Haloalkanes and Haloarenes or Alkyl Halides and Aryl Halides

The replacement of hydrogen atom(s) in hydrocarbon, aliphatic or aromatic, by

halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide
(haloarene), respectively.

Classification of Halogen Derivatives

On the basis of number of halogen atoms present, halogen derivatives are classified as
mono, di, tri, tetra, etc., halogen derivatives, e.g., For example

On the basis of the nature of the carbon to which halogen atom is attached, halogen
derivatives are classified as 1°, 2°, 3°, allylic, benzylic, vinylic and aryl derivatives,
e.g.,

1o, 2o and 3o halides: halogen atom is bonded to primary, secondary or tertiary carbon

atom of an alkyl group.
Allylic halides: Halogen atom is bonded to an sp3-hybridised carbon atom next to
carbon-carbon double bond (C=C) i.e. to an allylic carbon.

Benzylic halides: Halogen atom is attached to a sp3 - hybridised carbon atom next to

an aromatic ring.

Vinylic halides: Halogen atom is bonded to a sp2-hybridised carbon atom of a carbon-

carbon double bond (C = C).

Aryl halides: Halogen atom is bonded to the sp2-hybridised carbon atom of an

aromatic ring.

Note: Here X represents a halogen atom, i.e., X= F, Cl, Br, I.

Nature of C-X bond in haloalkanes

X is more electronegative than carbon. So, the C-X bond is polarized with C having a
partial positive charge and X having a partial negative charge.
Preparation of alkyl halides

I) From Alcohols:

i) Reaction with Hydrogen halide: The hydroxyl group of an alcohol is replaced by

halogen on reaction with concentrated halogen acids

ROH+HX→RX+H2O

Reaction with hydrogen chloride – Preparation of alkyl chloride

The reactions of primary and secondary alcohols with HCl are too slow and require
the presence of a catalyst, ZnCl2 .

With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with
concentrated HCl at room temperature.

Replacing -OH by bromine -Preparation of alkyl bromide

Rather than using hydrobromic acid, the alcohol is typically treated with a mixture of
sodium or potassium bromide and concentrated sulfuric acid. This produces hydrogen
bromide, which reacts with the alcohol.

CH3CH2OH+HBr→CH3CH2Br+H2O
Replacing -OH by iodine – Preparation of alkyl iodide

In this case, the alcohol is reacted with a mixture of sodium or potassium iodide and
concentrated phosphoric acid, H3PO4. The mixture of the iodide and phosphoric acid
produces hydrogen iodide, which reacts with the alcohol.

CH3CH2OH+HI→CH3CH2I+H2O

Phosphoric acid is used instead of concentrated sulfuric acid because sulfuric acid
oxidizes hydrogen iodide formed to iodine and produces hardly any hydrogen iodide.
A similar phenomenon occurs to some extent with bromide ions in the preparation of
bromo alkanes but not enough to interfere with the main reaction.

 The order of reactivity of alcohols is 3° > 2° > 1° methyl.

 The order of reactivity of the hydrogen halides is HI > HBr > HCl 

ii) Reacting Alcohols with Phosphorus Halides

Alcohols react with liquid phosphorus(III) chloride (also called phosphorus

trichloride) to yield chloroalkanes.

3CH3CH2CH2OH+PCl3→3CH3CH2CH2Cl+H3PO3

Alcohols also violently react with solid phosphorus(V) chloride (phosphorus

pentachloride) at room temperature, producing chloroalkanes.

CH3CH2CH2OH+PCl5→CH3CH2CH2Cl+POCl3+HCl
Similarly halo alkanes and iodo alkanes are prepared by the action of PBr3and PI3
respectively.

Since phosphorus(III) bromide and phosphorus(III) iodide are not very stable
compounds ,they are prepared in situ by the action of  red phosphorus and either
bromine or iodine. The phosphorus first reacts with the bromine or iodine to give the
phosphorus(III) halide.

2P(s)+3Br2→2PBr3

2P(s)+3I2→2PI3

These then react with the alcohol to give the corresponding halogenoalkane.

3CH3CH2CH2OH+PBr3→3CH3CH2CH2Br+ H3PO3

3CH3CH2CH2OH+PI3→3CH3CH2CH2I+ H3PO3

iii) Reacting alcohols with Thionyl Chloride

Thionyl chloride (SOCl2) reacts with alcohols at room temperature to produce a

chloroalkane. Sulfur dioxide and hydrogen chloride are given off.

