Download 10 Haloalkanes and Haloarenes

10 Haloalkanes and Haloarenes
Chapter 10: Haloalkanes and Haloarenes
;
**10.7
10.0 Introduction
Stereochemistry
10.1 Classification
10.8
Nucleophilic substitution mechanism
10.2 Monohalogen derivatives of alkanes
10.9
Haloarenes
10.10 Nature of C  X bond in haloarenes
10.3 Nomenclature of haloalkanes
*10.11 Preparation of haloarenes
10.4 Nature of C  X bond in haloalkanes
10.12 Physical and chemical properties of
haloarenes
10.5 Preparation of haloalkanes
10.6 Physical and
haloalkanes
chemical
* marked section is only for JEE (Main)
properties
of
10.13 Uses and environmental effects of some
haloalkanes and haloarenes
** marked section is for NEET UG
10.0 Introduction

Halogen derivatives of alkanes or of arenes:
When one or more hydrogen atoms of alkanes or arenes are replaced by corresponding number of halogen
atoms, the resulting compounds are called halogen derivatives of alkanes (haloalkanes) or halogen
derivatives of arenes (haloarenes) respectively.

Haloalkanes:
The halogen derivatives of saturated aliphatic hydrocarbons are called as haloalkanes or alkyl halides.
OR
Haloalkanes are obtained by replacing one or more hydrogen atom(s) of an alkane with the corresponding
number of halogen atom(s).
eg. H3C  Cl (Chloromethane)
In haloalkanes, halogen atom(s) is/are bonded to sp3 hybridised carbon atom(s) of an alkyl group.

Haloarenes:
The halogen derivatives of aromatic hydrocarbons are called as haloarenes or aryl halides.
OR
Haloarenes are obtained by replacing one or more hydrogen atom(s) of an arene with corresponding
number of halogen atom(s).
In haloarenes, halogen atom(s) is/are bonded to sp2 hybridised carbon atom(s) of an aryl group.
Note:
Several organic compounds containing halogen exist in nature and some of them are clinically useful.
i.
Substance
Chloramphenicol (Antibiotic)
Contains halogen atom
Chlorine
ii.
Thyroxine (Hormone)
Iodine
iii. Chloroquine (Synthetic halogen
compound)
iv. Halothane
v.
Certain
fully
compounds
Chlorine

fluorinated Fluorine
a.
b.
a.
b.
Description
Produced by soil micro-organisms.
Used in treatment of typhoid fever.
Produced inside our body.
Deficiency causes goiter.
Used in treatment of malaria.
Used as an anaesthetic during
surgery.
Being considered as potential blood
substitutes in surgery.
1
Chemistry Vol ‐ 2.2 (Med. and Engg.)
10.1 Classification
Haloalkanes and haloarenes are classified as monohalogen derivatives or polyhalogen (di-, tri-, etc.) derivatives of
alkanes and arenes respectively, based on the number of halogen atoms in their structure.

Classification of haloalkanes on the basis of the number of halogen atoms:
Haloalkanes
Monohaloalkanes
(Monohalogen derivatives of alkanes)
One hydrogen atom of an alkane is
substituted by one halogen atom.
General formula:
CnH2n+1X [n is an integer]
R  X [X = F, Cl, Br, I and R = alkyl group]
eg. CH3  CH2  Br
Polyhaloalkanes
(Polyhalogen derivatives of alkanes)
More than one hydrogen atom of alkanes are
substituted by corresponding number of
halogen atoms.
Ethyl bromide (Bromoethane)
Dihalogen derivatives
Two hydrogen atoms of an
alkane are substituted by
two halogen atoms.
General formula:
CnH2nX2
[X = F, Cl, Br, I and ‘n’ is
an integer]
Trihalogen derivatives
Three hydrogen atoms of
an alkane are substituted
by three halogen atoms.
General formula:
CnH2n1X3
[X = F, Cl, Br, I and n is an
integer]
eg. CHI3
Iodoform
Geminal dihalides
Both the halogen atoms are
attached to same C-atom.
eg.
H

H3C  C  Br

Br
Ethylidene bromide
(1,1-Dibromoethane)
2
Vicinal dihalides
Both the halogen atoms are
attached
to
adjacent
(vicinal) C-atom.
eg. H2C  CH2
 
Br Br
Ethylene dibromide
(1,2-Dibromoethane)
Tetrahalogen derivatives
Four hydrogen atoms of an
alkane are substituted by
four halogen atoms.
General formula:
CnH2n2X4
[X = F, Cl, Br, I and n is an
integer]
eg. CCl4
Carbon tetrachloride
Chapter 10: Haloalkanes and Haloarenes

Classification of monohalocompounds on the basis of nature of CX bond:
Monohalocompounds
Compounds containing
sp2CX bond
Compounds containing
sp3CX bond
Alkyl halides
(Haloalkanes)
Halogen atom is
bonded to an
alkyl group.
General formula:
CnH2n+1X.
Allylic halides
Halogen atom is
bonded to sp3hybridised carbon
atom next to C=C
i.e., to an allylic
carbon.
eg.
Benzylic halides
Halogen atom is
bonded to sp3hybridised carbon
atom next to an
aromatic ring.
eg.
CH2X
Aryl halides
(Haloarenes)
Halogen atom is bonded
to sp2-hybridised carbon
atom of an aro-matic
ring.
X
eg.
Vinylic halides
Halogen atom is
bonded to sp2hybridised carbon
atom of C=C.
eg.
X
CH2X
10.2 Monohalogen derivatives of alkanes
Monohalogen derivatives of alkanes (alkyl halides) are obtained by substituting one hydrogen atom of an alkane
by one halogen atom and are further classified as follows:
Alkyl halides
Primary
alkyl halide (1)
Halide group is attached to
primary carbon atom of an
alkyl group.
Secondary
alkyl halide (2)
Halide group is attached to secondary
carbon atom of an alkyl group.
eg.
CH3

H3C  C  Br

H
eg. CH3CH2CH2Br
n- Propyl bromide
(1-Bromopropane)
Isopropyl bromide
(2-Bromopropane)
Tertiary alkyl halide (3)
Halide group is attached to tertiary
carbon atom of an alkyl group.
eg.
CH
3

H3C  C  Br

CH3
tert-Butyl bromide
(2-Bromo-2-methylpropane)
10.3 Nomenclature of haloalkanes

