Haloalkanes & Haloarenes

VINYLIC
HALIDE
X
ARYL
HALIDE
BENZYLIC
HALOALKANE AND HALOARENE Soluble In
X HALIDE organic
CH2 – X
solvent
HALOALKANE, ALLYLIC PROPERTIES
REACTIVITY ORDER
X R-X HALIDE OF
HALOALKANES R-I > R-Br >
X
PHYSICAL R-Cl > R-F
Compounds containing
Sp2 C-X bond Compounds containing
PROPERTIES
Sp3 C-X bond
ORDER OF M.P.
AND B.P. IS POLARITY ORDER
CLASSIFICATION PROPERTIES R-I > RBr > RF > RCl >
NATURE OF OF RCl > RF RBr > R-I
MONOHALALKANE C-X BOND HALOARENES
EG. CH3X B.P. α size of
aryl group HALOARENES
DIHALOALKANE ON THE BASIS δ+ δ–
M.P. OF NUCLEOPHILIC
OF NUMBER OF C–X
EG. CH2X2 SUBSTITUTION
HALOGEN ATOMS Where X = Cl, Br, F, I ORDER OF B.P. p-ISOMER >
O-ISOMER AND REACTION
TRIHALOALKANE size of Halogen α C-X Bond m-ISOMER
Length Ar-I > Ar-Br >
EG. CHX3 Ar-Cl > Ar-F
Stablization of
molecule by
CHEMICAL Instability of delacalization
METHODS OF PREPARATIONS PROPERTIES phenyl cation of electron
Less reactive
than alkyl
PREPARATION OF Halide ELECTROPHILIC
HALOARENES SUBSTITUTION
PREPARATIONS OF REACTION
HALOALKANE HALOGEN EXCHANGE From amines
ELIMINATION METHOD
By sandmeyer's reaction Haloarene are
Finkelstein reaction NH2 β- Elimination ortho and para-directing
FORM ALCOHOLS e.g. Halogenation
ZnCl2 R – X + Nal → R – I + NaX + NaNO2 + HX H
R – OH + HCl R – Cl + H2O HALOALKANES β α
Sulphonation

Base 
SWARTS REACTION
From Hydrocarbon  C  C  →    + BH + X–


R – OH + PCl5 R – Cl + POCl3 + HCl

H3C–Br + AgF → AgBr + H3C–F
By Electrophilic Substitution 273-278 K X WURTZ FITTIG
R – OH + SOCl2 R – Cl + SO2+ HCl CH3
REACTION
+ X2 NUCLEOPHILIC SUBSTITUTION
N2+ X −
REACTION
SAYTZEFF RULE X R
FROM HYDROCARBON FROM ALKENE Preferred product is the alkene

+ Na + R–X
Ether
Fe which has greater no. of Alkyl
dark group attached to it.
Benzene Diazonium Salt
CH3
Addition of Addition of SN1 REACTION
Hydrogen Halide Halogens SN2 REACTION
X Cu2X2
FITTIG
– –
H H Nu + —C—X — C — Nu + X
From Alkane By free Radical (o-halotoluene) • Rate = k [Rx][Nu-] REACTION


C=C + HX


Halogenation C=C + Br2 + • Inversion of Configuration takes place

H H • Reactivity of Halides: 1°>2°>3°
←

X


CH3
Cl2 X • Rate = k [RX]
←
CH4 CH3Cl + CH2Cl2 + CHCl3 + Cl4

 

CCl4 • Follows First order Kinetics. 2 + 2Na

C=C
ether
H
hν + N2 Nu +
H
Cl
–δ
Nu
–δ
Cl Nu — C
H
H + Cl–
• Reactivity order of Alkyl Halide:



H H
Br – CH2 – CH2 –Br

H H
+ 2Nax
3° (halide) > 2° (halide) > 1° (halide) H
H X X
(p-halotoluene)
Where X = Cl, Br
HALOALKANES AND HALOARENES (FULLY SOLVED) FOR CBSE (IIT-JEE) EXAMS (2021 - 2022)
HALOALKANES AND
HALOARENES
Introduction:
Compounds derived from hydrocarbons by the replacement of one or more hydrogen atoms
by the corresponding number of halogen atoms are termed as halogen derivatives.
They are the compounds which have the general formula ‘RX’ , where
‘R’ is an alkyl or substituted alkyl group & ‘X’ is the halogen (F, Cl, Br, I). Likewise, haloarenes
or aryl halides are the compounds containing halogen attached directly to an aromatic ring.
They have the general formula ‘ArX’ (where ‘Ar’ is phenyl or substituted phenyl).
R–X Ar – X
An alkyl halide An aryl halide
(Haloalkane) (Haloarene)
R = Alkyl or substituted alkyl Ar = Phenyl or substituted phenyl
X = F, Cl, Br, I X = F, Cl, Br, I
Halogen containing organic compounds occur in nature & some of these are medicinally useful.
For exp, the chlorine containing antibiotic chloromycetin or chloramphenicol, produced by soil
microorganism, is very effective for the treatment of typhoid fever.
Our body produces an iodine containing hormone called thyroxine, the deficiency of which
causes the disease goiter.
Some synthetic halogen containing compounds are very useful in health–care and medicine.
For example, chloroquine is used for the treatment of malaria fever.
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Classification:
Halogen derivation of hydrocarbons broadly classified in to two types:
A. Aliphatic halogen compounds:
These are obtained by replacement of one or more hydrogen atoms of an aliphatic
hydrocarbon by an equal number of halogen atoms. Depending upon the nature of the
aliphatic hydrocarbon, whether alkane, alkene, alkyne aliphatic halogen compounds are of the
following three types:
a) Haloalkanes: are the halogen derivatives of alkanes. They derived from alkanes by the
replacement of one or more hydrogen atoms by the corresponding number of halogen
atoms (fluorine, chlorine, bromine, or iodine) are called as halo–alkanes.
These are named as fluro, chloro, bromo or iodo compounds according to the type of halogen
present in them.
CH3 Cl C2 H5 I C3 H7 Br
Ex:
Methyl chloride Ethyl Iodide Propyl Bromide
Depending on the number of halogen of atoms present in the halogen derivative, these are
termed as mono-, di-, tri-and tetra halogen derivatives.
• Monohalogen derivatives are formed by the replacement of one hydrogen atom by one
halogen atom
CH3 Cl CH3 CH2 Cl
Methyl chloride Ethyl chloride
Monohalogen derivatives or alkyl halides are classified as primary (1°), secondary (2°) or
tertiary (3°) depending upon whether the halogen atom is attached to primary, secondary
or tertiary carbon atoms.
i] Primary (𝟏°) alkyl halide: When the carbon atom containing halogen atom is linked to only
one carbon atom (i.e., primary carbon atom).
e.g. CH3 – CH2 – Cl, CH3 – CH2 – CH2 – CH2 – Cl, CH3 – CH – CH2 – Cl
Chloroethane Chlorobutane CH3 or
1-chloro-2-methyl propane
ii] Secondary (𝟐°) alkyl haide

When the carbon atom having halogen atom is linked to two carbon atoms (i.e., secondary
carbon atom) H
e.g CH3 – CH – Cl CH3 – C – CH2 – CH3
CH3 Cl
2-chloropropane 2-chloro-3-methylbutane
iii] Tertiary (𝟑°) alkyl halide
When the carabon atom having halogen atom is killed to three carbon atoms (i.e.,tertiary
carbon atoms). Cl Cl
e.g or CH3 – CH – CH3 CH3 – CH – CH2 – CH3
CH3 CH3
2-chloro-2-methylpropane 2-chloro-2-methylbutane

• Dihalogen derivatives: These are formed by the replacement of two hydrogen atoms by
two halogen atoms. The dihalogen derivatives are mainly three types.
a] Geminal or Gem-dihalides- Here both the halogen atoms are attached to the same
carbon atom. These are also called alkylidene halides
H Cl H H H
H–C–C–C–H H – C – C – Cl
H Cl H H Cl
(Isopropylidene chloride) (Ethylidene chloride)
(2,2-Dichloro propane) (1,1-Dichloro ethane)
b] Vicinal or vic-dihalides- Here both the halogen atoms are attached to adjacent carbon
atom. These are also called alkylene halides
Cl
Cl – CH2 – CH2 – Cl CH3 – CH – CH2 – Cl
(Ethylene chloride) (Propylene chloride)
(1,2-Dichloride ethane) (1,2-Dichloropropane)
c] 𝛂 − 𝛚 Halides (terminal dihalides)- Here both the halogen atoms are attached to
terminal carbon atoms. These are also called polymethylene halides.
Cl – CH2 – CH2 – CH2 – Cl
(1,3-Dichloropropane)
• Trihalogen derivatives- These are formed by the replacement of three hydrogen atoms
from three halogen atoms. These are also known as haloforms
Cl
Cl – C – H
Cl
1,1,1-trichloropropane(Chloroform)
• Tetrahalogen derivatives
These are formed by the replacement of four hydrogen atoms from four halogen atoms
CHCl3
Cl – C – H CCl4
CHCl3 Tetrachloromethane
1,1,2,2-tetrachloroethane
(b) Haloalkenes or Alkenyl halides are the halogen derivates of alkenes. The mono halogen
derivates of alkenes with formula is CnH2n–1X .
where X = F, Cl, Br or I and n = 2,3,4….., etc.
(c) Haloalkynes of Alkynyl halides are the halogen derivatives of alkynes. The monohalogen
derivatives of alkynes have the general formula CnH2n–3X,where X = F, Cl, Br or I and n = 2, 3,
4…., etc. For example,

