replaced naphthalene in the manufacture of moth balls as it
has a less obnoxious odour
 It is used for increasing the illuminating power of coal gas
 Naphthalene is used in the manufacture of phthalic anhydride,
 carbaryl for insecticides.
 2-naphihol dyes, and several medicinal products,
 1-Naphthol is used in the manufacture of insecticides and dyes.
 2-Naphthol is used
 (I) for making dyes
(2) as an antioxidant in the manufacture of synthetic rubber
(3) as an antiseptic in the treatment of skin diseases ; and
(4) for preparing its methyl and ethyl ethers (nerolins) which are extensively used in perfumery.
 1-Naphthylamine is used in the manufacture of dyes.
 2-Naphthylamine is used in the manufacture of dyes.
 A number of 1,4-naphthaquinone derivatives have been isolated from plant and animal sources.
For example, juglone (5-hydroxy. 1,4naphthaquinqe) is found in the shells of walnuts.
Plumbagin (2-methyl-5-hydroxy-1,4.naphthaquinone) is isolated from the roots of various
species of plumbogo.
 lawsone (2hydroxy-1,4.naphthaqujnone) is obtained from the henna plant and is used in the
manufacture of henna hair dyes.
 α-Naphthylacetic acid is used as an 'apple set' spray to reduce the premature dropping of fruit
from the tree ; one application near the end of growing season holds the apples on the tree for
one or two weeks.
 α-Naphthylacetic acid is also used as a spray to delay the flowering of fruit trees until the
danger of frost is past, and as a spray to keep potatoes from sprouting.
 When sprayed on tomatoes and cucumbers, it causes the production of seedless fruits.

ANTHRACENE AND PHENANTHRENE:

The positions in anthracene and phenanthrene are designated by numbers as Shown:

Structure of anthracene and phenanthrene:

Like naphthalene, anthracene and phenanthrene are classified as aromatic on the basis of their
properties. Consideration of atomic orbitals follows the same pattern as for naphthalene, and leads to
the same kind of picture: a flat structure with partially overlapping IT clouds lying above and below the
plane of the molecule. In terms of valence bonds, anthracene is considered to be a hybrid of structures
I-IV,
and phenanthrene, a hybrid of structures V-IX.

Heats of combustion indicate that anthracene has resonance energy of 84 kcal/mole, and that
phenanthrene has resonance energy of 92 kcal/mole.
For convenience we shall represent anthracene as the single structure X, and phenanthrene as
XI, in which the circles can be thought of as representing partially overlapping aromatic sextets.

All fourteen carbon atoms in anthracene are sp2 hybridized. The sp2 hybrid orbitals overlap with
each other and with s orbitals of the ten hydrogen atoms forming C-C and C-H bonds. Since the bonds
result from the overlap of trigonal sp 2 orbitals, all carbon and hydrogen atoms in anthracene lie in the
same plane. This has been confirmed by X-ray diffraction studies
Also each carbon atom in anthracene possesses an unhybridized p orbital containing one
electron. These p orbitals are perpendicular to the plane of the σ bonds. The lateral overlap of these p
orbitals produces a п molecular orbital containing fourteen electrons. One half of this it molecular
orbital lies above and the other half lies below the plane of the σ bonds. Anthracene shows aromatic
properties because the resulting it molecular orbital satisfies the Huckel's rule (n=3 in 4n+2).
 X-Ray diffraction studies show that, like naphthalene, all carbon-carbon bonds in anthracene are
not of the same length. In particular, the Cl—C2 bond is considerably shorter (I3.7A) than the C2—C3
bond (14.2 A). This difference in bond lengths can be understood if we examine the four resonance
forms given above. Notice that the Cl—C2 bond is double in three structures (A, B and C) and single in
only one (D) ; whereas the C2—C3 bond is single in three structurres (A, B and C), and double in only
one (D). We would, therefore, expect the CI—C2 bond to have more double-bond character (shorter
bond length), and the C2—C3 bond to have more single-bond character (longer bond length).