Common and IUPAC names of some monohalogen derivatives:
No.
i.
ii.
iii.
iv.
v.
vi.
vii.
Formula
CH3Br
CH3CH2Cl
CH3CH2CH2Br
Br
|
CH3  CH  CH3
CH3CH2CH2CH2Cl
CH3CH  CH2CH3
|
Br
CH3
CHCH2Cl
CH3
Common name
Methyl bromide
Ethyl chloride
n-Propyl bromide
Isopropyl bromide
(sec-Propyl bromide)
IUPAC name
Bromomethane
Chloroethane
1-Bromopropane
2-Bromopropane
n-Butyl chloride
sec-Butyl bromide
1-Chlorobutane
2-Bromobutane
Isobutyl chloride
1-Chloro-2-methylpropane
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Chemistry Vol ‐ 2.2 (Med. and Engg.)
viii.
ix.
x.
xi.
xii.
xiii.
xiv.
xv.
xvi.
xvii.
Br
|
CH3  C  CH3
|
CH3
Br
|
CH3  C  CH2  CH3
|
CH3
CH3
CHCH2Br
CH3
CH3
|
H3C  C  CH2I
|
CH3
CH2 = CHCl
CH2 = CH  CH2  Br
CH2Cl2
CHCl3
CCl4
CH2Cl
tert-Butyl bromide
2-Bromo-2-methylpropane
tert-Pentyl bromide
2-Bromo-2-methylbutane
Isobutyl bromide
1-Bromo-2-methylpropane
Neopentyl iodide
1-Iodo-2,2-dimethylpropane
Vinyl chloride
Allyl bromide
Chloroethene
3-Bromopropene
Methylene chloride
Chloroform
Carbon tetrachloride
Benzyl chloride
Dichloromethane
Trichloromethane
Tetrachloromethane
Chlorophenylmethane
10.4 Nature of C  X bond in haloalkanes
i.
In an alkyl halide, highly electronegative halogen atom is bonded to less electronegative carbon atom.
Therefore, C  X bond in alkyl halide is polar in nature.
The carbon atom carries partial positive charge (+) as it is less electronegative than halogen and halogen
atom carries a partial negative charge ().
ii.
+
CX
iii.
iv.


In the formation of CX bond, sp3 hybrid orbital of carbon atom overlaps with half filled p-orbital of
halogen atom.
C  X bond strength decreases down the group 17 of the periodic table because orbital overlap is most
efficient between orbital of same principle quantum number (i.e., in the same row of periodic table) and
efficiency decreases as difference in principle quantum number increases.
Halogen atom
Its overlapping orbital in C–X bond
F
2pz
Cl
3pz
Br
4pz
I
5pz
The size of the halogen atom increases from F to I, as a result of which, the bond length also increases and
the bond formed is weaker. Hence, C  X bond strength in CH3  X decreases in the order: CH3F > CH3Cl >
CH3Br > CH3I as the 2sp3 orbital of carbon cannot penetrate into the larger p-orbitals sufficiently to form
strong bonds.
v.
4
Chapter 10: Haloalkanes and Haloarenes
Bond enthalpy, bond length and dipole moment of CX bond in CH3X:
Bond
CH3  F
CH3  Cl
CH3  Br
CH3  I
Bond Enthalpy (kJ/mol)
452
351
293
234
Bond Length (Å)
1.42
1.77
1.91
2.12
Dipole moment (Debye)
1.847
1.860
1.830
1.636
10.5 Preparation of haloalkanes
Monohalogen derivatives of alkanes (haloalkanes) can be prepared by the following methods:
i.
From halogenation of alkanes:
a.
Direct halogenation of alkanes in the presence of UV light, heat or suitable catalyst gives the
corresponding alkyl halides.
b.
The displacement of H-atom from hydrocarbon during halogenation follows the order:
Benzylic  allylic > 3 H-atom > 2 H-atom > 1 H-atom > H-atom of methane > vinylic  arylic
c.
The reactivity of halogens decreases in the order: F2 > Cl2 > Br2 > I2
d.
Fluorination of alkanes is highly exothermic and violent, resulting in the cleavage of CC bonds.
Chlorination is fast and exothermic while bromination is slow, as bromination of alkanes is less
exothermic than chlorination. Direct iodination is not possible as reaction is reversible and highly
endothermic.
1.
Chlorination: Alkanes react with chlorine in the presence of UV light or diffused sunlight or at high
temperature to yield the corresponding alkyl chlorides.
h , UV light
R  H + Cl2 
R  Cl + HCl
or Δ
Alkane
eg.
Alkyl
chloride
h , UV light
CH3  H + Cl2 
 CH3  Cl + HCl
or 
Methane
2.
Methyl
chloride
Bromination: Alkanes are heated with bromine in the presence of anhydrous aluminium tribromide
to give the corresponding alkyl bromides.
Anhydrous AlBr3
R  H + Br2 
 R  Br + HBr
Alkane
eg.
Alkyl
bromide
Anhydrous AlBr3
CH3  CH2  H + Br2 
 CH3  CH2  Br + HBr
Ethane
Ethyl bromide
Note:
i.
Direct halogenation of an alkane is a chain reaction and follows free radical mechanism.
ii.
This method of preparation gives the mixture of mono, di, tri and tetra halogen derivatives of an
alkane and it is difficult to separate each component in pure form.
eg. Preparation of methyl chloride by direct halogenation method results in the formation of mono,
di, tri and tetra chloromethane derivatives.
Cl2
Cl2
Cl2
h
+ Cl2 
 CH3  Cl 
 CH2Cl2 
 CHCl3 
 CCl4
CH4
 HCl
 HCl
 HCl
 HCl
`
Methane
Methyl
chloride
Dichloromethane
Trichloromethane
Tetrachloromethane
Therefore, halogenation (chlorination and bromination) of an alkane is not useful for laboratory preparation
of alkyl halide, because it gives mixture of different alkyl halides which are difficult to separate.
Consequently, the yield of any one component is less due to the formation of other component.
5
Chemistry Vol ‐ 2.2 (Med. and Engg.)
3.
Iodination:
i.
Alkanes react with iodine to form the corresponding alkyl iodides.
ii.
This reaction is reversible and endothermic because hydroiodic acid (HI) during the course of
reaction acts as strong reducing agent and reduces alkyl iodide back to alkane.