B) Aromatic Halogen Compounds- These are obtained by replacement of one or more

hydrogen atoms of an aromatic hydrocarbon by an equal number of halogen atoms.
These have been further classified into the following two major categories:
(i) Nuclear halogen derivatives (aryl halides). Halogen derivatives of aromatic hydrocarbons
in which the halogen atom (F, Cl, Br or I) is directly attached to an aromatic ring are
called aryl halides. Their general formula is Ar–X where Ar (short name for aryl)
represents a phenyl, a substituted phenyl or any other aryl group such as naphthyl, etc.
Some examples of aryl halides are:
(ii) Side chain halogen derivatives (aralkyl halide). Halogen derivatives of aromatic
hydrocarbons in which the halogen atom is linked to one of the carbon atoms of the
side chain carrying the aryl group are called aralkyl halides. For example,
Cl CH2 CH 2 Br
chlorobenzene (2-bromoethyl)benzene
Classification based on hybridization of the carbon atom linked to the halogen:
(A) A compound containing C–X bond, where carbon is sp3-hybridized.

i) Alkylhalides or Haloalkanes: In alkyl halides, the halogen atom is bonded to an alkyl group.
The homologous series is represented by CH2n+1X.
They are further classified as primary (1°), secondary (2°) tertiary (3°) halides depending
upon whether the halogen atom is attached to a primary, secondary and tertiary carbon atom.
Examples: H R2 R2
R –C–X
1
R –C–X
1
R –C–X
1
H H R3
(Primary haloalkane) (Secondary haloalkane) (Tertiary haloalkane)

(ii) Allylic halides: In these halides, the halogen atom is attached to allylic carbon like
C = C – C* – X. i.e., carbon atom next to C=C.
Allylic halides may be primary secondary or tertiary.
CH2 = CH – CH2 – X X
(Allyl halide)
(3-Halo-1-propene)
(3-Halocyclohex-1-ene)
(iii) Benzylic halides: In these halides, the halogen atom is attached to a benzylic carbon
i.e.,the carbon atoms of the side chain carrying
,
the aryl group.
CH2X X R1
C–X
R2
(Benzyl halide) (1-Halo-1,2,3,4-tetra- (Dialkylbenzyle halide)
hydro naphthalene)
✓ NOTE :- Benzylic halides may be primary, secondary or tertiary.
(B) Compounds containing C–X bond, where carbon is sp2 hybridized

(i) Arylhalides: In these halides, the halogen atom is directly attached to the carbon
atom of an aromatic ring. These halides are also called haloarenes.
Cl I Br
CH 3
Chlorobenzene Iodobenzene 2-bromotoluene
(ii) Vinylic halides:In these halides, the halogen atom is attached to vinylic carbon
i.e., one of the carbon atoms of C= C.
CH2 = CH – X X
(Vinyl halide)
(1-Halocyclohex-1-ene)
Nomenclature of Halogen Compounds

Alkyl halides: In the common or trivial system, the monohalogen derivatives of alkanes are
called alkyl halides.
• Their individual names are derived by naming the alkyl group followed by the name of the
halogen as halide, i.e., fluoride, chloride, bromide or iodide. The complete name of any alkyl
halide is always written as two separate words.
• In the IUPAC system, the monohalogen derivatives of alkanes are named as haloalkanes.

COMMON NAME STRUCTURE IUPAC NAME

Methyl chloride CH3 – Cl Chloromethane
n-propyl fluoride CH3 – CH2 – CH2 – F 1-Fluoropropane
Iso-propyl chloride CH3 – CH – Cl 2-chloropropane
CH3
n-Butyl chloride CH3 – CH2 – CH2 – CH2 – Cl 1-chlorobutne
Sec-butyl chloride CH3 – CH2 – CH – CH3 2-chlorobutane
Cl
Iso-butyl chloride CH3 – CH – CH2 – Cl 1-chloro-2-methyl propane
CH3
Tert-butyl chloride CH3
CH3 – C – CH3 2-Bromo-2-methyl propane
Br
Neo-pentyl bromide CH3
CH3 – C – CH2 – Br 1-Bromo-2,2-dimethyl propane
CH3
Iso-pentyl chloride CH3 – CH – CH2 – CH2 – Cl 1-Chloro-3-methyl butane
Vinyl chloride CH2 = CH – Cl Chloro ethane
Allyl bromide CH2 = CH – CH2 – Br 3-Bromoprop-1-ene
Methylene chloride CH2 – Cl2 Dichloromethane
Chloroform CHCl3 Trichloromethane
Carbon tertrachloride CCl4 Tetrachloromethane
O-chloro toluene CH3
Cl 2-chlorotoluene or,
1-chloro-2-methyl benzene
Benzyl chloride CH2 – Cl Chlorophenyl methane
Alkyl halides: CH3CH2CH2Br Benzylic halides:

(Propyl bromide) CH2Br
Cycloalkyl halides:
Memories
Br
Allylic halides: CH2 = CH – CH2 – Cl

Cyclohexyl bromide Aryl halides: Cl
Vinylic halides: CH2 = CH – Cl

METHODS OF PREPARATION OF HALOALKANES

(1) From Alkanes: [Halogenation of Alkanes]
Alkanes react with halogen (Cl2, Br2) in presence of sunlight at ordinary temperature or by
heating at 400–500° or heating in presence of free radical initiators such as peroxides, TEL,
etc give RX.
𝑿𝟐
R-H → R-X + HX
𝒉𝝂
The reactivity of halogens decreases in the order: F2> Cl2> Br2> I2
This is a chain process and direct halogenation proceeds through free radical mechanism.
This is not a suitable laboratory methods since polyhalogen derivatives are formed along
with monohalogen derivatives.
For example, chlorination of methane gives four products,
𝐂𝐥𝟐 −𝐇𝐂𝐥 𝐂𝐥𝟐 −𝐇𝐂𝐥 𝐂𝐥𝟐 −𝐇𝐂𝐥 𝐂𝐥𝟐 −𝐇𝐂𝐥
i.e., 𝐂𝐇𝟒 → 𝐂𝐇𝟑 𝐂𝐥 → 𝐂𝐇𝟐 𝐂𝐥𝟐 → 𝐂𝐇𝐂𝐥𝟑 → 𝐂𝐂𝐥𝟒
Mechanism: Reaction proceeds via free radical formation.It takes place in the following steps:
• Initiation Step: The reaction is initiated by the breaking of chlorine molecule into
chlorine-free radicals in presence of UV light.
𝐇𝐨𝐦𝐨𝐥𝐲𝐭𝐢𝐜 𝐟𝐢𝐬𝐬𝐢𝐨𝐧
𝐂𝐥𝟐 → 𝐂𝐥• + 𝐂𝐥• (Chain initiation) … (i)
𝐡𝐯 𝐨𝐫 𝐡𝐞𝐚𝐭
• Propagation Step:The chlorine-free radicals attach methane molecule (𝐶𝑙 • is a substituent)
̇ 𝟑 + 𝐇𝐂𝐥 (Chain propagation) … (ii)
𝐂𝐇𝟒 + 𝐂𝐥• → •𝐂𝐇
Each methyl-free radicals, in turn, reacts with chlorine molecule to form methyl chloride and
at the same time chlorine-free radical is produced.
𝐂̇𝐇𝟑 + 𝐂𝐥𝟐 → 𝐂𝐇𝟑 𝐂𝐥 + 𝐂𝐥• (Chain propagation).
The chlorine-free radical can react with fresh methane molecule as in next step or methyl
chloride.This process may extend further till all the replaceable hydrogen atoms in the
methane have been substituted by chlorine atoms.
• Chain termination on step: The chain reaction is terminated when any two free radicals.
𝐂𝐥̇ + 𝐂𝐥̇ → 𝐂𝐥𝟐
𝐂̇𝐇𝟑 + 𝐂𝐥̇ → 𝐂𝐇𝟑 𝐂𝐥
𝐂̇𝐇𝟑 + 𝐂𝐇̇ 𝟑 → 𝐂𝐇𝟑 − 𝐂𝐇𝟑
Direct iodination is not possible as the reaction is slow, reversible, endothermic. Hence it
is done by heating alkane with I2 in presence of oxidising agents like conc. HIO3, HNO3, HgO.
𝐂𝐇𝟒 + 𝐈𝟐 ⇌ 𝐂𝐇𝟑 𝐈 + 𝐇𝐈
𝟓𝐇𝐈 + 𝐇𝐈𝐎𝟑 → 𝟑𝐈𝟐 + 𝟑𝐇𝟐 𝐎
𝟐𝐇𝐈 + 𝟐𝐇𝐍𝐎𝟑 → 𝐈𝟐 + 𝟐𝐇𝟐 𝐎 + 𝟐𝐍𝐎𝟐
𝟐𝐇𝐈 + 𝐇𝐠𝐎 → 𝐇𝐠𝐈𝟐 + 𝐇𝟐 𝐎
Fluorination of alkanes is highly exothermic or explosive hence it cannot be controlled under
ordinary conditions.
• In case of higher alkanes, even monohalogenation gives a mixture of all the possible
isomeric haloalkanes indicating the replacement of all the types of possible H atoms
present in a given alkane.