Synthesis of Anthracene:
(1) By Friedel-Crafts Reactions:
Benzyl chloride reacts with itself to form 9,10-dihydroanthracene, which readily loses two
hydrogen atoms to yield anthracene,

Anthracene may also be prepared by the Friedel Crafts reaction between benzene and 1,1,2,2-
tetrabromoethane (acetylene tetrabromide), or between benzene and dibromomethane.
(2) By Haworth Synthesis:
This involves the treatment of benzene with phthalic anhydride in the presence of aluminium
chloride to form o-benzoylbenzoic acid. This is then heated with concentrated sulphuric acid to give
9,10-anthraquinone. Distillation of the anthraquinone with zinc dust yields anthracene.

(3) By EIbs Reaction:

The conversion of a diaryl ketone containing a methyl or methylene group ortho to the carbonyl
function is known as the Elbs Reaction. For example, when o-methylbenzophenone is heated at 450°C,
anthracene is formed.
(4) By Diels-Alder Reacton:
This involves the reaction of l,4-naphthaquinone with 1,3.butadiene. The product of this
reaction is oxidised with chromium trioxide in glacial acetic acid to form 9,10-anthraquinone.
Distillation of anthraquinone with zinc dust yields anthracene.

PROPERTIES OF ANTHRACENE
Physicalproperties:
Anthracene is a colourless solid. It melts at 218 0C and boils at 340°C. Anthracene is insoluble
in water, but dissolves in benzene. It shows a strong blue fluorescence when exposed to ultraviolet
light. This fluorescent property of anthracene is used in criminal detection work, since a small amount
of finely powdered anthracene on clothing, skin, money, etc., is not detected under ordinary light but
easily noticed when exposed to ultraviolet light.
Chemical properties:
Anthracene undergoes addition and electrophilic substitution reactions. These reactions
preferentially occur at the C-9 and C-10 positions. This can be understood if we examine the
intermediate carbonium ions obtained from attack at C-1, C-2, and C-9 (all other positions are
equivalent to either 1 or 2 or 9 by symmetry). E+ in the following equations represents an electrophile.
Attack at C-9 yields a carbonium ion intermediate in which two benzene rings are retained
(retention of aromaticity and resonance energy)
whereas attack at C-1 or C-2 yields an intermediate in which a naphthalene system is retained.
The former intermediate is more stable and its formation favored because the resonance energy of two
benzene rings (2 x 36 = 72 Kcal) exceeds that of naphthalene (61 Kcal).
 This carbonium ion intermediate can lose a proton to give the corresponding substitution
product, or it can react with a nucicophile to form the 9,10-addition product. Nu - in the following
equations represents a nucleophile.

Substitution at C-I or C-2 occurs only when the reaction is reversible (as in the case of sulphonation).

The main chemical reactions of anthracene are described below.

(1) Reaction with Sodium:
Anthracene reacts with metallic sodium in liquid ammonia to form a deep blue
9,10-disodioanthracene.
When the disodio-derivative is heated with an alkyl halide, it gives the corresponding
9,10-dialkylanthracene.

(2) Reaction with Halogens:

Anthracene reacts with chlorine in carbon tetrachloride at room temperature to give
9,10-dichloro-9,10-dihydroanthracene (an addition product). On heating, this addition product loses a
molecule of hydrogen chloride by 1,4-elimination to form 9-chIoroanthracene (a substitution product).

9-Chloroanthracene may also be obtained by chlorinating anthracene at 100°C, or by treating

anthracene with cupric chloride in CCl4. Bromine reacts similarly.

(3) Friedel-Crafts Acylatlon:

Anthracene undergoes acylation with acetyl chloride and aluminium chloride in benzene to
form 9-acetylanthracene,

(4) Nitration:
Anthracene undergoes nitration with concentrated nitric acid in acetic anhydride at room
temperature to yield a mixture of 9-nitroanthracene and 9,10-dinitroanthracene. The usual nitrating
mixture (HNO2 + HSO4) is not used because it leads to the formation of 9,10-anthraquinone by
oxidation.
(5) Sulphonation:
Anthracene undergoes sulphonation with concentrated sulphuric acid to yield a mixture of
1-anthracenesulphonic acid and 2-anthracenesulphonic acid. At lower temperatures
I -anthracenesulphonic acid is the major product ; whereas at high temperatures 2-anthracenesuiphonic
acid is the major product.

(6) Reduction:
Anthracene undergoes reduction with sodium and ethyl alcohol to form 9,10-dihydroanthracene.