R  H + I2 
Alkane
eg.
RI
iv.
2CH4
Methyl
iodide
5C2H5  H + 2I2 + HIO3  5C2H5  I
Ethane
Ethyl
iodide
Iodic
acid
+ 3H2O
In the presence of dilute nitric acid (HNO3):
eg.
8C2H5  H + 4I2 + dil.HNO3  8C2H5I + 3H2O + NH3
Ethane
ii.
Mercuric
oxide
In the presence of iodic acid (HIO3):
eg.
4.
HI
Hydroiodic acid
 2CH3  I + HgI2 + H2O
+ 2I2 + HgO
Methane
vi.
+
Ethyl iodide
So, this reaction is carried out in the presence of oxidising agent like mercuric oxide (HgO),
iodic acid (HIO3), dilute nitric acid (HNO3), etc., which reacts with HI and prevents backward
reaction.
In the presence of mercuric oxide (HgO):
eg.
v.
HI
Hydroiodic acid
 C2H5  I
C2H5  H + I2 
Ethane
iii.
+
Alkyl iodide
Nitric
acid
Ethyl
iodide
Note: Iodination stops at monoiodo stage.
Fluorination: Alkanes react with fluorine in an explosive manner. Halogenation of alkanes is not a
suitable method for preparing alkyl fluorides as the byproduct formed (hydrofluoric acid) is
poisonous and corrosive.
From halogenation of alkenes:
a.
When alkenes are treated with Br2 or Cl2 in the presence of solvent like CCl4, the addition reaction
takes place across the double bond to give vic-dihalides.
CCl4
C = C + X2 
 XC  C X
Alkene
eg.
CCl4
H2C = CH2 + Br2 
 Br  CH2  CH2  Br
Ethene
b.
Vic-dihalide
1,2-Dibromoethane
This reaction is used for detection of unsaturation (multiple bond) in an organic compound. The
disappearance of reddish brown colour of bromine due to formation of colourless vic-dibromide
indicates the presence of a multiple bond.
Note:
i.
The reaction of alkenes (except ethylene) with Cl2 or Br2 at higher temperature (about 773 K) gives
substitution reaction product instead of addition reaction product. This is because at higher
temperature, the addition reaction is reversible and the substitution reaction is irreversible. The
hydrogen atom of allylic carbon is replaced with the halogen atom to form allylic halides and the
reaction is called as allylic halogenation.
773K
eg. H3C  CH = CH2 + Cl2 
 Cl  CH2  CH = CH2 + HCl
Propene
6
3-Chloropropene
(Allyl chloride)
Chapter 10: Haloalkanes and Haloarenes
ii.
eg.
Allylic halogenation is also carried out by using specific reagents like N-bromosuccinimide (NBS)
and sulphuryl chloride at 473 K in the presence of light and peroxide as initiator.
a.
O
O
h
peroxide
N  Br 
+
Cyclohexene
Br
O
473K
h , Peroxide
H3C  CH = CH2 + SO2Cl2  Cl  CH2  CH = CH2 + SO2 + HCl
Propene
iii.
O
3-Bromocyclohexene Succinimide
NBS
b.
NH
+
Sulphuryl
chloride
3-Chloropropene
By addition of hydrogen halides to alkenes:
a.
Alkyl halides can be obtained by the electrophilic addition of hydrogen halides like HCl, HBr, HI
across the double bond of alkene.
H
C = C + HX
Alkene
b.
  C  C 
Hydrogen
halide
But-2-ene
d.
 H3C  CH2  CH  CH3

Cl
2-Chlorobutane
In the case of unsymmetrical alkenes, carbon atoms involved in double bond are non-equivalent, so
the addition of HX in unsymmetrical alkene takes place according to Markownikoff’s rule.
According to Markownikoff’s rule, “during addition of an unsymmetrical reagent across the double
bond of an unsymmetrical alkene, the negative part of reagent attacks on the carbon atom with less
number of hydrogen atom(s) (more substituted carbon) and positive part of the reagent attacks on
carbon atom with more number of hydrogen atom(s) (less substituted carbon)”.
eg.
Cl
Cl


HCl
H3C  CH = CH2 
 H3C  CH  CH3 + CH3  CH2  CH2
1.
Markownikoff ’s rule
Propene
2.
2-Chloropropane
(Major product)
1-Chloropropane
(Minor product)
I
H
I
H
H
2
2 1
4 3
1
2 3 4
1
3 4
HI
H3C  C = C  CH3 
 H3C  C  C  CH3 + H3C  C  C  CH3
Markownikoff ’s rule





CH3
CH3 H
CH3 H
2-Methylbut-2-ene
e.
Alkyl
halide
Order of reactivity of hydrogen halides is: HI > HBr > HCl > HF
In case of symmetrical alkenes, alkyl group or number of hydrogen atoms present on either side of the
doubly bonded carbon atoms is same, therefore during addition of HX, only one type of product is
formed.
eg. H3C  CH = CH  CH3 + HCl
c.
X
2-Iodo-2-methylbutane
(Major product)
2-Iodo-3-methylbutane
(Minor product)
But the addition of HBr to an unsymmetrical alkene in the presence of peroxide like Na2O2, H2O2,
benzoyl peroxide (C6H5CO)2O2 follows Anti-Markownikoff’s rule.
7
Chemistry Vol ‐ 2.2 (Med. and Engg.)
f.
According to Anti-Markownikoff’s rule, “during addition of HBr in the presence of peroxide, the
negative part of reagent attacks on C-atom with more number of hydrogen atom(s) while positive part
of reagent attacks on C-atom with less number of hydrogen atom(s)”.
This rule is also known as Peroxide effect or Kharasch effect or Kharasch-Mayo effect.
Br
eg.
3
2
1
3 2
1
Peroxide
 H3C  CH2  CH2  Br + H3C  CH  CH3
1. H3C  CH = CH2 + HBr 
Anti- Markownikoff ’s rule
2.
2-Bromopropane
(Minor product)
1-Bromopropane
(Major product)
Propene
CH3
CH3
CH3
3
3
2 1
3 2 1
2 1
(C6 H5CO)2 O 2
CH3  C = CH2 + HBr 


H
C

C

CH

Br
+
H
C
3
2
3  C  CH3
Anti-Markownikoff ’s rule


2-Methylpropene
H
Br
1-Bromo-2-methylpropane
(Major product)
2-Bromo-2-methylpropane
(Minor product)
Note: Peroxide effect is observed only in case of HBr. HI and HCl follow Markownikoff’s rule even
in the presence of peroxide.
iv.
From Alcohols:
Alkyl halides can be prepared from alcohols by substituting the hydroxy group of alcohols with halogen
atom. Following three types of reagents can be used to carry out this reaction.
a.
halogen acids
b.
phosphorus halides or
c.
thionyl chloride
a.
Reaction with halogen acids:
1.
Chloroalkanes:
i.
Alcohols react with Lucas reagent (solution of concentrated HCl and anhydrous zinc chloride)
to form the corresponding alkyl chlorides.
Anhydrous ZnCl2
R  OH + HCl 
 R  Cl + H2O