Monohalogenation does not depend upon the type of carbon atoms present in given alkanes,
but on type of replaceable H atoms in the given structures.
The relative amounts of the isomeric haloalkanes depends upon the nature of halogen and the
number type of hydrogens (1°, 2°, 3°) being substituted. The ease of substitution of various
H atoms generally follows the order 3° > 2° > 1°, but their relative rate of abstraction varies
with the nature of halogens.
The abstraction of H is in the order: Benzylic  allylic > 3° > 2° > 1° > CH4> Vinylic = aryl
Chlorination of n–alkanes (C4 and above) gives a mixture of d– and l–optical isomers i.e., racemic
mixture (optically inactive)
Allylic substitution: When alkenes are heated with Cl2 or Br2 at high T, H atom of allylic
carbon is substituted with the halogen atom without breaking double bond and forms allyl
halide.
𝟓𝟎𝟎°𝐂 𝐂𝐥 − 𝐂𝐇𝟐 − 𝐂𝐇 = 𝐂𝐇𝟐
𝐂𝐇𝟑 − 𝐂𝐇 = 𝐂𝐇𝟐 + 𝐂𝐥𝟐 → + 𝐇𝐂𝐥
𝐚𝐥𝐥𝐲𝐥 𝐜𝐡𝐥𝐨𝐫𝐢𝐝𝐞
Allylic substitution can also be carried out by heating the alkene with NBS or SO2Cl2 at 200°C
in presence of light and traces of organic peroxides.
(2) From Alkenes:

Halogen acids add over alkene readily to give RX. The addition of HX to alkene is electrophilic
addition forming carbocation as reactive intermediate.
𝐇𝐈
𝐑 − 𝐂𝐇 = 𝐂𝐇𝟐 → 𝐑 − 𝐂𝐇𝟐 − 𝐂𝐇𝟐 𝐈
The unsymmetrical alkens follow Markownikoff’s rule during addition forming 2° or 3° RXs
predominantly through the formation of most stable carbocation.
‘In the addition reactions of unsymmetrical alkenes the −ve part attaches to the carbon
atom having lesser number of H-atoms’.

𝐇𝐈
𝐂𝐇𝟐 = 𝐂𝐇𝟐 → 𝐂𝐇𝟑 − 𝐂𝐇𝟐 𝐈
 −
CH3−CH=CH2 ⎯⎯
⎯→ CH3−CH−CH3 ⎯⎯
HX
⎯→
X
CH3−CH−CH3
(2° carbocation)
X
[Resonance stabilized carbocation is major more stable than hyperconjugation carbocation]
In presence of peroxides such as benzoyl peroxide (C6H5CO–O–O–COC6H5), the addition

of HBr (but not of HCl or HI) to unsymmetrical alkenes takes place opposite to
Markovnikov’s rule. This is known as Peroxide effect or Kharash effect. Thus,
• •
CH3− CH = CH2 ⎯⎯⎯→ CH3− CH−CH2− Br ⎯⎯
HBr
⎯→ CH3− CH2− CH2Br +
H Br
Peroxide
(2° radical)
Br
Mechanism: The addition of HBr to alkenes in presence of peroxides occurs by a free radical
mechanism. It consists of the following three steps:

a. Initiation
b. Propagation: During the first step, a Br adds to the double bond in such a way to give the
more stable free radical. In the second step, the free radical thus produced abstracts a
H from HBr to complete the addition.
c. Termination: It involves the following three steps:
Exceptional behaviour of HBr: Peroxide effect is effective only in the case of HBr,
because HF and HCl are held by strong electrostatic force hence they cannot be broken
into free radicals.
H–I also give corresponding halogen free radicals. I° free radical being larger in size is less
reactive towards C= C bond but readily combines with another I° to give I2 molecule.
(3) From Alcohols:

Alkyl halides are prepared from alcohols by replacing –OH group of alcohols by a halogen atom.
R–OH + H–X ⎯→ R–X + H2O
The replacement of –OH group can be done with HX, phosphorus halides and SOCl2.
a. By action of halogen acids (Groves process)
This reaction is carried out in presence of a dehydrating agent as ZnCl2 or conc. H2SO4.
The 1:1 ratio of of HCl and anhydrous ZnCl2 is known as Lucas’s reagent.

𝐙𝐧𝐂𝐥𝟐
𝐑 − 𝐎𝐇 + 𝐇𝐂𝐥(𝐠)𝐨𝐫𝐇𝐂𝐥(𝐜𝐨𝐧𝐜. ) → 𝐑𝐂𝐥 + 𝐇𝟐 𝐎
𝐙𝐧𝐂𝐥𝟐
𝐂𝐇𝟑 𝐂𝐇𝟐 𝐎𝐇 + 𝐇𝐂𝐥 → 𝐂𝐇𝟑 𝐂𝐇𝟐 𝐂𝐥 + 𝐇𝟐 𝐎
𝐞𝐭𝐡𝐲𝐥𝐜𝐡𝐥𝐨𝐫𝐢𝐝𝐞
The reactivity of halogen acids is in the order: HI > HBr > HCl.
The order of reactivity of alcohols for halogen acid is as: 3° > 2° > 1°.
Reason: This reaction is an example of a nucleophilic substitution reaction in which the
nucleophile i.e., halide ion attacks the protonated alcohol molecule with the expulsion of water–
a good leaving group.

H+ SN1 +X−
R −OH R−O−H R R−X + (Some rearranged
−H2O
product, if possible)
H
X− SN2
R − X + H2O
∵ the reaction of alcohol with HX proceeds via the formation of a carbocation, hence 𝐶𝑙 −
attacks the most stable carbocation.
Ex: and not

Because of the strong tendency of neopentyl cation to rearrange to the more stable 3°
carbocation, neopentyl chloride cannot be prepared by the action of HCl on neopentyl alcohol.
Instead 2–chloro–2–methylbutane is formed as shown below :
(b) By action of Phosphorus halides
Alkyl chlorides are formed by the action of PCl3 or PCl5 on alcohols.
R−OH + PCl5 ⎯⎯→ R−Cl + POCl3 + HCl
3R−OH + PCl3 ⎯
⎯→ 3R−Cl + H3PO3
In case of preparation of alkyl bromides and iodides, PBr3 or PI3 required for the reaction is
generally produced in sites by the action of red P ON Br2 or I2.
3R−OH + PBr3 ⎯
⎯→ 3R−Br + H3PO3
3R−OH + PI3 ⎯⎯→ 3R−I + H3PO3
This method gives a good yield of 𝟏° RX but poor yield of 2° and 3° alcohols on heating forms
alkenes. This method is used in preparation of lower RBr and RI in the lab.
(c) By action of SOCl2 [Darzen’s method]
Alkyl chlorides are prepared by refluxing alcohols with SOCl2 in presence of pyridine.
⎯→ R−Cl + SO2 + HCl
R−OH + SOCl2 ⎯⎯⎯
Pyridine
The best method of preparation of haloalkanes from alcohols is using SOCl 2. In this method,
both the by-products (SO2 & HCl) are gaseous. They escape, leaving behind the product. Hence,
the product, does not need purification.
Corresponding bromides and iodides cannot be obtained by this method because thionyl bromide
is unstable while thionyl iodide does not exist.