Catalytic reduction using nickel at 225˚C first gives 9,10-hydroanthracene, and on continued
hydrogenation this is converted into 1,2,3,4-tetrahydroanthracene and 1,2,3,4,5,6,7,8-octahydro
anthracene. Notice that the 9,10-hydrogen atoms migrate to the neighboring ring.

(7) Oxidation:
Anthracene undergoes oxidation with sodium dichromate and sulphuric acid to form
9, 10-anthraquinone. Other oxidising agents like nitric acid and air in the presence of V 2O5 also lead to
the formation of 9, 10-anthraquinone.
(8) Diels-Alder Reaction:
Anthracene undergoes a Diels-Alder reaction with maleic anhydride to yield the corresponding
adduct.

Important derivatives of anthracene

Uses:
 Anthracene is used in the manufacture of anthraquinone
 Anthraquinone is used in the manufacture of alizarin and several other dyes.
Alizarin (1,2-dihydroxyanthraquinone)

Phenanthrene is an isomer of anthracene. It may be obtained, along with anthracene, from the
green oil fraction of coal-tar. On cooling this fraction we get a solid mass which contains phenanthrene,
anthracene, and carbazole. Treatment of this with solvent naphtha dissolves phenanthrene. Evaporation
of this, solution yields crude phenanthrene, which may be purified by recrystallization of the picrate
from ethanol.
In naming derivatives of phenanthrene, the numbering system shown below is used. As before,
the numbers selected to denotethe position of a substituent on phenanthrene rings should be as small as
possible.
Similar to anthracene, phenanthrene is a planar molecule. All fourteen carbon atoms are sp2
hybridized. The sp2 orbitals overlap with each other and with s orbitals of ten hydrogen atoms to form C
—C and C—H a bonds. Each carbon atom also possesses a p orbital and these are perpendicular to the
plane containing the σ bonds. The lateral overlap of these p orbitals produces a п molecular orbital
containing ten electrons. Phenanthrene shows aromatic properties because the resulting п molecular
orbital satisfies the Huckel's rule (n=3 in 4n+2).

Synthesis of Phenanthrene:
Phenanthrene may be obtained from naphthalene and succinIc anhydride by Haworth synthesis
as follows.

Properties of Phenanthrene:
Physical Properties:
Phenanthrene is a colourless solid, mp 100˚C. It is insoluble in water, but dissolves readily in
ethanol, benzene, and ether. Phenanthrene gives blue fluorescence in benzene solution.

ChemicalProperties:
Phenanthrene undergoes oxidation, addition and electrophilic substitution reactions. As with
anthracene, these reactions preferentially occur at the C-9 and C-10 positions. Some of the important
reactions of phenanthrene are described below.

(1) Reaction with Halogens:

 Phenanthrene reacts with chlorine in carbon tetrachloride at room temperature to give
9,10-dichloro-9,10-dihydrophenanthrene (an addition product). On heating this loses a molecule of
hydrogen chloride to yield 9-chlorophenanthrene (a substitution product).

9-Chlorophenanthrene may also be obtained by treatment of phenanthrene with chlorine in the

presence of FeCl3. Bromine reacts similarly.

(2) Friedel crafts Acylatlon:

Phenanthrene undergoes acylation with acetyl chloride in the presence of aluminium chloride at
0˚C to give 9-acetylphenanthrene.

(3) Nitration:
Phenanthrene undergoes nitration with concentrated nitric acid and sulphuric acid to yield
9-nitrophenanthrene.

(4) Sulphonation:
Phenanthrene reacts with concentrated sulphuric acid at 12˚C to give a mixture of
2-phenanthrenesufphonic acid and 3-phenanthrenesulphonic acid. Notice that, like anthracene,
substitution does not occur at C-9.
(5) Reduction:
Phenanthrene undergoes reduction with sodium and isopentanol to form
9,10-dihydrophenanthrene,

(6) Oxidation:
Phenanthrene undergoes oxidation with potassium dichromate and sulphuric acid or chromium
trioxide in acetic acid to form 9,l0-phenanthraquinone (mp 206˚C). Further oxidation of this with
hydrogen peroxide in acetic acid gives diphenic acid.

Uses:
 Phenanthrene is of little industrial importance but phenanthrene ring systems are found widely
distributed in natural compounds such as carcinogenic (meaning cancer-producing)
hydrocarbons, bile acids, sex harmones, morphine alkaloids, and cholesterol.