Alcohol
ii.
(conc.)
Alkyl
chloride
Primary and secondary alcohols react with concentrated HCl and anhydrous ZnCl2 to give the
corresponding alkyl chlorides. This process is called “Groove’s process”.
Anhydrous ZnCl2
eg. a.
CH3  OH + HCl 
 CH3  Cl + H2O

Methyl
Methanol
(conc.)
chloride
b.
iii.
CH3
CH3


Anhydrous ZnCl2
CH3  C  OH + HCl 
CH

3  C  Cl
Room temperature
(conc.)


H
H
Propan-2-ol
2-Chloropropane
Tertiary alcohols readily react (simply by shaking) with concentrated HCl even in the absence
of anhydrous ZnCl2.
eg.
CH3
CH3


Room temperature
H3C  C  OH + HCl 
 H3C  C  Cl + H2O


(conc.)
CH3
CH3
tert-Butyl alcohol
8
+ H2O
tert-Butyl chloride
Chapter 10: Haloalkanes and Haloarenes
Note: Anhydrous ZnCl2 acts as a catalyst by helping in cleavage of C  O bond. It is Lewis acid so it easily
abstracts hydroxyl group of an alcohol by coordinating with oxygen of OH group. Due to this, weakening
of C  O bond takes place and it finally breaks to form carbocation. Chloride ion then readily reacts with
carbocation to form chloroalkanes.
2.
Bromoalkanes:
i.
Alkyl bromides are prepared by heating alcohol with hydrobromic acid (generated in situ by
treating sodium bromide or potassium bromide with conc. H2SO4).
NaBr  Conc.H 2SO 4
R  CH2  OH 
 R  CH2  Br + H2O + NaHSO4
Re flux
Alkyl bromide
Alcohol
eg.
C2H5OH
NaBr  Conc.H 2SO 4


Re flux
Ethyl alcohol
ii.
C2H5Br
+
H2O + NaHSO4
Ethyl bromide
In the preparation of secondary and tertiary bromides from respective alcohols, conc. H2SO4 is
not used (as it may result in the dehydration of secondary and tertiary alcohols to form alkenes);
instead dil. H2SO4 is used.
OH
Br


KBr  dil.H 2SO 4
R  CH  R 

 R  CH  R + H2O + KHSO4
Secondary
alcohol
Secondary
alkyl bromide
R
R


KBr  dil.H 2SO 4
R  C  OH 

 R  C  Br + H2O + KHSO4


R
R
Tertiary
alcohol
Tertiary alkyl
bromide
where R, R and R can be same or different alkyl groups.
eg. a.
OH
Br


KBr  dil.H 2SO4
H3C  CH  CH3 
 H3C  CH  CH3 + H2O + KHSO4
b.
Isopropyl alcohol
Isopropyl bromide
CH3

H3C  C  OH

CH3
CH3

KBr  dil. H 2SO4

 H3C  C  Br + H2O + KHSO4

CH3
tert-Butyl alcohol
3.
tert-Butyl bromide
Iodoalkanes:
i.
Alkyl iodides are prepared by heating respective alcohols with conc. hydroiodic acid (57 %).
R  OH
Alcohol
ii.
+ HI
(conc.)
(57%)


 RI
+ H2O
Iodoalkane
Hydroiodic acid can be prepared in situ by reacting potassium iodide with 95% phosphoric
acid.
9
Chemistry Vol ‐ 2.2 (Med. and Engg.)
eg.
a.

 CH3CH2CH2I + KH2PO4 + H2O
CH3CH2CH2OH + KI + H3PO4 
1-Iodopropane
Phosphoric
acid (95%)
Propan-1-ol

CH3  CH  CH2  CH3 + KI + H3PO4 
 CH3  CH  CH2  CH3 + KH2PO4 + H2O


Phosphoric
I
acid
(95%)
OH
b.
Butan-2-ol
4.
2-Iodobutane
Fluoroalkanes:
Fluoroalkanes are not practically prepared by this method as hydrogen fluoride is least reactive.
Note:
i.
The order of reactivity of alcohols in this reaction: Allyl alcohol > tertiary > secondary > primary.
This is because of +I effect of alkyl group(s) attached to the  - carbon atom of an alcohol, which
facilitates the cleavage of C  O bond of an alcohol and increases the reactivity of alcohol.
ii.
The order of reactivity of halogen acids with alcohols is: HI > HBr > HCl > HF.
This order is in accordance with bond dissociation energies. (Bond dissociation energy of HI is less
than that of HBr which is in turn less than that of HCl).
b.
1.
Reactions with phosphorus halides:
Haloalkanes are prepared by heating alcohols with phosphorus trihalides or phosphorus pentahalides.
Chloroalkanes:
Alkyl chlorides are prepared by treatment of phosphorus pentachloride (PCl5) or phosphorus
trichloride (PCl3) on respective alcohols.

R  OH + PCl5 
 R  Cl + POCl3 + HCl
Alcohol
eg.

CH3  OH + PCl5 
 CH3  Cl +
Methanol
Methyl
chloride

3R  OH + PCl3 
 3R  Cl +
Alcohol
eg.
Alkyl
chloride
3C2H5OH
+ HCl
H3PO3
Phosphorus
acid
+
Ethyl chloride
H3PO3
Phosphorus acid
Bromoalkanes and iodoalkanes:
i.
Alkyl bromides and iodides are prepared by action of phosphorus tribromide (PBr3) or
phosphorus triiodide (PI3) on alcohols.
ii.
PBr3 is unstable and can be generated in situ by action of red phosphorus on Br2.
2P
+ 3Br2
 2PBr3
Red
phosphorus
Bromine
Phosphorus
tribromide
red P  Br2
3R  OH + PBr3 

Alcohol
eg.