CH3CH2CH2OH+SOCl2→CH3CH2CH2Cl+SO2+HCl

The advantage that this reaction has over the use of either of the phosphorus chlorides
is that the two other products of the reaction (sulfur dioxide and HCl) are both gases.
That means that they separate themselves from the reaction mixture.

II) From Alkanes

 By free radical halogenation:

Free radical chlorination or bromination of alkanes with Cl2 or Br2 in presence of

sunlight gives a complex mixture of isomeric mono- and polyhaloalkanes,

When alkanes larger than ethane are halogenated, even monohalogenation gives a
mixture of all possible isomeric haloalkane.

Mixture of mono halo alkane is formed due to the substitution of all different types of
hydrogen atoms like primary, secondary, tertiary present in higher alkanes.

The ease of substitution of various hydrogens follows the order 3° > 2° > 1°

CH3-CH2-CH3 + Cl2  CH3-CH2-CH2Cl (45%) +

CH3-CHCl-CH3(55%)

III) FROM ALKENE

i) Addition of Hydrogen Halides

Reaction type: Electrophilic Addition

Hydrogen halide reactivity order : HI > HBr > HCl > HF

Addition of Hydrogen halides to unsymmetrical alkene takes place according to

Markovnikov's rule : which states that "For addition of hydrogen halides to
unsymmetrical alkene the H atom adds to the C with the most H atoms already
present"

CH3−CH=CH2+H−Br→CH3−CH(Br)−CH3

When HBr is treated with unsymmetrical alkene in presence of peroxide ,then addition
takes place against the Markownikoff’s rule i.e. negative part of reagent adds to that
carbon ot carbon-carbon double bond which carries more number of hydrogen atoms.
This is known as peroxide effect.

ii) Addition of halogens

Bromination or chlorination of alkenes is a reaction in which Cl2 or Br2 is added to a

molecule after breaking the carbon to carbon double bond resulting in the formation of
a vicinal (neighboring) dihalide.
Addition of bromine in CCl4 to an alkene resulting in discharge of reddish brown
colour of bromine constitutes an important method for the detection of double bond in
a molecule.

iii) Allylic halogenation

When alkenes except ethene are heated with Cl2 or Br2 at a high temperature a
substitution reaction occurs at the allylic position rather than addition at the double
bond. The product is an allylic halide (halogen on carbon next to double bond
carbons), The reaction takes place through a free radical chain mechanism.

Such a reaction in which halogenation occurs at allylic position of an alkene are called
allylic substitution reaction.

IV) Halogen Exchange Reactions

i) Finkelstein's reaction

Finkelstein's reaction is a method of preparation of alkyl iodides from alkyl chlorides

or alkyl bromides by treating them with NaI in presence of dry acetone.

In acetone, sodium iodide is soluble, but NaCl or NaBr are insoluble. Hence NaCl or
NaBr so formed get precipitated, thus do not interfere the reaction on formation.

R-X +NaI —————-> R-I + NaX

ii) Swarts reaction :

It is a reaction to produce fluoroalkane. When alkyl chloride / alkyl bromide is heated in
presence of metallic fluoride such as AgF, Hg2F2, CoF2, SbF3 alkyl fluoride is produced

 CH3CH2Cl + AgF→ CH3CH2F + AgCl

PREPARATION OF ARYL HALIDES

i) Nuclear halogenations( Electrophilic substitution reaction)

This method can be used to prepare aryl chlorides and bromides. This is done by
treatment of arene with chlorine or bromine in the presence of halogen carrier like
AlCl3, FeCl3 etc. at low temperatures.
Ex 1

In the above reaction Fe is not the catalyst , it gets changed to FeCl3 by the following
reaction

Fe + 3Cl2 FeCl3

Ex 2
The ortho and para isomers can be easily separated due to large difference in their
melting points

In the case of electrophilic substitution reaction of aryl iodide, HI is produced which is

a reducing agent. It reduces aryl iodide back to aromatic hydrocarbon and iodine as
this reaction is reversible. Hence oxidising agents such as HNO3, HIO4 are used to
remove HI produced in the reaction as follows:
4HI + 2HNO3  → 2N2O3   + 2I2  

ii) Side chain halogenation:

When chlorine is passed through toluene in presence of sunlight and absence of
halogen carriers , Hydrogen atoms on the CH3 group is substituted with Chlorine
atom.