4. By Hunsdieker reaction or Borodine – Hunsdiekar reaction

Alkyl chlorides or alkyl bromides are prepared by the action of Cl2 or Br2 in CCl4 on silver salt
of fatty acids.
RCOOAg + X2 ⎯⎯
⎯4 → R−X + AgX + CO2
CCl
(X2 = Cl2 or Br2)
This reaction gives the product with one carbon atom less than the fatty acid.
The yield of RX by this method is in the order: 1° > 2° > 3°.
5. By halide exchange method: This is a convenient method for the preparation of RI.
• The corresponding RBr or RCl are heated with a sol. of NaI in acetone or methanol. This is
called Finkelstein reaction.
𝑵𝒂𝑰
R−Cl → R−I + NaCl
𝑨𝒄𝒆𝒕𝒐𝒏𝒆
𝑵𝒂𝑰
R−Br → R−I + NaBr
𝑨𝒄𝒆𝒕𝒐𝒏𝒆
• Alkyl fluorides may also be obtained by treating an RCl or RBr with Hg2F2 or SbF3 or AgF.
This is called Swart’s reaction.
𝟐𝐂𝐇𝟑 𝐂𝐥 + 𝐇𝐠𝐅𝟐 → 𝟐𝐂𝐇𝟑 𝐅 + 𝐇𝐠𝐂𝐥𝟐
𝐂𝐇𝟑 𝐁𝐫 + 𝐀𝐠𝐅 → 𝐂𝐇𝟑 𝐅 + 𝐀𝐠𝐁𝐫
𝟑𝐂𝐇𝟑 𝐂𝐂𝐥𝟐 𝐂𝐇𝟑 + 𝟐𝐒𝐛𝐅𝟑 → 𝟑𝐂𝐇𝟑 𝐂𝐅𝟐 𝐂𝐇𝟑 + 𝟐𝐒𝐛𝐂𝐥𝟑
Physical Properties:
1. CH3F, CH3Cl, CH3Br and C2H5Cl are gases at room temperature, while CH3I is a liquid at room
temperature. Haloalkanes upto C18 are colourless liquids while higher members are
colourless solids.
2. RXs being polar in nature are water insoluble.
3. They burn on Cu wire with green edged flame (Beilstein test for halogens)
4. RBrs and RIs are heavier than water, RCls and RFs are lighter than water.
5. RIs become brown or violet in colour on exposure to light.
𝐥𝐢𝐠𝐡𝐭
𝟐𝐑𝐈 → 𝐑 − 𝐑 + 𝐈𝟐
The I2 thus liberated dissolves in RX to impart dark colour.
6. The B.Ps of RXs are in the order: RI > RBr > RCl > RF.
Greater the molecular mass, stronger the vanderwaal’s forces of attraction and hence higher
is the M.P and B.P. For a given halogen, the B.Ps of RXs increases with the increase of the size
of ‘R’ group.
In isomeric RXs, as branching increases, surface area is decreased and hence B.P is decreased.
This it is in the order: 1° > 2° > 3°.
7. Decreasing order of density among RXs is in the order: RI > RBr > RCl > RF.
High density is observed for CH3I. Thus, for RI, the decreasing order of density is as follows:
CH3I > CH3CH2I > CH3CH2CH2I >……

8. Stability of C–X bond decreases as the strength of C–X bond decreases. Bond strength
of C–X bond decreases as the size of halogen atom increases.
∴ bond strength of C–X bond is in the order: CH3–F > CH3–Cl > CH3–Br > CH3–I
9. Dipole moment of RXs decreases as E.N. of halogen atom decreases from Cl to Br to I.
fluorides have lower ′𝜇′than chlorides due to very small size of F which out weights the
effect of greater E.N. The actual order is: CH3Cl > CH3F > CH3Br > CH3I
Chemical Properties:
Nature of C–X Bond
The high reactivity of RXs can be explained in terms of the nature of C–X bond which is highly
polarized covalent bond due to large difference in E.N of C and halogen atoms.
- or −𝐶 𝛿+ − 𝑋 𝛿−
This polarity gives rise to 2 types of reactions namely nucleophilic substitution reactions and
elimination reactions.
Such reactions in which a stronger nucleophile displaces a weaker nucleophile are called
nucleophilic substitution reactions and the atom or group (halide ion in the present case) which
departs with its bonding pair of electrons is called the leaving group. Better the leaving group,
more facile is the nucleophilic substitution reaction.
Mechanism of nucleophilic substation reactions:
2 types of nucleophilic substitution reaction namely sN2 and sN1 .
1] Substitution nucleophilic bimolecular reactions 𝐬𝐍 𝟐
If the rate of the reaction depends on the concentration of alkyl halide as well as
nucleophile, then the reaction is known as 𝑠N2 reaction.
Rate ∝ [alkyl halide] [nucleophile]
sN2 Reactions are occurred in one step. During the reaction the nu attacks from the back
side (opposite side) of the halogen atom, the carbon-halogen band starts breaking and a
new carbon–nu bond starts forming. These two processes take place simultaneously in a single
step and a transition state is formed.
In the transition state, the carbon atom is simultaneously bonded to the incoming
nucleophile and the outgoing leaving group. This is unstable state, it ultimately decomposes
to form the product (CH3OH) and the leaving group (Cl– ion).

H H
HO C I H
-
H HO C I HO C I
H H
H H H
iodomethane transition state methanol
Since the nucleophile attacks from the back side, hence optical property is inversion of
configuration so 𝐬𝐍 𝟐 reaction also called as Walden inversion.
Note-Since nucleophile attacks from the back side hence most favorable substrate will be
primary alkyl halide because less no. of alkyl groups will hinder the approach of nucleophile to
the carbon atom C-X bond among the 𝟏° alkyl halide, the most favorable R – X is methyl halide
Thus, the overall order of reactivity of different alkyl halides towards 𝑠N2 reaction is:
Methyl halides > 𝟏°halides > 𝟐°halides > 𝟑°halides.
ii] Substitution nucleophilicuni molecular reaction 𝐬𝐍 𝟏 :

Here rate of reaction of depends on only the concentration of alkyl halide only, then the
reaction is known as sN1 reaction
Rate ∝ [alkyl halide]
𝐬𝐍𝟏 Reaction takes place in two steps

Step1: The formation of carbocation
The alkyl halide undergoes heterolytic cleavage forming a carbocation. This is slow and rate
determining step.

CH3 CH3
H3C C Br H3C C + Br .....Slow
CH3 CH3
Carbocation
Step 2: The nucleophile can attack the planar carbocation from either side to give the
substituted product. This step is fast and hence does not affect the rate of the reaction
Thus, reactivity of different alkyl halides towards 𝑠𝑛1 reaction is:

𝟑°Alkyl halide>𝟐° Alkyl halide>𝟑° Alkyl halide > Methyl halide.
Stereochemistry of nucleophilic substitution reactions: As the reaction involves the
formation of a planar carbocation, it allows the attack of nucleophile from both sides and
leads to the formation of enantiomers. So optical property observed is racemization

COMPARISON OF SN1 AND SN2

SN1 SN2
(a) Number of steps 2 steps : 1 step :
(i) R : L ⎯⎯⎯→ R+ + :L−
slow R : L + : Nu− → R:Nu + :L−
Or
(ii) R+ + :NuH ⎯⎯
⎯→ R:Nu + H+
fast
R : L + :NuH → R : N+uH +:L−
(b) Reaction rate & order Rate = k1[RL]; first order Rate = k1[RL] [:Nu−]; second order
(c) Molecularity Unimolecular Bimolecular
−
(d) TS of slow step + − Nu……C…… L− (with : Nu−)
R …… L ……HNu: +
HNu……C…. L− (with : HNu)
(e) Stereochemistry Inversion and retention (Partial racemization) Inversion of configuration (backside
attack)
(f) Reacting nucleophile Nucleophilic solvent; stable R+ may react with added Added nucleophile
nucleophile
(g) Structure of R 3° > 2° > 1° > Me Me > 1° > 2° > 3°
(h) Nature of Leaving group Weakest base is best leaving group, i.e. Weakest base is best leaving group,
I− > Br− > Cl− > F− i.e. I− > Br− > Cl− > F−
(i) Nature of nucleophile For HNu: (solvent), In protic solvents,
rate  basicity of HNu: (i) Within a periodic table group, rate
 polarizability of Nu
(ii) For same nucleophilic site, rate 
basicity of Nu
In polar aprotic solvents, rate
 basicity of Nu
(j) Solvent effect Rate  H−bonding ability and dielectric constant Depends on charge type. Polar aprotic
solvents leave “freest” most reactive
Nu.
(k) Determining factor Stability of R+ Steric hindrance
(l) Rearrangement Observed Not observed, except for allylic
(m) Catalysis Lewis and Bronsted acids: No specific catalyst
Ag+, AlCl3 , ZnCl2 etc.

The nucleophilic substitution reactions of haloalkanes are discussed below.

1. Substitution by hydroxyl group :
Haloalkanes on treatment with boiling aqueous alkalies or moist 𝐀𝐠 𝟐 𝐎 undergo hydrolysis to
form alcohols.
(𝑏𝑜𝑖𝑙)
𝑅 − 𝑋 + 𝐻𝑂𝐻 → 𝑅 − 𝑂𝐻 + 𝑋 −
𝛥
𝑅 − 𝑋 + 𝐾𝑂𝐻(𝑎𝑞. ) → 𝑅 − 𝑂𝐻 + 𝐾𝐵𝑟
𝛥
𝑅 − 𝑋 + 𝐴𝑔2 𝑂(𝑚𝑜𝑖𝑠𝑡) → 𝑅 − 𝑂𝐻 + 𝐴𝑔𝐵𝑟
2. Substitution by alkoxy group:
Haloalkanes on treatment with sodium or potassium alkoxides or dry Ag 2 O from ethers. If the
attacking nucleophile is OR− , the reaction is called williamson’s ether synthesis.
Δ
R − X + NaOR′ → R − OR′ + NaX
Δ
2RX + Ag 2 O → R − O − R + 2Ag X
Δ
Ex: C2 H5 Br + NaOCH3 → C2 H5 OCH3 + NaBr
Δ
Ex: 2C2 H5 Br + Ag 2 O → C2 H5 OC2 H5 + 2AgBr
3] Substitution by hydro-sulphide group:
Haloalkanes on Heating with aqueous alcoholic sodium or potassium hydrogen Sulphide gives
thioalcohols.
C2 H5 OH / H2 O
RX + NaSH→ RSH + NaX
Δ
C2 H5 OH / H2 O
Ex: C2 H5 Br + NaSH → C2 H5 SH + NaBr
Δ
4] Substitution by cyano group :
Haloalkanes on heating with alcoholic solution of KCN (ionic natured) gives alkyl cyanides as
the major product along with a small amount of alkyl isocyanide.
alochol
RX + KCN → RCN + KX
Δ
alochol
EX- C2 H5 I + KCN→ C2 H5 CN + KI
Δ
5] Substitution by isocyanide group:

Alkyl halides on heating with aqueous ethanolic solution of AgCN(covalent natured),gives alkyl
isocyanides (carbylamines) as the major product along with a small amount of alkyl cyanide
alochol
RX + AgCN→ RNC + AgX
Δ
alochol
Ex: C2 H5 Cl + AgCN→ C2 H5 NC + AgCl
Δ

Explanation: The formation of cyanides or isocyanides from alkyl halides involves nucleophilic
substitution reaction.
• Alkali metal cyanides (KCN or NaCN) are predominantly ionic. Therefore, both carbon and
nitrogen atoms are free to donate electron pair. Since C–C bond is relatively stronger than
C–N bond, therefore, in this case attack mostly occurs through the carbon atom of the
cyanide group and alkyl cyanides are the major product.
• Silver cyanide is predominantly covalent. Consequently, only nitrogen electron pair is available
for bond formation, and the attack mostly occurs through the nitrogen atom of cyanide group
giving alkyl isocyanides as the major product.
6] Substitution by nitrite group:
Alkyl halides react with aq, ethanolic solution of 𝐾𝑁𝑂2,alkyl nitrite major product formed along
with a small amount of nitro alkane.
alochol
RX + K + O− − N = 0→ R − O − N = O + KX
Δ
alochol
Ex: C2 H5 Cl + K + O− − N = O→ C2 H5 − O − N = O + KCl
Δ
7] Substitution by nitro group:
Haloalkanes react with aqueous ethanolic solution of AgNO2 gives nitro alkane as the major
product with a small amount of alkyl nitrite
alochol
Ex: C2 H5 Br + AgNO2 → C2 H5 − NO2 + AgBr
Δ
Explanation: Nitrite ion (–O–N=O) like cyanide ion is an ambident nucleophile since it has two
sites (oxygen and nitrogen) through which it can attack an alkyl halide. Whereas attack through
nitrogen gives nitro compounds, attack through oxygen gives nitrites.
Alkali metals nitrites are ionic compounds and hence have a negative charge on one of the
oxygen atoms. Hence, it gives alkyl nitrites.
In contrast, silver nitrite is a covalent compound and hence does not have a negative charge
on the oxygen atom, therefore, lone pair of electrons on the nitrogen atom is more easily
available for bond formation or nucleophilic attack occurs through nitrogen and hence silver
nitrite predominantly gives nitro compounds

8] Substitution by carboxylate group: -

Alkyl halides on heating with ethanolic solution of silver salts of fatty acids give esters.
C2 H5 OH
R′ COOAg + RX→ R′ COOR + AgX
Δ
C2 H5 OH
Ex: CH3 COOAg + C2 H5 Br → CH3 COOC2 H5 + AgBr
Δ
9] Substitution by aminogroup:
Haloalkanes on heating with ethanolie solution of NH3 in a sealed tube at100°𝐶, gives a mixture
of 𝟏°, 𝟐°, 𝟑° amines and quaternary ammonium salts . This reaction is called Hoffmann
ammonolysis.
C2 H5 OH
Ex: C2 H5 Br + NH3→ C2 H5 NH2 + HBr
Δ
1° amine
C2 H5 OH
C2 H5 NH2 + C2 H5 Br → (C2 H5 )2 NH + HBr
Δ
2° amine
C2 H5 OH
(C2 H5 )2 NH + C2 H5 Br → (C2 H5 )3 N + HBr
Δ
3° amine
C2 H5 OH
(C2 H5 )3 N + C2 H5 Br → (C2 H5 )4 N+ Br −
Δ
4° ammonoium salt
10] Substitution of chlorine or bromine by iodine:
Alkyl chlorides or bromides on treating with NaI or KI in acetone give alkyl iodides. This
reaction is called finkelstein reaction.
acetone
C2 H5 Br + NaI → C2 H5 I + NaBr.
B] Elimination Reaction: (Dehydrohalogenation)

When alkyl halide boiled with conc.Alc KOH, they undergo dehydrohalogenation to give alkenes.
These reactions are called 𝜷- elimination reactions because the hydrogen atom of haloalkane
which is eliminated comes from a 𝛽 −carbon

Orientation of dehydrohalogenation: If the halogen is present on the terminal C atom of

chain, dehydrohalogenation can occur only in one direction to give only the terminal alkene.
For example,1-chlorobutane on dehydrohalogenation gives but-1-ene
In the dehydrohalogenation of secondary and tertiary haloalkenes, when there is a possibility

of formation of two isomers, the hydrogen atoms is preferentially eliminated from the
adjacent carbon atom with lesser number of hydrogen atoms. This generalization is known as
Saytzeff’s rule, i.e., the more substituted alkene is more stable. For example,

H 3C CH CH CH 3
CH 3 But- 2-ene
alc. KOH (Major ) (80%)
H 3C
H 3C CH 2 CH CH 2
Br
But- 1-ene
(Minor) (20%)
but-1-ene
(minor)
but-2-ene(Major)
can exist as cis and trans isomers
The ease of dehydrohalogenation is in the order :

Tertiary alkyl halide > Secondary alkyl halide > Primary alkyl halide.
C] Other reaction:
1] Reduction: Alkyl halides on reduction with ZN/HCL, No/alcohol, LiAlH4 or ZnCU
couple/alcohol gives corresponding alkanes.
Zn − Cu couple
RX + 2H→ R − H + HX
alcohol
Zn − Cu couple
Ex: C2 H5 CL + 2H→ C2 H6 + HCl
alcohol
2] Reaction with sodium (Wurtz reaction):
Alkyl halides react with metallic sodium in presence of dry ether to form symmetrical
alkanes containing double the number of carbon atoms present in the alkyl halide.
R – X + 2Na + X – R Dry ether R – R + 2NaX
(Alkane)
H5C2 – X + 2Na + X – C2H5 Dry ether CH3 CH2 CH2 CH3 + NaCl
(n-Butane)
3] Reaction with magnesium:
Alkyl halides form Grignard reagents when treated with dry magnesium powder in dry ether.
R–X + Mg Dry ether RMgX
(Alkyl halide) (Alkyl magnesium halide)
(Grignard reagent)
CH3 – CH2 – Br + Mg Dry ether CH3 – CH2 - MgBr
(Ethyl magnesium bromide)
Grignard reagents are used for making a very large

number of organic compounds.
Reactivity order is RI > RBr > RCl.

4. Reaction with other metals: Organometallic compounds are formed.

• When heated with zinc powder in ether, alkyl halides form dialkyl zinc compounds. These
are called Frankland reagents.
Ether
2C2 H5 Br + 2Zn → (C2 H5 )2 Zn + ZnBr2
Heat
• When heated with lead–sodium alloy, ethyl bromide gives tetra ethyl lead (TEL) which is
used as antiknock compound in petrol.
dryether
4CH3 CH2 Br + 4Pb(Na) → (CH3 CH2 )4 Pb + 4NaBr + 3Pb
• Alkyl halides reacts with lithim dialkyl cuprate (R2CuLi) to form unsymmetrical alkanes
(Corey –House Synthesis).
+ KOH (aq.) + KNO2

R – OH + KX R – O – N = O + KX
(Alcohol) (Alkyl nitrite)
+ Na+OR- + AgNO2
R – O – R + NaX
1
R – NO2 + AgX
(Ether) (Nitroalkane)
+ KCN (alc.) + LiAlH4
R – CN + KX R–H
(Alkyl cyanide) R–X (Alkane)
ALKYL HALIDE
+ AgCN + NH3
R – NC + AgX R – NH2 + H – X
(Alkyl isocyanide) (Alkyl amine)
+ NaSH O
R – SH + NaX + R – C – O – Ag
1
(Thioalcohol)
+ Na+C-≡ CH O
R – C ≡ CH + NaX R – C – O – R1 + AgX
(Higher Alkyne) (Ester)
Dihalogen compounds
General methods of preparation of gem-dihalides
1] From aldehydes and ketones:
Aldehydes and ketones on treating with PCl5 gives gem-dehalides.
𝐶𝐻3 𝐶𝐻𝑂 + 𝑃𝐶𝑙5 → 𝐶𝐻3 𝐶𝐻𝐶𝑙2 + 𝑃𝑂𝐶𝑙3
Ethylidene chloride
𝐶𝐻3 𝐶𝑂𝐶𝐻3 + 𝑃𝐶𝑙5 → 𝐶𝐻3 𝐶𝐶𝑙2 𝐶𝐻3 + 𝑃𝑂𝐶𝑙3

Isopropylidene chloride

2] From alkynes: Addition of halogen acids to alkynes gives gem-dihalides

𝐻𝐶𝑙
𝐶𝐻 ≡ 𝐶𝐻 + 𝐻𝐶𝑙 → 𝐶𝐻2 = 𝐶𝐻𝐶𝑙 → 𝐶𝐻3 − 𝐶𝐻𝐶𝑙2
General methods of preparation of vicinal dihalides:

1. From glycols : Glycols on reacting with halogen acids, PCl3 , PCl5 or SOCl2 gives Vic-
dihalides.
2. From alkenes: Addition of halogens to alkenes gives ric-dihalides.