3R  Br
Alkyl bromide
+
H3PO3
Phosphorus acid
red P  Br2
3CH3  CH2  CH2  OH + PBr3 
 3CH3  CH2  CH2  Br + H3PO3
Propan-1-ol
(n-Propyl alcohol)
10
POCl3
Phosphorus
oxychloride

+ PCl3 
 3C2H5  Cl
Ethanol
2.
Phosphorus
oxychloride
Alkyl
chloride
Phosphorus
tribromide
1-Bromopropane
(n-Propyl bromide)
Phosphorus
acid
Chapter 10: Haloalkanes and Haloarenes
iii.
PI3 is also unstable and it can be generated in situ as follows:
2P
+ 3I2  2PI3
Red
phosphorus
Phosphorus
triiodide
3R  OH +
red P  I2


Phosphorus
triiodide
Alcohol
eg.
PI3
3R  I
+
H3PO3
Alkyl iodide
3CH3  (CH2)3  CH2  OH +
Pentan-1-ol
(n-Pentyl alcohol)
PI3
Phosphorus
acid
 3CH3(CH2)3  CH2I + H3PO3
Phosphorus
triiodide
1-Iodopentane
(n-Pentyl iodide)
Phosphorus
acid
Note:
i.
Good yield of primary alkyl halides is obtained by this method.
ii.
Secondary and tertiary alcohols undergo dehydration to form alkenes and hence good yield of
secondary and tertiary alkyl halides is not obtained by this method.
iii. In laboratory, lower alkyl bromides and iodides are prepared by this method.
iv. PBr5 and PI5 does not exist.
c.
1.
Reactions with thionyl chloride (sulphonyl chloride):
This reaction is used for the preparation of alkyl chlorides. When alcohols are refluxed with thionyl
chloride in the presence of pyridine base, corresponding alkyl chlorides are formed.
Pyridine
R  OH + SOCl2 
Re flux
Alcohol
eg.
Thionyl
chloride
CH3  OH
+ SO2 + HCl
Alkyl chloride
Pyridine
+ SOCl2 
CH3  Cl
Re flux
Methanol
2.
R  Cl
Thionyl
chloride
+ SO2 + HCl
Methyl chloride
Chloro compounds obtained by this method can be easily isolated as both the byproducts of reaction
(SO2 and HCl) are gases and escape easily leaving behind pure alkyl chloride.
Note:
i.
This process is also known as “Darzen’s procedure”.
ii.
Thionyl bromide is unstable and thionyl iodide does not exist, thus alkyl bromides and alkyl iodides
cannot be prepared by this method.
v.
By Halogen Exchange:
a.
This method is used for the preparation of alkyl iodides. Alkyl chlorides or bromides are heated with
solution of sodium iodide in dry acetone to give corresponding alkyl iodide. This reaction is known as
“Finkelstein reaction”.
b.
Sodium bromide and sodium chloride are less soluble in dry acetone and thus they get precipitated.
c.
These precipitates are removed by filtration and thus backward reaction is also prevented.
d.
Primary alkyl bromides and chlorides give best results by this reaction.
R  X + NaI
Alkyl
halide
Dry acetone

 RI
Sodium
iodide
Alkyl
iodide
+
NaX
Sodium
halide
(X = Cl, Br)
eg.
Dry acetone
CH3  CH2  Br + NaI 
 CH3  CH2  I + NaBr
Ethyl
bromide
Sodium
iodide
Ethyl iodide
11
Chemistry Vol ‐ 2.2 (Med. and Engg.)
e.
Alkyl fluorides can also be prepared by this method; by the action of mercurous fluoride (Hg2F2),
silver fluoride (AgF), cobalt fluoride (CoF2) or antimony trifluoride (SbF3) on alkyl chloride or
bromide.
2R  X
Alkyl
halide
+
Hg2F2

Mercurous
fluoride
2R  F
+ Hg2X2
Alkyl fluoride
(Fluoroalkane)
(X = Cl, Br)
This reaction is known as “Swarts Reaction”.
+ Hg2F2
 CH3  F
eg. 2CH3  Cl
Methyl chloride
Mercurous
fluoride
+ Hg2Cl2
Methyl fluoride
Note: When the organic halides contain two or three halogen atoms on the same carbon atom, SbF3 or
CoF2 are used.
Cl

3CH3  C  CH3

Cl
eg.
2,2-Dichloropropane
+
2SbF3
F

 3CH3  C  CH3

F
+ 2SbCl3
2,2-Difluoropropane
10.6 Physical and chemical properties of haloalkanes

i.
ii.
iii.
iv.
v.
vi.

i.
12
Physical properties:
Physical state:
a.
Lower members of haloalkanes (when pure) are colourless gases at room temperature while higher
members are liquids or solids.
b.
Bromides and iodides develop colour when exposed to light.
Smell: Lower members of haloalkane series are sweet smelling liquids.
Solubility: Alkyl halides are very slightly soluble in water but readily soluble in organic solvents like
methanol, acetone, etc.
Density of haloalkanes:
a.
Bromoalkanes, iodoalkanes and polychloro derivatives of alkanes are heavier than water whereas
chloroalkanes and fluoroalkanes are lighter than water.
1
b.
Density of haloalkane  size of halogen atom and density of haloalkane 
.
size of the alkyl group
c.
Densities increase in the order: Fluoride < Chloride < Bromide < Iodide.
Melting and boiling points of haloalkanes:
a.
Melting and boiling points of alkyl halides are greater than corresponding hydrocarbons.
b.
Due to the polarity of C  X bond and high molecular mass, intermolecular forces of attraction
(dipole-dipole-London force and van der Waal’s force) between molecules of haloalkanes are
stronger and results in increase in melting and boiling point.
c.
Hence, boiling points of haloalkanes having same alkyl group increase in the order:
H3C  F < H3C  Cl < H3C  Br < H3C  I
d.
In case of isomeric haloalkanes, branching results in decrease in boiling point.
Inflammable nature: Haloalkanes are less inflammable than hydrocarbons. They give green edged flame
with copper wire on heating (Beilstein test).
Chemical properties:
Reactivity of an alkyl halide (for the same alkyl group) decreases in the order given below:
R  I > R  Br > R  Cl > R  F
Chapter 10: Haloalkanes and Haloarenes
ii.
iii.