When the side chain is larger than the methyl group as in Ethylbenzene halogenation
occurs at the benzylic location exclusively. The hydrogens bonded to the aromatic
ring have relatively high bond dissociation energies and are not substituted.

C6H5CH2CH3 + Cl2 → C6H5CHClCH3 + HCl

iii) Sandmayer reaction

Treating diazonium salt with copper (I) chloride (Cu2Cl2) or copper (I) bromide
(Cu2Br2) leads to the formation of corresponding haloarene. This reaction is known
as Sandmeyer reaction.

Benzene diazonium salt needed for the above reactions is prepared by treating primary
aromatic amine (aniline) with nitrous acid (generated in situ from sodium nitrite and a
strong acid, such as hydrochloric acid ) at 0-50C. This reaction is called diazotization
rection.

iii) Gatterman reaction.

 
Chlorine or bromine can be introduced in the benzene ring by treating the benzene
diazonium salt solution with corresponding halogen acid in the presence of copper
powder. This is referred as Gatterman reaction.
 

Benzene diazonium salt needed for the above reactions is prepared by treating primary
aromatic amine (aniline) with nitrous acid (generated in situ from sodium nitrite and a
strong acid, such as hydrochloric acid ) at 0-50C

Replacement by iodide ion: Iodine is introduced in the benzene ring when diazonium

salt solution is treated with potassium iodide, iodo benzene is formed.

Balz-Schiemann reaction: Flurobenzene is prepared by treating an aqueous solution

of diazonium salt with fluoroboric acid under cold conditions to give diazonium
fluoroborate as precipitate, followed by gentle heating .This reaction is called Balz-
Schiemann reaction 
 Physical properties of haloalkanes:
a) Solubility
The haloalkanes are very slightly soluble in water. In order to dissolve haloalkane in
water, energy is required to overcome the attractions between the haloalkane
molecules and break the hydrogen bonds between water molecules. Less energy is
released when new attractions are set up between the haloalkane and the water
molecules as these are not as strong as the original hydrogen bonds in water. As a
result, the solubility of haloalkanes in water is low.

b) Density
Simple fluoro and chloroalkanes are lighter than water while bromides and poly
chloro derivatives are heavier than water.
With the increase in number of carbon atoms, the densities go on increasing. With the
increase in number of halogen atoms, the densities go on increasing. The densities
increase in the order: Fluoride < chloride < bromide < iodide
The density also increases with increasing number and atomic mass of the halogen.

c) Boiling Points
Molecules of organic halogen compounds are generally polar.Due to the polarity as
well as higher molecular mass as compared to the parent hydrocarbon, the
intermolecular forces of attraction (dipole – dipole and van der Waals) between the
molecules are stronger in halogen derivatives of alkanes. As a result melting and
boiling points of chlorides, bromides and iodides are considerably higher than those of
the parent hydrocarbon of comparable molecular mass

For the same alkyl group the boiling points of alkyl chlorides, bromides and iodides
follow the order RI >RBr>RCl> RF where R is an alkyl group.
This is because with the increase in the size of the halogen, the magnitude of van der
Waals force increases.

In general, the boiling points of chloro, bromo and iodo compounds increase with
increase in the number of halogen atoms.
For the same halogen atom, the boiling points of haloalkanes increase with increase in
the size of alkyl groups.

For isomeric alkyl halides, the boiling points decrease with branching. This is because
branching of the chain makes the molecule more compact and, therefore, decrease the
surface area. Due to decrease in surface area, the magnitude of van der Waals forces
of attraction decreases and consequently, the boiling points of the branched chain
compound is less than those of the straight chain compounds.
The para isomers of dihalobenzenes have high melting point as compared to their
ortho and meta isomers. It is due to symmetry of para isomers that fits in crystal lattice
as compared to ortho and meta isomers
For example

d) Dipole moment
The dipole moment of methyl halides follows the order: CH3Cl > CH3F > CH3Br >
CH3I.
Usually, dipole moment decreases with decrease in electronegativity. Flourides have
lower dipole moment than chlorides dure to the very small size of fluorine which
outweighs the effect of greater electronegativity.es.
REACTIONS OF HALO ALKANES

I) Substitution reaction in halo alkanes

Nucleophilic substitution reactions are most common reactions of alkyl halides.