𝐶𝐻2 = 𝐶𝐻2 + 𝐶𝑙2 → 𝐶𝐻2 𝐶𝑙 − 𝐶𝐻2 𝐶𝑙
Trihalogen derivatives
Chloroform(𝐶𝐻𝐶𝑙3): It was discovered by Liebig and named chloroform by Dumas. Earlier it
was extensively used as anesthesia for surgery. But now it is rarely used as it causes liver
damage.
Preparation of chloroform- By distillation of ethyl alcohol and bleaching powder:
Chloroform is prepared by the distillation of ethyl alcohol with bleaching powder and water.
The yield is about 40% the available chlorine from bleaching powder serves as oxidising agent
and chlorinating agent.
The reaction takes place in 4 steps.
a] Hydrolysis of bleaching powder, 𝐶𝑎𝑂𝐶𝑙2 + 𝐻2 𝑂 → 𝐶𝑎(𝑂𝐻)2 + 𝐶𝑙2
b] Oxidizing of ethyl alcohol to acetaldehyde by Cl2 , 𝐶𝐻3 𝐶𝐻2 𝑂𝐻 + 𝐶𝑙2 → 𝐶𝐻3 𝐶𝐻𝑂 + 2𝐻𝐶𝑙
c] Chlorination of acetaldehyde to chloral, 𝐶𝐻3 𝐶𝐻0 + 3𝐶𝑙2 → 𝐶𝐶𝑙3 𝐶𝐻𝑂 + 3𝐻𝐶𝑙
d] Hydrolysis of chloral with Ca(OH)2, 2𝐶𝐶𝑙3 𝐶𝐻𝑂 + 𝐶𝑎(𝑂𝐻)2 → (𝐻𝐶𝑂𝑂)2 𝐶𝑎 + 2𝐶𝐻𝐶𝑙3
Physical properties:
1] It is a colorless sweet-smelling liquid
2] It is a heavier than water. (d= 1.485 g/cc)
3] It is less soluble in water but more soluble in organic solvents
4] It is a non-inflammable liquid. But vapors burn with green flame
Chemical properties:
1] Oxidation: When chloroform is exposed to sunlight and air it is slowly oxidized to phosgene
a colourless poisonous gas.
1 ℎ𝜈
𝐶𝐻𝐶𝑙3 + 𝑂2 → 𝐶𝑂𝐶𝑙2 + 𝐻𝐶𝑙
2
The oxidation of chloroform is prevented by
a] Strong chloroform in dark brown-colored bottles filled up to the brim.
b] Adding 1% ethyl alcohol- Ethyl alcohol prevents the oxidation chloroform and converts
phosgene into harm less ethyl carbonate.
COCl2 + 2C2 H5 OH → (C2 H5 O)2 CO + 2HCl

Phosgene is extremely poisonous gas. To use chloroform as an anesthetic agent, it is necessary

to prevent the above reaction. The following precautions are taken when chloroform is stored.
(a) It is stored in dark blue- or brown-colored bottles which are filled up to the brim.
(b) 1% ethyl alcohol is added. This retards the oxidation and converts the phosgene formed
into harmless ethyl carbonate.
Carbylamine reaction (isocyanide test): This reaction is actually a test of primary amines.
Chloroform, when heated with primary amine in presence of alcoholic potassium hydroxide
forms a derivative called isocyanide (carbylamines) which has a very offensive smell.
This reaction is also used for the test of chloroform.

Uses:
(i) It is used as a solvent for fats, waxes, rubber, resins, iodine, etc.
(ii) It is used for the preparation of chloretone (a drug) and chloropicrin (insecticide).
(iii) It is used in laboratory for the test of primary amines, iodides and bromides.
(iv) It can be used as anaesthtic but due to harmful effects it is not used these days for this
purpose. It causes liver damage when inhaled in excess (SO is CCl4).
(v) It may be used to prevent putrefaction of organic materials, i.e., in the preservation of
anatomical species.
Test of chloroform:
(i) It gives isocyanide test (carbylamines test).
(ii) It forms silver mirror with Tollen’s reagent.
(iii) Pure chloroform does not give white precipitate with silver nitrate.
Iodoform: Iodoform resembles chloroform in the methods of preparation and properties.

Iodoform reaction: Organic compounds having methyl ketone (CH3 – C O )
Or gives methyl ketone group on oxidation, when heated with I2 in presence of alkali gives
iodoform. Basic requirement for a compound to give iodoform reaction is presence of
(or) . Group. Compounds having group. undergo oxidation with
halogens to give group

p - p - Dichloro diphenyl tri chloro ethane (DDT)

It is manufactured by condensation of chloro benzene with trichloro acetaldehyde (chloral) in
the presence of sulphuric acid.
Cl
Cl
Cl 1 2 Cl (Structure of DDT)
Cl H
Preparation of DDT:
Cl H Cl Cl Cl
Cl – C – CH O + H+ Cl – C – CH
Cl H Cl Cl Cl
DDT
Properties It is a white powder insoluble in water but soluble in oils.
Uses It is a powerful insecticide. However, it is highly stable & is not easily decomposed in the
environment. Therefore, its long-term effect could be potentially dangerous & its use
is banned in many countries
FREONS These are poly chlorofluoro derivative of alkane.
Preparation of freons :
CCl4 + HF ⎯SbCl
⎯⎯→ CCl3F + HCl
5
C2Cl6 + 2HF ⎯SbCl

⎯⎯→ C2F2Cl4 + 2HCl
5
hexachloro ethane freons–112

Properties & uses of freons :
(a) Freons are colourless, odourless, unreactive & non-combustible liquids.
(b) Having very low boiling points (e.g CF2Cl2= – 29.8ºC). They easily converted from gaseous
state to liquid state, therefore they are used as a coolant in A.C. & Refrigerator.
(c) Used as a aerosol propellant in aeroplane & rockets.
(d) Also used as a solvent.
Note: Main cause of Ozone layer decay (CFC – chlorofluoro car
Nomenclature of Freons:
• The common name of freons is F-cba, Where c = number of carbon atoms – 1
b = number of hydrogen atoms + 1
a = total number of atoms of fluorene
Example: CFCl3 : C – 1 ⇒ 1 – 1 = 0, H + 1 ⇒ 0 + 1 = 1, F = 1 ⇒ F-11
CF2Cl2 : C – 1 ⇒ 1 – 1 = 0, H + 1 ⇒ 0 + 1 = 1, F = 2 ⇒ F-12

Haloarenes
Compounds in which halogen is directly attached to an aromatic ring are known as aryl halides
or haloarenes and are represented as Ar –X, where 𝐴𝑟 is phenyl, substituted phenyl or a group
derived from other aromatic system.
General methods of preparation:
1. By direct halogenation of aromatic hydrocarbons: This method is used for the
preparation of chloro and bromo derivatives. Halogens react with aromatic hydrocarbons in
presence of catalysts or halogen carriers such as iron, iodine or anhydrous ferric or
aluminium chloride (Lewis acid) at room temperature in absence of direct sunlight.
H
+ X2 Fe or FeX3 / dark + HX (X = Cl, Br)
H
+ I2 HIO3 or HNO3 + HX (X = Cl, Br)
Cl
AlCl3
+ Cl2 + HCl
Br
Fe or
Br 2 HBr
FeBr 3
(Bromobenzene)
CH 3 CH 3
CH 3
Cl
FeCl 3
Cl 2
dark
(o-chlorotoluene)
Cl
(p-chlorotoluene)
For further halogenation, more halogen is used,
The function of the Lewis acid is to carry the halogens to the aromatic hydrocarbon.
If toluene is used instead of benzene, a mixture of o–and p–chlorotoluenes is obtained since –
CH3 group is o, p–directing.
CH 3
CH 2Cl
Cl 2 338 K HCl
Sunlight
1-chloro-1-phenylmethane
(b) Side chain halogenation-Benzylic halogenation:

The most convenient method for the preparation of side chain substituted aryl halides or
aralkyl halides is by the direct halogenation of a suitable arene. For example, when Cl2 is passed

through boiling toluene in presence of sunlight and absence of halogen carrier,

phenylchloromethane (benzyl chloride) is formed.
CH 3
CH 2Cl
Cl 2 338 K HCl
Sunlight
1-chloro-1-phenylmethane
If Cl2 is passed for a longer time, the initially formed benzyl chloride reacts further to form
first benzal dichloride and then benzo trichloride.
2. From diazonium salts

NH2 N+≡ 𝑁𝐶𝑙 −
H
NaNO2 /HCl (273-278K) + HX (X = Cl, Br)
CuCl/HCl Cl + N2
Sandmayer’s reaction
CuBr/HBr Br + N2
N+≡NC𝑙 −
(Benzene diazoniu + KI I + KCl + N2

chloride)
+ NaBH4 F + BF3 + NaCl + N2
(i) By Sandmeyer reaction:

NH 2
N2 Cl-
273-277 K
HNO 2 HCl 2H2 O
Aniline Benzenediazonium chloride
The diazonium compound is treated with CuCl and HCl or CuBr and HBr to give the
corresponding haloarene. This reaction is known as Sandmeyer reaction.
-
N2 Cl Cl
CuCl
N2
-
N2 Cl Br
CuBr
N2 CuCl

(ii) By Gattermann reaction:

Haloarenes particularly chloro-and bromoarenes can also be prepared by Gattermann reaction.
It is a modification of the Sandmeyer reaction. In this reaction, a mixture of freshly
prepared copper powder in the presence of HCl or HBr is used. The yields are often around
40%. Thus,
• From silver salt or aromatic acids–Hunsdiecker reaction: Like alkyl bromides, aryl
bromides can also be prepared by refluxing the silver salt of aromatic acids with bromine
in boiling carbon tetrachloride.
C6H 5COOAg X 2 ⎯⎯⎯⎯⎯
CCl4 / Xylene
→ C6H 5 X CO 2 AgX
(Cl 2 or Br 2)
Chemical properties of haloarenes:- Aryl halides are less reactive than that of alkyl halides
towards nucleophilic substitution reactions.
This can be explained as follows:
(i) Resonance Effect: In haloarenes (e.g., chlorobenzene), the lone pairs of electrons on the
halogen atom are delocalized on the benzene ring as shown below:
+ + +
X X X X X
CANONICAL FORM
𝛿+ X
𝛿− 𝛿−
𝛿−
As a result, C–Cl acquires some double bond character. On the other hand, in case of alkyl
halides (say methyl chloride) carbon is attached to chlorine by a pure single bond.
Consequently, C–X bond in aryl halides is little stronger than in alkyl halides, and hence cannot
be easily broken.
(ii) Bond energies due to difference in hybridization:
In alkyl halides, the carbon holding halogen is 𝑠𝑝3 −hybridised. In aryl halides, carbon is
𝑆𝑃2 −hybridized; the carbon −halogen bond is shorter and stronger, and the molecule is more
stable.

(iii) Polarity (or Nature) of the carbon halogen bond: Another reason for the low reactivity
of aryl/vinyl halides over alkyl halides is their lesser polar character.
The sp2–hybrid carbon due to greater s–character is more electronegative than a sp3–hybrid
carbon. Therefore, the sp2–hybrid carbon of C–X bond in aryl halides or vinyl halides has less
tendency to release electrons to the halogen than a sp3–hybrid carbon in alkyl halides. As a
result, the C–X bond in aryl halides or vinyl halides is less polar than in alkyl halides. This is
supported by the observation that the dipole moment of chlorobenzenes just1.69D as
compared to the dipole moment of methyl chloride i.e., 1.86 D. Consequently, the halogen atom
present in aryl halides cannot be easily displaced by nucleophiles.
1] Substitution reactions: Nucleophilic substitution reactions of chlorobenzene given below
(a) Replacemet of –Cl by –OH: When chlorobenzene is heated at 350°C under high pressure
with caustic soda, phenol is formed (Dow process).
Cl ONa OH
623 K, 300 atm H

NaOH Na
(b) Replacement by methoxy group: Ether is formed when chlorobenzene is heated with
sodium methoxide at 200°C in presence of copper salts.
Cl OR
Cu salt (catalyst)
NaOR, High pressure, 573 K
Ether
(c) Replacement by amino group: When chlorobenzene is treated with aqueous ammonia at
200°C under a pressure of 60 atmospheres in presence of cuprous oxide or cuprous chloride,
aniline is formed.
475 K
2 Cl 2NH 3 Cu 2O 2 NH 2 2 CuCl H 2O
60 atm
(d) Replacement by –CN group: When chlorobenzene is treated with cuprous cyanide in
pyridine or DMF at 200°C, phenyl cyanide is formed.
DMF
Br CuCN CN CuBr
470 K
Phenyl cyanide

2] Reaction with sodium: Alkyl halides when heated with sodium in presence of dry either
gives diaryls. This reaction is called fitting reaction.
2 Cl 2Na ether 2 NaCl
When a mixture of aryl halide and alkyl halide is heated with sodium in presence of dry ether
gives alkyl derivatives of benzene. This reaction is called Wurtz –fitting reaction.
Cl 2Na ether
CH 3Cl CH 3 2 NaCl
Toluene
3] Reaction with magnesium: Bromo–and iodoarenes from Grignard reagents when their
ethereal solution is treated with magnesium turnings. Chloroarenes from Grignard
reagents only if the reaction is carried out in dry tetrahydrofuran (THF) as solvent.
Br MgBr
ether
Mg
Phenyl magnesium bromide

THF
Cl Mg MgCl
Phenyl magnesium chloride

4] Reaction with lithium: Bromo– and iodoarenes also react with lithium metal in presence of
dry ether to form the corresponding organometallic compounds. For example,
Br 2Li dry ether Li LiBr
Phenyl lithium
5] Reduction: Chlorobenzene undergoes reduction with LiAlH4 or Ni–Al alloy in NaOH solution
or Na–Mg/H2O to give benzene.
Ni Al alloy
C6H 5Cl 2H C6H 5 HCl
NaOH
Benzene
6] Reactions of benzene ring:
–Cl group in chlorobenzene is o–, p–director and deactivating. However, because of steric
hindrance at the o–position, the p–product usually predominates over the o–product. The
benzene ring undergoes halogenation, nitration, sulphonation (Electrophilic substitution)
reactions.
+ + +
X X X X X
CANONICAL FORMATS
Due to resonance, the electron density increases more at ortho & para positions than at meta
position. Therefore, electrophilic substitution reaction take place at ortho & para position.

(i) Halogenation
Cl Cl Cl
Cl
Fe or FeCl 3
Cl 2 or anh. AlCl
3
1,2-dichlor o benzene
Cl
1, 4-dichloro benzene
(ii) Nitration
Cl Cl Cl
NO 2
HNO 3 H 2SO4 
(conc.) (conc.)
l-chloro 2 nitro benzene
NO 2
l -chloro
benzene
(iv) Sulphonation
Cl Cl Cl
SO3H
H 2SO4 
(conc.)
2-chloro benzene
sulphonic acid SO3H
4-chloro benzene
sulphonic acid
(iv) Friedel–Crafts reaction
Cl Cl Cl
CH 3
anhy. AlCl 3
CH 3Cl
2-chlorotoluene
CH 3
4-chlorotoluene
Cl Cl Cl
COCH 3
anlcy AlCl 3
CH 3COCl
o-chloro acetophenone
COCH 3
(Minor)
p-chloro acetophenone
(Major)
(v) Reaction with chloral: When chlorobenzene is heated with chloral (trichloracetaldehyde)
in the presence of conc. H2SO4, a powerful insecticide, DDT (p, p'–
dichlorodiphenyltrichloroethane) is formed.
H Cl Cl
Conc. H 2SO4
CCl 3 CHO CCl 3 CH

H Cl - H 2O Cl
[l, l, l, trichloro- 2, 2-bis

(p-chlorophenyl) ethane

BASIC STEREOCHEMICAL PRINCIPLES AND NOTATION

OPTICAL ISOMERISM Certain organic compounds, when their solutions are placed in the
path of a plane polarized lights have the remarkable property of rotating its plane through a
certain angle which may be either to the left or to the right. This property of a substance of
rotating the plane polarized light is called optical activity and the substance passing it is said
to be optically active.
CONDITIONS FOR OPTICAL ACTIVITY:

Asymmetric or Chiral carbon atom: A carbon atom is described as being asymmetric when four
different atoms or groups are bonded to it thus an asymmetric carbon in formulas is usually indicated
by on asterik (*) placed near it.
All organic compounds containing an asymmetric carbon atom are optically active.
OH CH3
| |
CH3–C*–COOH CH3–CH2–C*–CH2OH
| |
H H
lactic acid amyl alcohol
(optically active) (optically active)
Asymmetric or Dissymmetric molecules: Molecules which have asymmetric or dissymmetric molecular
structure in tetrahedral perspective, are called asymmetric molecules. The two features of such
structures are -
(i) No plane of symmetry : A 'Plane' of symmetry' is a plane which divides an object in such a way
that the part of it on one side of the plane is the mirror image of that on the other side for eg. a ball
is symmetrical while a hand is asymmetric
Plane of symmetry Plane of symmetry

(absent) (present)
Similarly, an organic molecule is asymmetric if there is no plane (or centre) of symmetry for example.
COOH C COOH
C
H OH H OH
H H F
OH H Br H
H HO
Br
COOH COOH
Symmetric symmetic asymmetic asymmetric

(ii) Non superimposable on its mirror image:

An asymmetric object cannot be superimposed on its mirror image thus right hand produces a mirror
image which is identical with your left hand. The two hands are non-superimposable which is clearing
evident if your right and in the left-handed glove. On the other hand, asymmetric object like a ball can
be superimposed its mirror image which is another similar ball.
Mirror
Mirror
COOH COOH
Cl | |
| Cl
| C C
C
C H OH HO H
H F F H CH3 CH3
Br Br
Chirality: - This term has been recently used to describe such molecules as have no elements of
symmetry, thus symmetrical molecules are also called chiral molecules and optical activity is attributed
to certain chiral centres in them. An asymmetrical carbon is a chiral center.
Chirality is lost when the two atoms bonded to an asymmetric carbon become similar thus lactic acid
is optically active but propionic acid is inactive.
Chirality or molecular dissymmetry cause of optical Isomerism

• The necessary condition for a molecule to exhibit optical isomerism is dissymmetry or chirality .
Thus, all organic compounds which contain one asymmetric carbon are chiral and exist in two
tetrahedral forms.
• Although the two forms (I and II) shown in fig. (a) have the same structure, they have different
arrangements of groups a,b,d,e about the asymmetric carbon in fact, they represent asymmetric
molecules they do not have a plane of symmetry they are related to each other as an object to its
mirror image and are non-superimposable.
• The two models or structures (I and II) stand for dextro or (+) and laevo or (-) isomers. Since they
are related to each other as mirror images, they are commonly called Enantiomers (Gr, enantio =
opposite, morph - form) or enantiomers thus optical isomerism is now often reflected to as an
enantiomer.