Reactivity of alkyl halide depend on the polarity of C  X bond as electronegativity of halogen atoms
decreases in the order of F > Cl > Br > I; so strength of C  F bond is more due to large difference in
electronegativities (therefore it is more stable) whereas C  I bond is less stable and shows high reactivity
compared to other halogens.
The order of reactivity among 1, 2 and 3 alkyl halide is:
3 alkyl halide > 2 alkyl halide > 1 alkyl halide.
This is due to +I effect of an alkyl group which increases bond polarity of C  X bond.
Substitution reactions:
Reactions in which an atom or a group of atoms is substituted by another atom or a group of atoms
respectively are known as substitution reactions.
An alkyl halide shows nucleophilic substitution reaction due to polarity of C  X bond.
Y
 R  Y
+ X
RX +
Alkyl
halide
i.
Nucleophile
Substituted
alkane
Hydrolysis:
a.
Alkyl halides on boiling with aqueous alkali hydroxide (KOH/NaOH) undergo hydrolysis to form the
corresponding alcohols.
RX
+
Alkyl halide
b.
Boil
KOH 
 R  OH
KX
Potassium
halide
Alcohol
(aq.)
During the course of reaction, X group of an alkyl halide gets substituted by OH to form an alcohol.
Boil
eg. CH3  Cl
+ KOH 
 CH3  OH + KCl
Methyl chloride
c.
+
(aq.)
Methyl alcohol
Alkyl halides on boiling with moist silver oxide undergo hydrolysis to form the corresponding alcohols.
RX
moist Ag 2 O
+ AgOH 

 R  OH
Boil
Alkyl halide
eg.
+ AgX
Alcohol
CH3  (CH2)2  I
moist Ag 2 O
+ AgOH 

 CH3  (CH2)2  OH + AgI
Boil
nPropyl iodide
nPropyl alcohol
Note:
i.
Silver hydroxide does not exist.
ii.
Silver oxide suspended in water behaves as silver hydroxide.
ii.
Formation of alkyl cyanides (alkane nitriles):
a.
Alkyl halides on boiling with alcoholic potassium cyanide form corresponding alkyl cyanides or
alkane nitriles.
RX
Alkyl halide
b.
boil
+ KC  N 
RCN
(alc.)
+ KX
Alkyl cyanide
Halogen atom is substituted by nucleophile cyanide (CN) to form product, because of strong basic
nature of KCN, cyanide attacks through C-atom.
eg.
Cl
CN


boil
CH3  CH  CH3 + K  C  N 
H3C  C  CH3
+ KCl
|
(alc.)
2-Chloropropane
H
2-Methylpropanenitrile
13
Chemistry Vol ‐ 2.2 (Med. and Engg.)
c.
iii.
The product formed in the above reaction has one more carbon atom than the haloalkanes. Thus, the
reaction is a good method for increasing the length of carbon chain.
Formation of alkyl isocyanides (R  N  C):
a.
Alkyl halide reacts with alcoholic silver cyanide (AgCN) to form corresponding alkyl isocyanide.
RX

Ag  C  N 
 RNC
+
Alkyl halide

+ Ag  C  N 

Ethyl bromide
iv.
Alkyl isocyanide
(alc.)
C2H5  Br
eg.
+ AgX
C2H5  NC
+ AgBr
Ethyl isocyanide
(Carbylaminoethane)
(alc.)
b.
In this reaction halide group is substituted by nucleophilic cyanide group to form product.
c.
In the presence of silver salt, nucleophilic attack takes place through N-atom of cyanide.
Formation of amines (ammonolysis):
a.
Alkyl halide on heating with alcoholic ammonia under pressure undergoes substitution reaction to
give corresponding primary amine.
b.
In this reaction, halide group is substituted by an amino (–NH2) group.

+ H  NH2 
 R  NH2 + HX
under pressure
RX
Alkyl halide
eg.
Primary
amine
(alc.)

CH3  Cl + H  NH2 
 CH3  NH2 + HCl
under pressure
Methylamine
Methyl
(alc.)
(Primary amine)
chloride
Note: ‘R’ group in alkyl halide can be primary, secondary or tertiary.
c.
d.
Order of reactivity of haloalkanes (for the same alkyl group) with NH3 is RI > RBr > RCl.
When an alkyl halide is in excess, mixture of primary amine, secondary amine, tertiary amine and
quaternary ammonium salt is obtained.
 , under
R X
R X
R X
pressure
R  X + NH3 
 R  NH2 

 R2NH 

 R3N 
[R4N]+X
 , under pressure,
 , under pressure,
 , under pressure
 HX
 HX
Alkyl
halide
 HX
1 amine
2 amine
Quaternary
ammonium
salt
3 amine
eg.
C2H5  Cl
 , under
C2 H5  Cl
C2 H5  Cl
pressure
+ NH3 
 C2H5  NH2 
(C2H5)2NH 
(C2H5)3N
 , under pressure
 , under pressure
 HCl
Ethyl
chloride
Ethylamine
(1 amine)
 HCl
Diethylamine
(2 amine)
(1 Alkyl halide)
 HCl
Triethylamine
(3 amine)
, under
pressure C2H5  Cl
[(C2H5)4N]+Cl
Tetraethyl ammonium chloride
(Quaternary salt)
e.
f.
v.
14
This reaction is known as “Hoffmann’s ammonolysis reaction” or alkylation of ammonia.
When excess of ammonia is used, primary amine is obtained as a major product.
Formation of ethers (Williamson’s synthesis):
a.
Alkyl halide is heated with alkali alkoxide (KOR/NaOR) to give corresponding ether. This reaction is
known as “Williamson’s synthesis”.
Chapter 10: Haloalkanes and Haloarenes
RX

Na OR 
 R  O  R
+
b.
c.
In this reaction, halide group undergoes substitution with alkoxy OR group.
Sodium alkoxide can be prepared by action of sodium metal on alcohol.
2R  OH + 2Na  2R  ONa + H2
Alcohol
eg.
Sodium alkoxide

CH3  I + NaOCH3 
 CH3  O  CH3 + NaI
Methyl
iodide
d.
+ NaX
Ether
Sodium
alkoxide
Alkyl halide
Dimethyl ether
Sodium
methoxide
When haloalkanes are heated with dry silver oxide, symmetrical ethers are obtained.
2R  X
Alkyl halide
eg.