In this type of reaction, a nucleophile reacts with haloalkane (the substrate) having a
partial positive charge on the carbon atom bonded to halogen. A substitution reaction
takes place and halogen atom, called leaving group departs as halide ion. Since the
substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution
reaction.

The products formed by the reaction of haloalkanes with some common nucleophiles
are given in Table
Ambident nucleophiles.

Groups like cyanides and nitrites possess two nucleophilic centres and are called
ambident nucleophiles.

For example, cyanide group is a hybrid of two contributing structures and therefore
can act as a nucleophile in two different ways , i.e., linking through
carbon atom resulting in alkyl cyanides and through nitrogen atom leading to
isocyanides.

Nitrite ion also represents an ambident nucleophile with two different points of

linkage . The linkage through oxygen results in alkyl nitrites while through
nitrogen atom, it leads to nitroalkanes

Note: Haloalkanes react with KCN to form alkyl cyanides as main product while
AgCN forms isocyanides as the chief product. This is because KCN is predominantly
ionic and provides cyanide ions in solution. Although both carbon and nitrogen atoms
are in a position to donate electron pairs, the attack takes place mainly through carbon
atom and not through nitrogen atom since C—C bond is more stable than C—N bond.
However, AgCN is mainly covalent in nature and nitrogen is free to donate electron
pair forming isocyanide as the main product.

Similarly Haloalkanes react with KNO2 to form alkyl nitrite as main product while
AgNO2 forms nitro alkane as the chief product. This is because alkali metal nitrites are
ionic compounds and have negative charge on one of the oxygen atoms of NO2- group.
Nucleophilic attack through this negatively charged oxygen atom gives alkyl nitrites.

In contrast AgNO2 is a covalent compound. And hence does not have negative charge
on oxygen atom instead both nitrogen and oxygen carry lone pair of electrons. Lone
pair of electrons on N is easily available for bond formation as it is less
electronegative which means nucleophilic attach takes place through nitrogen atom
giving alkyl nitrite as the product.

Mechanism of nucleophilic substitution reactions.

Nucleophilic substitution may take place in two ways:

1. SN1 Mechanism
SN1 reactions are basically unimolecular nucleophilic substitution reactions. The
mechanism of SN1 reaction for tertiary butyl bromide and hydroxide ion is as follows.
It occurs in two steps.
1) The polarized C --- Br bond undergoes slow cleavage to produce a carbocation and
a bromide ion.
2) The carbocation thus formed is attacked by the nucleophile to complete the
substitution reaction.
 
These reactions are carried out in polar protic solvents such as water, alcohol, acetic
acid, etc.
The increasing order of reactivity is1o halide < 2o halide < 3o halide. The reason
behind the above trend is as follows.
Greater the stability of a carbocation, more easily the alkyl halide is formed and
hence, faster is the reaction rate. Since 1 o halide forms 1 o carbocation, 2 o halide forms
2 o carbocation, and 3 o halide forms 3 o carbocation. Therefore, the increasing order of
reactivity is 1 o halide < 2 o halide < 3 o halide.
Allylic and benzylic halides show high reactivity towards the SN1 reaction. The
carbocation thus formed gets stabilized through resonance as shown below.

 
2. SN2 Mechanism: SN2 reactions are known as bimolecular nucleophilic substitution
reactions.

The mechanism for this type of reaction between CH3Cl and hydroxide ion is as
follows.
The incoming nucleophile interacts with the alkyl halide causing the carbon halide to
break while forming a new a new carbon – OH bond. The two processes take place
simultaneously and no intermediate is formed. This results in an unstable transition
state with five bonds. Then the leaving group is expelled, and results in a product with
a complete inversion of configuration.

Inversion of configuration SN2 creates a product with an inverted stereo structure to

that of the substrate (Reactant). This happens because the nucleophile attacks from the
direction opposite to that of the leaving group, inverting the molecule’s original
stereochemistry. This process called inversion of configuration.

Of the simple alkyl halides, methyl halides react most rapidly in SN 2 reactions
because there are only three small hydrogen atoms. Tertiary halides are the least
reactive because bulky groups hinder the approaching nucleophiles. Thus, the order of
reactivity followed is: Primary halide > Secondary halide > Tertiary halide.