Unit-V:
In the compounds that we have studied till now, the carbon atoms are attached to one another to
form chains; these are called open-chain compounds, In many compounds, however, the carbon atoms
are arranged to form rings; these are called cyclic compounds. In this chapter we shall take up the
alicyclic hydrocarbons (aliphatic cyclic hydrocarbons). Much of the chemistry of cycloalkanes and
cycloalkenes we already know, since it is essentially the chemistry of open-chain alkanes and alkenes.
But the cyclic nature of some of these compounds confers very special properties on them. It is because
of these special properties that, during the past fifteen years, alicyclic chemistry has become what
Professor Lloyd Ferguson, of the California State College at Los Angeles, has called "the playground
for organic chemists." It is on some of these special properties that we shall focus our attention.

Nomenclature
Cyclic aliphatic hydrocarbons are named by prefixing cyclo- to the name of the corresponding
open-chain hydrocarbon having the same number of carbon atoms as the ring. For example:

Substituents on the ring are named, and their positions are indicated by numbers, the lowest
combination of numbers being used, in simple cycloalkenes and cycloalkynes the doubly- and triply-
bonded carbons are considered to occupy positions 1 and 2. For example:

For convenience, aliphatic rings are often represented by simple geometric figures: a triangle for
cyclopropane, a square for cyclobutane, a pentagon for cyclopentane, a hexagon for cyclohexane, and
so on. It is understood that two hydrogens are located at each corner of the figure unless some other
group is indicated. For example:
 Polycyclic compounds contain two or more rings that share two or more carbon atoms. We can
illustrate the naming system with norbornane, whose systematic name is bicyclo[2.2.1]heptane: (a)
heptane, since it contains a total of seven carbon atoms; (b) bicyclo, since it contains two rings, that is,
breaking two carbon-carbon bonds converts it into an open-chain compound; (c) [2.2.1], since the
number of carbons between bridgeheads (shared carbons) is two (C-2 and C-3), two (C-5 and C-6), and
one (C-7).

Preparation:
1. From Dihalogen Compounds:
Suitable dibalogen compounds on treatment with sodium or zinc give corresponding
cycloalkanes. For example,

This reaction is an extension of Wurtz Reaction and may be regarded as an Internal Wurtz Reaction.
The reaction is useful for the preparation of three- to six-membered rings. With higher homologues
various side reactions occur.
2. From Calcium or Barium salts of Dicarboxylic acids:
When the calcium or barium salt of adipic, pimelic, or suberic acid is heated, a cyclic ketone is
formed.

Cyclic ketones may be readily converted into the corresponding cycloalkanes by means of the
Clemmensen Reduction.
Cyclopropane cannot be prepared by this method.
3. From Esters of Dicarboxylic acids (Dieckmann Reaction):
The diester of adipie, pimelic, or suberic acid when treated with sodium undergoes
intramolecular acetoaectic ester condensation and α -ketoester is formed. The ketoesters on hydrolysis
give corresponding cyclic ketones. Cyclic ketones on reduction yield the corresponding cycloalkanes.

4. From Aromatic Compounds:

Benzene may be catalytically hydrogenated at elevated temperature and pressure to yield
cyclohexane.

PHYSICAL PROPERTIES
(1) Cyclopropane and cyclobutane are gases at ordinary temperatures; the remaining cycloalkanes are
liquids. Their melting points and boiling points show a gradual rise with the increase in molecular
weight.
(2) They are all lighter than water; the series has a limiting density of less than 0.9.
(3) They are insoluble in water but are soluble in organic solvents such as ethers and alcohols.
The physical constants of some selected cycloalkanes are given below
CHEMICAL PROPERTIES:
Cycloalkanes resemble alkanes in their chemical behaviour.
(a) They do not react with acids, bases, oxidizing agents and reducing agents.
(b) They undergo substitution reactions with halogens. However, cyclo propane and cyclobutane are
the exceptions.
With certain reagents they undergo ring-opening and give addition products. The more important
reactions of cycloalkanes are given below.

Substitution reactions With C12 and Br2

1. Cycloalkanes react with Cl, and Br, in the presence of diffused sunlight or ultraviolet light to
give a monochlorination and monobromination product respectively.
 In each case only one monosubstitution product is possible because all hydrogens are
chemically identical.
Special ReactionsoOf Cyclopropane and Cyclobutane

2. Addition of C12 and Br2:

Cyclopropane reacts with C12 and Br2 at room temperature and in the absence of diffused
sunlight to produce I,3-dichlorocyclopropane and 1,3-dibromocyclopropane respectively. One of the
carbon-carbon bonds of cyclopropane is broken and the two halogen atoms appear at the ends of the
propane chain.