Ag2O 
 ROR
+
Dry
silver oxide
+ 2AgX
Ether

2C2H5  CH2Cl + Ag2O 
 C2H5  CH2  O  CH2  C2H5 + 2AgCl
n-Propyl chloride
(1-Chloropropane)
Dry
silver oxide
Dipropyl ether
(1-Propoxypropane)
Note: Silver oxide used should be completely dry, as traces of moisture may result in alcohol formation.
vi.
Formation of esters:
a.
Ethanolic solution of silver salt of a fatty/carboxylic acids on heating with haloalkanes give
corresponding esters.
O
RX
+
Alkyl halide
C2 H5OH
Ag  O  C  R 
 R  O C  R + AgX

Ester
O
Silver salt
of carboxylic acid
b.
c.
In this reaction, halogen group is substituted by carboxylate (R  COO ) group.
During this reaction, carboxylate ion (R  COO) acts as a nucleophile.
O
eg.
CH3  I
C2 H5OH
+ Ag  O  C  CH3 
 CH3  O  C  CH3 + AgI

Methyl iodide
vii.
Methyl acetate
O
Silver acetate
Formation of alkyl nitrite and nitroalkanes:
a.
Alkyl halide (R  X) on treatment with KNO2 forms alkyl nitrite (R  O  N = O) whereas on
treatment with AgNO2 forms nitroalkane (R  NO2).
b.
The nitrite ion possesses two nucleophilic centres (i.e., it is an ambident nucleophile).
c.
The linkage through oxygen results in the formation of alkyl nitrites whereas the linkage through
nitrogen results in the formation of nitroalkanes.
Heat
R  X + K+O – N = O 
 R  O  N = O + KX
Alkyl
halide
Potassium
nitrite
Alkyl nitrite
R  X + AgNO2 
 R  NO2 + AgX
Alkyl
halide
Silver
nitrite
Nitroalkane
15
Chemistry Vol ‐ 2.2 (Med. and Engg.)
Note: Nucleophilic substitution of alkyl halides (R – X):
Reagent
i.
KOH / NaOH /
moist Ag2O
ii. Alcoholic KCN
iii. Alcoholic AgCN
iv. Alcoholic NH3
v.
NaOR
vi. H2O
vii. Dry Ag2O
Product
R – OH
Reagent
viii. RCOOAg
Product
RCOOR
R – CN
R – NC
R – NH2
R – O – R
R – OH
R–O–R
ix.
x.
xi.
xii.
xiii.
R–I
R–O–N=O
R – NO2
R–H
R – R
NaI
KNO2
AgNO2
LiAlH4
R – M+

Elimination Reactions:
Elimination reactions are those reactions in which a molecule loses two atoms or groups attached to
neighbouring carbon atoms with formation of double bond between carbon atoms.
OR
The reaction in which two atoms or groups are removed from adjacent carbon atoms in a molecule to form
an unsaturated compound is called an elimination reaction.

i.
Dehydrohalogenation (formation of alkenes):
When alkyl halides are heated with alcoholic solution of alkali hydroxide (KOH/NaOH), halogen atom
from -carbon atom and a hydrogen atom from adjacent -carbon atom gets eliminated to form
corresponding alkenes.
This reaction is also called as “dehydrohalogenation of an alkyl halide”.
ii.
H H
H H
 
 

R  C  C  X + K+OH 
 RC=C
 

(alc.)
H
H H
As hydrogen atom is eliminated from -carbon atom, it is also known as “-elimination reaction”.
v.
vi.




K+OH 
 CH3  CH = CH2 + H2O + KI
n-Propyl iodide
Propylene
(alc.)
In dehydrohalogenation of secondary and tertiary alkyl halides there is possibility of formation of two
isomers of alkene, in such a case elimination takes place according to Saytzeff’s rule.
According to Saytzeff’s rule, “when there is a possibility of formation of two types of alkenes by
dehydrohalogenation of alkyl halide, then H-atom is eliminated preferentially from C-atom having least
number of H-atom(s)”. In other words, in dehydrohalogenation reaction, more substituted double bond
formation is always preferred.
In dehydrohalogenation, reactivity of alkyl halide is in the following order:
RI > RBr > RCl > RF (when same alkyl group is present).
Ease of dehydrohalogenation in case of haloalkanes follows the order:
Tertiary > secondary > primary (when same halogen group is present).
alc.KOH
eg. H3C  CH2  CH  CH3 
 H3C  CH = CH  CH3 + CH3  CH2  CH = CH2
  HBr
eg.
iv.
H2O + KX
Alkene
Alkyl halide
iii.
+
CH3  CH2  CH2  I
Br
2-Bromobutane
+
But-2-ene
(80%)
But-1-ene
(20%)

Reaction with metals:
Alkyl halides react with metals such as sodium to form corresponding higher saturated hydrocarbon and
with magnesium to form organometallic compounds.
i.
Reaction with sodium or Wurtz synthesis:
a.
Haloalkanes when treated with metallic sodium in the presence of dry ether form corresponding
symmetrical higher alkanes.
16
Chapter 10: Haloalkanes and Haloarenes
2R  X
dry ether
+ 2Na 


Alkyl halide
b.
c.
RR
This reaction is called as “Wurtz synthesis”.
The product formed contains more number of carbon atoms than reactants; thus, this method is
preferably used for the preparation of higher alkanes.
dry ether
eg. 2CH3  CH  Br + 2Na 
 H3C  CH  CH  CH3 + 2NaBr



CH3
CH3 CH3
2,3-Dimethylbutane
2-Bromopropane
ii.
+ 2NaX
Higher alkane
d.
Tertiary halides do not undergo this reaction.
Reaction with magnesium or formation of Grignard’s reagent:
a.
Grignard’s reagent can be prepared by reaction of alkyl halide with pure and dry magnesium in the
presence of dry ether.
RX
+
Alkyl halide
b.
(Dry)
Alkyl magnesium halide
(Grignard’s reagent)
Grignard’s reagent is chemically known as alkyl magnesium halide and represented by general
formula R  Mg  X.
dry ether
eg. CH3  I + Mg 

 CH3  Mg  I
Methyl
iodide
c.
dry ether
Mg 
 R  Mg  X
(Dry)
Methyl magnesium
iodide
The Grignard reagents are very reactive compounds and react with any source of proton to form
corresponding hydrocarbons.
R  Mg  X + ZH  R  H + ZMgX
Grignard reagent
Alkane
where, Z = OH, OR, NH2,etc.
Note:
i.
Compound in which less electropositive carbon atom is directly attached to highly electropositive metal
atom is called “Organometallic compound”.
ii.
In this compound, C-atom has partial negative charge and metal atom has partial positive charge.
iii. In Grignard’s reagent, C  Mg bond is highly polar and Mg  X bond is ionic in nature.
Hence, Grignard’s reagent are highly reactive towards organic as well as inorganic reagents.