Stereo chemical aspects of SN reactions

Some basic stereochemical principles and notations

i) Optically active compounds. Certain compounds rotate the plane polarised light
(produced by passing ordinary light through Nicol prism) when it is passed through
their solutions. Such compounds are called optically active compounds.

ii) If the compound rotates the plane polarised light to the right, i.e., clockwise
direction, it is called dextrorotatory or the d-form

iii) If the light is rotated towards left (anticlockwise direction), the compound is said
to be laevorotatory or the l-form
iv) (+) and (–) isomers of a compound are called optical isomers and the phenomenon
is termed as optical isomerism.

v) asymmetric carbon or stereocentre. if all the substituents attached to a carbon in

an organic compound are different, such a carbon is called asymmetric carbon or
stereocentre.

vi) Chiral and chirality: The objects which are non-superimposable on their mirror
image are said to be chiral and this property is known as chirality.

vii) The objects, which are, superimposable on their mirror images are called achiral.

vii) Enantiomers: The stereoisomers related to each other as non-superimposable

mirror images are called enantiomers. Enantiomers possess identical physical
properties like melting point, boiling point, solubility, refractive index, etc. They only
differ with respect to the rotation of plane polarised light.

viii) Racemic mixture: A mixture containing two enantiomers in equal proportions

will have zero optical rotation, as the rotation due to one isomer will be cancelled by
the rotation due to the other isomer. Such a mixture is known as racemic mixture or
racemic modification. A racemic mixture is represented by prefixing dl or (±) before
the name, for example (±) butan-2-ol. The process of conversion of enantiomer into a
racemic mixture is known as racemisation.

Retention: Retention of configuration is the preservation of integrity of the spatial

arrangement of bonds to an asymmetric centre during a chemical reaction or
transformation.

For example, the reaction that takes place when (–)-2-methylbutan-1-ol is heated with
concentrated hydrochloric acid proceeds with retention
During the reaction no bond at the stereo centre is broken hence the reaction proceeds
with retention of configuration even though the optical rotation has changed from (-)
to (+).

Stereochemical aspect of SN 2 reaction

In case of optically active alkyl halides, the product formed as a result of SN2
mechanism has the inverted configuration as compared to the reactant. This is because
the nucleophile attaches itself on the side opposite to the halogen atom. Thus, SN2
reactions of optically active halides are accompanied by inversion of configuration.

When (–)-2-bromooctane is allowed to react with sodium hydroxide, (+)-octan-2-ol is

formed with the –OH group occupying the position opposite to what bromide had
occupied.

Stereochemical aspect of SN 1 reaction

In case of optically active alkyl halides, SN1 reactions are accompanied by

racemisation. the carbocation formed in the slow step being sp2 hybridised is planar.
The attack of the nucleophile takes place from either side resulting in a mixture of
products, one having the same configuration (the –OH attaching on the same position
as halide ion) and the other having opposite configuration (the –OH attaching on the
side opposite to halide ion).
For example hydrolysis of optically active 2-bromobutane, which results in the
formation of (±)-butan-2-ol.

For both SN1 and SN2 reaction, the order of reactivity of halides is
R−F << R−Cl < R−Br < R−I

I) Elimination reactions

Haloalkanes also undergo elimination reactions. In these reactions, a haloalkane with

a (beta) β-hydrogen atom on heating with an alcoholic solution of potassium
hydroxide, loses a halogen atom from the (alpha) α- carbon and a hydrogen atom from
the β- carbon to form an alkene molecule. Since beta hydrogen atoms are involved in
the elimination, these reactions are often called "(beta) β- elimination reactions"

If there are more than one beta hydrogen atoms, according to Saytzeff's rule, in
dehydrohalogenation reactions, the preferred product is the alkene that has the greater
number of alkyl groups attached to the doubly bonded carbon atoms.
Ex:

Elimination versus substitution

An alkyl halide with β-hydrogen atoms when reacted with a base or a nucleophile has
two competing routes: substitution (SN 1 and SN 2) and elimination. Which route will
be taken up depends upon the nature of alkyl halide, strength and size of
base/nucleophile and reaction conditions.
a bulkier nucleophile will prefer to act as a base and abstracts a proton rather than
approaching a tetravalent carbon atom due to steric reasons and forms an alkene as the
product.
For example:
When 2- bromo propane is treated with a bulkier nucleophile like potassium tertiary
butoxide, elimination occurs to form alkene. However, when it is treated with a small
nucleophile like hydroxide ion substitution occurs to form an alcohol.