Cyclobutane and higher members of the family do not give this reaction.

3. Addition of HBr and HI:

Cyclopropane reacts with cone HBr and HI to give 1-bromopropane and 1-iodopropane
respectively.
 Cyclobutane and the higher members of the family do not give this reaction.

4. Addition of Hydrogen;
Calalytic Redaction. Cyclopropane and cyclobutane react with hydrogen in the presence of a
nickel catalyst to produce propane and n-butane respectively.

Hydrogenation of cyclobutane takes place at a higher temperature (200°C) than that required for
cyclopropane (80°C).
Cyclopentane and higher members of the family do not give this reaction.

Besides the free-radical substitution reactions that are characteristic of cycloalkanes and of
alkanes in general, cyclopropane and cyclobutane undergo certain addition reactions. These addition
reactions destroy the cyclopropane and cyclobutane ring systems, and yield open-chain products. For
example:
 In each of these reactions a carbon-carbon bond is broken, and the two atoms of the reagent

appear at the ends of the propane chain:

Baeyer strain theory:

In 1885 Adolf von Baeyer (of the University of Munich) proposed a theory to account for
certain aspects of the chemistry of cyclic compounds. The part of his theory dealing with the ring-
opening tendencies of cyclopropane and cyclobutane is generally accepted today, although it is dressed
in more modern language. Other parts of his theory have been shown to be based on false assumptions,
and have been discarded.
Baeyer's argument was essentially the following. In general, when carbon is bonded to four
other atoms, the angle between any pair of bonds is the tetrahedral angle 109.5. But the ring of
cyclopropane is a triangle with three angles of 60˚, and the ring of cyclobutane is a square with four
angles of 90. In cyclopropane or cyclobutane, therefore, one pair of bonds to each carbon cannot
assume the tetrahedral angle, but must be compressed to 60˚ or 90˚ to fit the geometry of the ring.
“These deviations of bond angles from the "normal" tetrahedral value cause the molecules to be
strained, and hence to be unstable compared with molecules in which the bond angles are tetrahedral.”

Cyclopropane and cyclobutane undergo ring-opening reactions since these relieve the strain and
yield the more stable open chain compounds. Because the deviation of the bond angles in cyclopropane
(109.5˚ - 60˚ = 49.5˚) is greater than in cyclobutane (109.5˚ - 90˚= 19.5˚), cyclopropane is more highly
strained, more unstable, and more prone to undergo ring-opening reactions than is cyclobutane.
Calculation of angle strain:
In cyclopropane, the three carbon atoms occupy the corners of an equilateral triangle. Thus
cyclopropane has C—C—C bond angles of 60° (the internal angle of an equilateral triangle). This
implies that the normal tetrahedral angle of 109°28' between any two bonds is compressed to 60°, and
that each of the two bonds involved is pulled in by ½(109°28'-60°) = 24°44'.

The value 24°44' then represents the angle strain or the deviation through which each bond
bends from the normal tetrahedral direction.
Angle strain = ½(109°28'- bond angle)

The angles of a regular pentagon (108˚) are very close to the tetrahedral angle (109.5˚), and
hence cyclopentane should be virtually free of angle strain. The angles of a regular hexagon (120˚) are
somewhat larger than the tetrahedral angle, and hence, Baeyer proposed (incorrectly), there should be a
certain amount of strain in cyclohexane. Further, he suggested (incorrectly) that as one proceed to
cycloheptane, cyclooctane, etc., the deviation of the bond angles from 109.5˚ would become
progressively larger, and the molecules would become progressively more strained.
Thus Baeyer considered that tings smaller or larger than cyclopentane or cyclohexane were
unstable; it was because of this instability that the three- and four- membered rings underwent ring-
opening reactions; it was because of this instability that great difficulty had been encountered in the
synthesis of the larger rings.
How does Baeyer's strain theory agree with the facts?
Heals of combustion and relative stabilities of the cycloalkanes:
We recall that the heat of combustion is the quantity of heat evolved when one mole of a
compound is burned to carbon dioxide and water. Like heats of hydrogenation, heats of combustion can
often furnish valuable information about the relative stabilities of organic compounds. Let us see if the
heats of combustion of the various cycloalkanes support Baeyer's proposal that rings smaller or larger
than cyclopentane and cyplohexane are unstable.