• It is obvious that optical isomers or enantiomers due to the presence of an asymmetric carbon atom
in a compound differ only in the arrangement or configuration of groups in tetrahedral perspective
this may be illustrated by taking a few examples of compounds which exist as (+) and (-) enantiomers.
COOH Mirror COOH H Mirror H
| | | |
C C C C
H OH H CH3 C6H5
HO C6H5 CH3
CH3 CH3 Cl Cl
(+) and (–)-Lactic acid (+) and (–)-1-Chloro-1-phenyl ethane

Number of optical isomers :
Case - 1 When the molecule is unsymmetrical. (It cannot be divided into two halves)
Number of d and  isomers = 2n
Number of meso form = 0 Where n is the number of chiral carbon atoms
Total number of optical isomers = 2n
e.g.
. Pentane-2, 3-diol
CH3
|
H––C*––OH Number of d and  isomers = 22 = 4
|
H––C*––OH Number of meso isomers = 0
|
C2H5
Case - 2 When the molecule is unsymmetrical and number of chiral carbon = even number
Number of d and  isomers =2(n – 1)
Number of meso isomers =2(n/2 – 1)
n 
 –1
Total number of optical isomers = 2 (n–1)
+ 2 2 
e.g. : Tartaric acid

COOH
| Number of d and  isomers = 2(2 – 1) =21 = 2
H––C*––OH
| Number of meso isomers = 2(2/2 – 1) =2º = 1
H––C*––OH Total optical isomers = 2 + 1 = 3
|
COOH
Case- 3. When the molecule is symmetrical number of chiral carbon = odd number
( n –1)
Number of d and  isomers = 2(n – 1) – 2 2
Number of meso isomers = 2

Total number of optical isomers = 2 n–1
e.g. Pentane – 2,3,4-triole

Number of d&  isomers = 22 – 21 = 4 – 2 = 2
* * *
CH3–CH–CH–CH–CH3  3 –1 
| | |  
OH OH OH Number of meso isomers = 2 2 
= 21 = 2
Total optical isomer = 23–1 = 22 = 4

D, L- SYSTEM (RELATIVE CONFIGURATION) :

Optically inactive
(does not rotate plane polarised light)
Organic compound Laevorotatory [rotates the plane polarised

(𝑙 or –) light towards left]
Optically active + & – isomers of an
(rotate plane polarised light) organic compound
Dextrorotatory [rotates the plane polarised are optical isomers
(𝑑 or +) light towards right]
It is applicable for Fischer projection formula.

It represents relative configuration with respect to glyceraldehydes.
Following configuration of glyceraldehyde is considered as standard configuration.
CHO CHO
H * OH HO * H
CH2OH CH2OH
D- glyceraldehyde L- glyceraldehyde
(– OH is on R.H.S.) (– OH is on L.H.S.)
• All molecules which could be chemically relative to D-glyceraldehyde are assigned the D-
configuration and those related to L-glyceraldehyde are designated L-configuration as illustrated
below -
COOH COOH COOH COOH
H * OH HO * H H2N * H H 2N * H
CH3 CH2OH CH3 CH2OH
(D-form) (L-form)
(L-form) (D-form)
CH2OH
CHO
CHO COOH C=O
H * OH H * OH
H * OH H * NH2 HO * H H * OH
HO * H HO * H
H * OH HO * H
CH2OH CH3 H * OH
CH2OH
(L-form) (D-form)
CH2OH
(D-form) (L-form)
Special Point:
The method of separating a racemic mixture into its enantiomeric constituents is called as Resolution.
There is no direct relation between D, L– configuration with d, l or (+), (–) configuration.
ENANTIOMERS: The stereoisomers related to each other as non-superimposable mirror

images are called enantiomers.
PROPERTIES OF ENANTIOMERS :
(1) Enantiomers have chiral molecule (optically active)
(2) Enantiomers have identical physical properties like BP, MP, refractive index, density etc.
(3) They rotate PPL in opposite direction but to the equal extent.
(4) They have identical chemical properties, However their reactivity i.e. rate of reaction will be differ
if they combine with other optically active reagent.

k3
k1
R + x ⎯⎯→ R + y ⎯⎯→ P
P active, then K3  K4
k2 inactive, then K1 = K2 k4
S + x ⎯⎯→ P S + y ⎯⎯→ P
(5) They have different biological properties i.e. (+) sugar play significant role in animal metabolism
while (–) sugar do not.
Meso compounds; These are the compounds containing the more than one asymmetric
carbon atom but due to the presence of plane of symmetry, do not show optical activity.
They are optically inactive compounds.
DIASTEROMERS: Such configurational isomers which are neither be mirror image nor be
superimpossible on each other, called as diastereoisomer.
e.g.
CH3 CH3
H OH H OH
HO H & H OH
CH3 CH3
CH3 CH3 CH3 H
C=C & C=C
H H H CH3
(cis) (trans)
Cis-trans isomer are geometrical isomer but they are not the mirror image thus, they are said to be
diastereomer
CHARACTERISTICS OF DIASTEREOMERS:
(1) They are generally optical active, however geometrical isomers are exception.
(2) They have different physical properties like MP, BP, density, solubilities & value of specific rotation.
(3) They are separated by fractional distillation, fractional crystallisation & chromatography etc.
(4) They exhibit similar
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Haloalkanes & Haloarenes

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VINYLIC

FROM HYDROCARBON FROM ALKENE Preferred product is the alkene

Halogenation C=C + Br2 + • Inversion of Configuration takes place

CH4 CH3Cl + CH2Cl2 + CHCl3 + Cl4

CCl4 • Follows First order Kinetics. 2 + 2Na

Br – CH2 – CH2 –Br

 ACTIVE SITE EDUTECH  CONTACT:  Page 1 of 38

ii] Secondary (𝟐°) alkyl haide

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 ACTIVE SITE EDUTECH  CONTACT:  Page 3 of 38

B) Aromatic Halogen Compounds- These are obtained by replacement of one or more

Classification based on hybridization of the carbon atom linked to the halogen:

(A) A compound containing C–X bond, where carbon is sp3-hybridized.

 ACTIVE SITE EDUTECH  CONTACT:  Page 4 of 38

(B) Compounds containing C–X bond, where carbon is sp2 hybridized

Chlorobenzene Iodobenzene 2-bromotoluene

Nomenclature of Halogen Compounds

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COMMON NAME STRUCTURE IUPAC NAME

Benzyl chloride CH2 – Cl Chlorophenyl methane

Alkyl halides: CH3CH2CH2Br Benzylic halides:

Allylic halides: CH2 = CH – CH2 – Cl

Vinylic halides: CH2 = CH – Cl

 ACTIVE SITE EDUTECH  CONTACT:  Page 6 of 38

METHODS OF PREPARATION OF HALOALKANES

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(2) From Alkenes:

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[Resonance stabilized carbocation is major more stable than hyperconjugation carbocation]

In presence of peroxides such as benzoyl peroxide (C6H5CO–O–O–COC6H5), the addition

 ACTIVE SITE EDUTECH  CONTACT:  Page 9 of 38

c. Termination: It involves the following three steps:

(3) From Alcohols:

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Ex: and not

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4. By Hunsdieker reaction or Borodine – Hunsdiekar reaction

(X2 = Cl2 or Br2)

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ii] Substitution nucleophilicuni molecular reaction 𝐬𝐍 𝟏 :

𝐬𝐍𝟏 Reaction takes place in two steps

 ACTIVE SITE EDUTECH  CONTACT:  Page 14 of 38

H3C C Br H3C C + Br .....Slow

Thus, reactivity of different alkyl halides towards 𝑠𝑛1 reaction is:

 ACTIVE SITE EDUTECH  CONTACT:  Page 15 of 38

COMPARISON OF SN1 AND SN2

R : L + :NuH → R : N+uH +:L−

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The nucleophilic substitution reactions of haloalkanes are discussed below.

5] Substitution by isocyanide group:

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8] Substitution by carboxylate group: -

B] Elimination Reaction: (Dehydrohalogenation)

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Orientation of dehydrohalogenation: If the halogen is present on the terminal C atom of

In the dehydrohalogenation of secondary and tertiary haloalkenes, when there is a possibility

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The ease of dehydrohalogenation is in the order :