Thus, a secondary alkyl halide may undergo SN 2 mechanism or elimination reaction

depends on the size of the nucleophile.
2) If the base is weak, substitution occurs predominantly. However, if the base is
stronger elimination predominates over substitution.
For example: With a weak base like sodium acetate, isopropyl bromide exclusively
gives substitution product. while with a strong base like sodium ethoxide, elimination
predominates and an alkene will be formed.

3) Primary alkyl halides can be made to undergo elimination reaction by using

stronger base than NaOH or KOH with less polar solvents such as alcohol rather than
water at high temperature.

For example:
When ethyl bromide is treated with ethanolic KOH (C2H5O- ion is a stronger base
than OH- ion and ethanol is less polar than water.) at about 473 to 573K undergoes
elimination to form alkene.

Reaction with metals:

Most organic chlorides, bromides and iodides will react with certain metals to form
compounds containing carbon-metal bonds. Such compounds are called
organometallic compounds.

One important class of organometallic compounds are the Grignard reagents. Grignard
reagents are alkyl magnesium halides with the general formula RMgX, where R is an
alkyl (or) aryl group and X is a chloride, bromide (or) iodide.
A Grignard reagent can be synthesised by combining a haloalkane with magnesium
metal in dry ether.
Ex: Bromoethane reacts with magnesium in dry ether to form methyl magnesium
bromide (Grignard reagent).

   CH3Br           +             Mg                      →                 CH3MgBr

Bromomethane               Magnesium                                Methyl magnesium bromide
In a Grignard reagent, the carbon-magnesium bond is covalent, but very polar, with
the carbon being more electronegative than magnesium. The magnesium-halogen
bond is essentially ionic in nature.

Grignard reagents are highly reactive compounds. They react with any source of
proton, even with weak proton donors such as water, alcohol to give corresponding
alkanes
Grignard reagents react with water, alcohol and amines to form alkanes.

E.g. Ethyl magnesium bromide liberates ethane gas when treated with water.

Wurtz reaction
 In this reaction alkyl halides are reacted with sodium metal in presence of dry
ether to give double the number of carbon atoms present in the halide.

2R–X + 2Na → R–R + 2Na+X− Where X = halogen

Wurtz reaction is limited to synthesis of symmetrical alkanes with even number

of carbon atoms only using only one type of alkyl halide. If dissimilar alky
halides are used, a mixture of alkanes is formed. E.g., The Wurtz reaction
between R-X and R'-X yields not only R-R' but also R-R and R'-R'. It is usually
difficult to separate the mixture and hence wurtz reaction not a suitable method
to synthesize unsymmetrical alkanes.
REACTIONS OF HALO ARENES
i) NUCLEOPHILIC SUBSTITUTION REACTIONS
Aryl halides are less reactive towards nucleophilic substitution reaction due to
several reasons.
Resonance concept: —

In case of aryl halides, the halogen is attached to SP2 hybridized carbon of the benzene
ring. Due to resonance, there is partial double bond characters in C-X bond. And C-X
bond length in haloarene is smaller than C-X bond length in Alkyl halide. It is
difficult to break a shorter bond.

Hybridisation concept: ——

In haloarenes,the carbon atom to which halogen is attached is sp2 hybridised .while

that in haloalkanes it is sp3 hybridised. Due to the greater percentage of s orbital
character in hybridization, sp2 hybridised carbon in haloarenes, tend to hold electron
pair of C-X bond. Thus, it is difficult to displace halogen in haloarenes by any
nucleophile.

Instability of phenyl cation: —

In haloarenes,the phenyl cation formed by self ionisation is not stabilised by

resonance. Because sp2 hybridised orbital of carbon bearing positive charge is
perpendicular to the p-orbitals of the phenyl ring. Hence substitution by formation of
cation (intermediate) is not possible.

Electronic repulsion——

Arenes being electron rich molecules, due to presence of pi bonds, repel electron rich
nucleophilic. Thus, attack of nucleophiles on aryl halides becomes less likely.

Examples of nucleophilic substitution reactions of aryl halides

The presence of electron withdrawing groups such as NO2, CN etc., at ortho and
para positions increase the reactivity of haloarenes.
 ii) Electrophilic substitution reactions
Halogen atom is slightly deactivating and o,p-directing; therefore, electrophilic
substitution in haloarenes occurs at ortho and para positions with respect to the
halogen atom.