Examination of the data for a great many compounds has shown that the heat of combustion of
an aliphatic hydrocarbon agrees rather closely with that calculated by assuming a certain characteristic
contribution from each structural unit. For open-chain alkanes each methylene group, -CH 2-,
contributes very close to 157.4 kcal/mole to the heat of combustion. Table lists the heats of combustion
that have been measured for some of the cycloalkanes.

We notice that for cyclopropane the heat of combustion per -CH 2- group is 9 kcal higher than
the open-chain value of 157.4; for cyclobutane it is 7 kcal higher than the open-chain value. Whatever
the compound in which it occurs, a -CH2- group yields the same products on combustion: carbon
dioxide and water.
If cyclopropane and cyclobutarie evolve more energy per -CH2- group than an open-chain
compound, it can mean only that they contain more energy per -CH2- group. In agreement with the
Baeyer angle-strain theory, then, cyclopropane and cyclobutane are less stable than open-chain
compounds; it is reasonable to suppose that their tendency to undergo ring-opening reactions is related
to this instability.
 According to Baeyer, rings larger than cyclopentane and cyclohexane also should be unstable,
and hence also should have high heats of combustion; furthermore relative instability and, with it, heat
of combustion should increase steadily with ring size. However, we see from Table that almost exactly
the opposite is true. For none of the rings larger than four carbons does the heat of combustion per
-CH2- deviate much from the open-chain value of 157.4. Indeed, one of the biggest deviations is for
Baeyer's "most stable" compound, cyclopentane: 1.3 kcal per -CH2- , or 6.5 kcal for the molecule.
Rings containing seven to eleven carbons have about the same value as cyclopentane, and when
we reach rings of twelve carbons or more, heats of combustion are indistinguishable from the
openchain values. Contrary to Baeyer's theory, then, none of these rings is appreciably less stable than
open-chain compounds, and the larger ones are completely free of strain. Furthermore, once they have
been synthesized, these large-ring cycloalkanes show little tendency to undergo the ring-opening
reactions characteristic of cyclopropane and cyclobutane.
What is wrong with Baeyer's theory that it does not apply to rings larger than four members?

Simply this: the angles that Baeyer used for each ring were based on the assumption that the
rings were flat. For example, the angles of a regular (flat) hexagon are 120˚, the angles for a regular
decagon are 144˚. But the cyclohexane ring is not a regular hexagon, and the cyclodecane ring is not a
regular decagon. These rings are not flat, but are puckered (see Fig below) so that each bond angle of
carbon can be 109.5˚.

A three-membered ring niust be planar, since three points (the three carbon nuclei) define a
plane. A four membered ring need not be planar, but puckering here would increase (angle) strain- A
five-membered ring need not be planar, but in this case a planar arrangement would permit the bond
angles to have nearly the tetrahedral value, All rings larger than this are puckered. (Actually, as we shall
see, cyclobutane and cyclopentane are puckered, too, but this is in spite of increased angle strain.)

If large rings are stable, why are they difficult to synthesize? Here we encounter Baeyer's second false
assumption. The fact that a compound is difficult to synthesize does not necessarily mean that it is
unstable. The closing of a ring requires that two ends of a chain be brought close enough to each other
for a bond to form. The larger the ring one wishes to synthesize, the longer must be the chain from
which it is made, and the less is the likelihood of the two ends of the chain approaching each other.
Under these conditions the end of one chain is more likely to encounter the end of a different chain, add
thus yield an entirely different product.

The methods that are used successfully to make large rings take this fact into consideration. Reactions
are carried out in highly dilute solutions where collisions between two different chains are unlikely;
under these conditions the ring-closing reaction, although slow, is the principal one. Five- and six-
membered rings are the kind most commonly encountered in organic chemistry because they are large
enough to be free of angle strain, and small enough that ring closure is likely.

Orbital picture of angle strain:

What is the meaning of Baeyer's angle strain in terms of the modern picture of the covalent
bond?
We have seen in alkanes that, for a bond to form, two atoms must be located so that an orbital of
one overlaps an orbital of the other. For a given pair of atoms, the greater the overlap of atomic orbitals,
the stronger the bond. When carbon is bonded to four other atoms, its bonding orbitals (sp 3 orbitals) are
directed to the corners of a tetrahedron; the angle between any pair of orbitals is thus 109.5˚. Formation
of a bond with another carbon atom involves overlap of one of these sp 3 orbitals with a similar sp3
orbital of the other carbon atom. This overlap is most effective, and hence the bond is strongest, when
the two atoms are located so that an sp3 orbital of each atom points toward the other atom. This means
that when carbon is bonded to two other carbon atoms the C-C-C bond angle should be 109.5˚. In
cyclopropane, however, the C-C-C bond angle cannot be 109.5˚, but instead must be 60˚. As a result,
the carbon atoms cannot be located to permit their sp 3 orbitals to point toward each other (see Fig
below). There is less overlap and the bond is weaker than the usual carbon-carbon bond. The decrease
in stability of a cyclic compound attributed to angle strain is due to poor overlap of atomic orbitals in
the formation of the carbon-carbon bonds.

(a) Orbital overlap in the carbon-carbon bonds of cyclopropane cannot occur perfectly end-on. This
leads to weaker “bent” bonds and to angle strain. (b) Bond distances and angles in cyclopropane. (c) A
Newman projection formula as viewed along one carbon-carbon bond shows the eclipsed hydrogens.

Coulson and Moffitt’s modification:

 On the basis of quantum mechanical calculations, C. A. Coulson and W, A. Moffitt (of Oxford
University) proposed bent bonds between carbon atoms of cyclopropane rings; this idea is supported by
electron density maps based on X-ray studies. Carbon uses sp2 orbitals for carbon-hydrogen bonds
(which are short and strong), and orbitals with much p character (sp 4 to sp5) for the carbon-carbon
bonds. The high p character of these carbon-carbon bonds, and their location largely outside the ring
seems to underlie much of the unusual chemistry of these rings. The carbon-carbon bond orbitals can
overlap orbitals on adjacent atoms; the resulting delocalization is responsible for the effects of
cyclopropyl as a substituent.
The carbon-carbon bond orbitals provide a site for the attack by acids that are the first step of
ring-opening. (Indeed, "edge-protonated" cyclopropanes seem to be key intermediates in many
reactions that do not, on the surface, seem to involve cyclopropane rings.) Ring-opening is due to the
weakness of the carbon-carbon bonds, but the way in which it happens reflects the unusual nature of the
bonds; all this stains ultimately from the geometry of the rings and angle strain.

SACHSE MOHR CONCEPT OF STRAINLESS RINGS:

In order to account for the stability of cycloalkanes beyond cyclopentane, Sachse and Mohr
(1918) pointed out that such rings can become absolutely free of strain carbons are not force into one
plane, if all the ring as was supposed by Baeyer. If the ring assumes a 'puckered' condition, the normal
tetrahedral angles of 109°28 are retained and as a result, the strain within the ring is relieved. Thus
cyclohexane can exist in two non-planar strain less forms, namely, the Boat form and the Chair form.
In the Boat form, carbons 1, 2, 4 and 5 lie in the same plane and carbons 3 and 6 above the
plane. In the chair form, carbons I, 2, 4 and 5 lie in the same plane, but carbon 6 is above the plane and
carbon 3 is below it.

Actually, only one form of cyclohexane is known and not two forms as shown above. We failure
to isolate the two forms is ascribed to rapid interconversions between them. Such non-planar strain less
rings in which the ring carbon atoms can have normal tetrahedral angles are also possible for larger ring
compounds.

CONFORMATIONS OF CYCLOHEXANE AND ITS DERIVATIVES:

 The cyclohexane ring can assume many shapes. A single cyclohexane molecule is in a
continuous state of flexing or flipping into different shapes or conformations Some of these shapes are
shown below.

These conformations arise due to rotation around carbon-carbon bonds. The chair form and the
boat form are the extreme cases.

Figure given above shows the energy requirements for the interconversion of the different
conformations of cyclohexane. Notice that the chair form has the lowest energy, while the half-chair
has ,the highest energy. At any given time, we would expect most of the cyclohexane molecules to be in
the chair form, Indeed, It has been calculated that about 99.9% of cyclohexane molecules are in the
chair form at any one time.