In haloarenes, electron density is more at ortho and para positions due to resonance
which is evident from the resonance structures of haloarenes given below:

therefore, further substitution occurs at ortho- and para -positions

Halogenation

In halogenation of haloarenes, ortho and para substitution occurs when dihalogen

reacts with haloarenes in the presence of lewis acids such as Fe, FeCl 3, AlCl3. The
following reaction is an example of chlorination:

Nitration

Nitration of haloarenes requires a mixture of conc. HNO3 and conc. H2SO4. The
mixture is used to generate nitronium ion (NO2+), which acts as electrophile
Sulphonation

Friedel-Crafts reactions

Friedel-Crafts reactions are the reactions of the benzene derivatives with alkyl halides
or acyl halides in the presence of Lewis acid catalysts (anhydrous AlCl3 for example).

Friedel-Crafts alkylation

Haloarenes and suitable alkyl halides in the presence of a Lewis acid catalyst form
alkyl derivatives of benzene.

 Friedel-Crafts acylation

In Friedel-Crafts acylation, benzene derivatives are treated with acid

chlorides(R−COCl) to produce aromatic ketones.
iii) Fittig reaction: Aryl halides give analogous compounds when treated with
sodium in dry ether. This reaction is called Fittig reaction

iv)  Wurtz – fittig reaction: A mixture of aryl halides and alkyl halides when treated
with sodium metal in the presence of dry ether to give alkyl arene. This reaction is
called Wurtz – fittig reaction.

Poly halogen Compounds

Dichloromethane (Methylene chloride)

o It is used as solvent, paint remover, propellant in aerosols, process solvent in the

manufacture of drugs.
o It is used as metal cleaning and finishing solvent.
o But human beings can be adversely affected when exposed to Methylene
o It causes harm to human mental health.
o Human exposure to even lower levels of methylene chloride in air can lead to
dizziness, nausea, tingling, numbness in fingers and toes, etc.
o Direct exposure to Methylene chloride can cause intense burning and mild redness
in the skin.
o Cornea of eyes can be adversely burnt on direct exposure to Methylene
 

Trichloromethane (Chloroform)

o Chloroform is a sweet smelling, heavy and colorless liquid. It has low B.P. of 61o
o It is insoluble in water but soluble in organic solvents.
o If it is inhaled it causes unconsciousness.
o It is used as anesthetic because when pure chloroform is inhaled it affects the heart
due to which after mixing with ether and other suitable anesthetics chloroform can
be used as anesthetic.
o Chloroform on oxidation in air leads to the formation of phosgene which is a
poisonous gas due to which it should be stored in a dark colored bottle.

o Before using chloroform as an anesthetic it is tested with AgNO3. Poisonous

chloroform gives white precipitate with AgNO3.
 

Triiodomethane (Iodoform)
o They are used as an antiseptic due to the liberation of free iodine. It is not because
of Iodoform itself.
o But due to the offensive smell it was replaced by some other solutions that contain
iodine.
 

Tetra chloromethane (Carbon tetrachloride)

o They are used in manufacturing refrigerants and propellants for aerosol cans.
o They are also used for the synthesis of chlorofluorocarbons, pharmaceutical etc.
o It was extensively used as cleaning agent in industry and as a degreasing agent at
home as well.
o It is also used as a spot remover and fire extinguisher.
o Exposure to CCl4 can adversely affect the heart beat and make it beat irregularly
or make it permanently stop.
o Exposure to eyes can cause irritation.
o Exposure to atmosphere can lead ozone depletion that may lead to rise in the level
of exposure to ultraviolet rays. This in turn leads to increased risk of skin cancer,
eye diseases and other disorders as well as weakened immune system.
 
Freons

o The chlorofluorocarbon compounds of methane and ethane are jointly called

freons.
o They are very stable, non-corrosive, non-toxic, and unreactive liquefiable gases.
o Freon 12 (CCl2F2) is most commonly used Freons in industrial sector.
o Freons are manufactured from tetra chloromethane using Swarts reaction.
o Freons are extensively used in aerosol propellants, refrigerants and air
conditioners.
 

p,p’-Dichlorodiphenyltrichloroethane(DDT)

o DDT stands to be the first chlorinated organic insecticides originally discovered in

1873 which was then further studied and it was 1939 when Paul Muller discovered
the effectiveness of DDT as an insecticide.
o It is highly poisonous to all living organisms as it does not get metabolized rapidly
by animals and gets deposited and stored in the fatty tissues.