iv.
+

R  Mg  X
a.
Traces of moisture (if remained during preparation of Grignard’s reagent) readily react with Grignard
reagent to form corresponding alkane; also Grignard’s reagent in free state is explosive in nature.
b.
Hence, Grignard’s reagent is never stored and always prepared at the time of requirement.
c.
It is used in the absence of air, under inert atmosphere like dry ether (as a solvent).
10.7 Stereochemistry
Stereochemistry plays an important role in deciding the product of any reaction specially nucleophilic substitution
reaction. Some basic stereochemical notations and concepts are given below:
Ordinary Light: Ordinary light consists of electromagnetic radiations of different wavelengths, vibrating in all
possible directions in space and perpendicular to direction of propagation of light.
Monochromatic Light:
i.
Ordinary light after passing through monochromator (prism or grating monochromator) emerges out as a
ray of single wavelength and is called as “Monochromatic ray of light”.
ii.
Monochromatic ray of light vibrates in different planes, perpendicular to the direction of propogation of light.
17
Chemistry Vol ‐ 2.2 (Med. and Engg.)
Plane Polarized Light:
i.
A beam of light vibrating in only one plane in space is called “Plane Polarized Light”.
ii.
Ordinary beam of light after passing through Nicol’s prism (crystalline calcium carbonate) emerges out as
plane polarized light.
iii. Nicol’s prism is called as polarizer in which vibrations in all other planes are cut off except one plane.
iv. Nicol’s prism is combination of two prisms made of calcite crystals and fused base to base by Canada balsam.
Nicol’s prism
(Polarizer)
Ordinary light
Plane Polarized light
Plane Polarized Light

i.
Optical Activity:
When solution of certain organic compounds come in contact with plane polarized light, they rotate the
plane of plane polarized light by some angle either in clockwise or anticlockwise direction. This property of
organic substance is called as “Optical activity”.
 or 
Plane polarized light
Sample containing
optically
active substance
Clockwise
rotation
by 
Anticlockwise
rotation
by 
Analyser
Rotation of Plane Polarized Light due to an Optically Active Substance
ii.
iii.


i.
Polarimeter is the instrument used to measure optical activity (i.e., to measure the magnitude and the
direction of the rotation of plane of plane polarized light) of an optically active compound.
The polarimeter consists of a light source, two nicol prisms and the sample tube to hold the substance. The
prism placed near the source of light is called polariser while the other placed near the eye is called analyser.
Optically Active Molecule:
If a molecule is capable of rotating plane of plane polarized light in either clockwise or anticlockwise
direction, it is called as “Optically Active Molecule”.
eg. Lactic acid, 2-Iodobutane, glucose, fructose, etc.
d - l configuration: Depending upon the behaviour of molecules of the compound towards plane polarized
light; they can be differentiated as follows:
Dextro Rotatory Molecules:
a.
If a molecule is capable of rotating plane of plane polarized light to the right i.e., in the clockwise
direction then it is called as “Dextro Rotatory Molecule”. (Latin, dexter = right)
b.
These are designated as (+) or (d).
eg. 1.
COOH
I
2.


H

C*
 OH
H3C  C*  C2H5


CH3
H
(+)/(d)–2-Iodobutane
ii.
18
(+)/(d)-Lactic acid
Laevo Rotatory Molecules:
a.
If a molecule is capable of rotating plane of plane polarized light to the left i.e., in the anticlockwise
direction then it is called as “Laevo Rotatory Molecule”.
(Latin, Laevus = left)
Chapter 10: Haloalkanes and Haloarenes
b.
These are designated as () or (l).
I
eg. 1.

H5C2  C*  CH3

H
2.
COOH

HO  C*  H

CH3
()/(l)-Lactic acid
()/(l)-2-Iodobutane
iii.
Racemic Mixture:
a.
Equimolar mixture of dextro and laevo form of the same compound is known as “Racemic Mixture”
or “Racemic modification” or “Racemate”.
b.
The process of conversion of enantiomer into a racemate is called as racemisation.
c.
Racemic mixture is optically inactive and does not rotate the plane of plane polarized light.
d.
When dextro and laevo forms of molecule cancel each other’s rotation (which is equal but in opposite
direction), it is known as “External Compensation”.
e.
It is designated as (dl) or ().
eg. dl -Lactic acid,  2-Iodobutane, etc.

Optically Inactive Molecules:
Optically inactive molecules are those which do not rotate the plane of plane polarized light.
eg. Ethyl chloride, water, etc.
Chirality:
Four valencies of carbon atom are arranged along the four corners of regular tetrahedron. If all 4 atoms or
groups attached to such carbon atom are different, then it is called as “Asymmetric” or “Chiral carbon
atom” or “Stereocentre” and it is denoted by star or asterisk (*) on it .
When molecule contains asymmetric carbon atom, the symmetry of molecule is lost, i.e., its mirror image is
non-superimposable with itself, such molecule is known as “Asymmetric molecule”.
eg. 2-Iodobutane
I
I

i.
ii.
C*
H5C2
C*
CH3
H3C
C2H5
H
iii.
iv.

i.
ii.
iii.
iv.
H
B
A
a.
2-Iodobutane contains asymmetric carbon atom, its mirror image is non-superimposable on each other.
b.
B is the mirror image of A. Position of CH3 group in A does not coincide with the position in mirror
image B.
c.
Same is the case for ethyl group also. Under such condition, mirror image is non-superimposable on
each other.
Therefore, it can be said that molecule on whole must be non-superimposable on its mirror image, such a
molecule is called as “Chiral Molecule” and the property of non-superimposability is called “Chirality”.
Chiral molecule exists as d and l form and is an optically active molecule whereas molecule which is
superimposable on its mirror image is called “Achiral molecule” and it does not exist as d and l form,
therefore are optically inactive.
Vant Hoff Le Bel Theory:
Phenomenon of optical activity was discovered by French physicist Biot in 1815.
Though optical activity was discovered, for many years direct correlation between optical activity and the
structure of molecule was not known.
First convincing explanation for this corelation was given by Dutch scientist J. Vant Hoff and French
scientist Le Bel in 1874.
They independently put forward the theory of optical activity.
19
Chemistry Vol ‐ 2.2 (Med. and Engg.)
v.
vi.
vii.
Almost all the scientists until 1874 believed that all the molecules are always two dimensional i.e., they are
flat entities.
Van’t Hoff and Le Bel for the first time proposed the three dimensional structure of molecules.
According to this theory:
a.
In sp3 hybridized carbon atom, all four valencies are pointed towards four corners of regular tetrahedron.
b.
If all the four valencies of carbon atom are satisfied by different atoms or groups of atoms, then Catom is known as asymmetric carbon atom.
c.
1.
In an asymmetric carbon atom, two bonds are on plane of the paper shown by ordinary line.
2.
One bond is below the plane of paper (i.e., away from observer) and is shown by dotted line.
3.
The other bond is above the plane of paper (i.e., towards the observer) and is shown by thick wedge.
eg. 2-Iodobutane
I
H5C2

i.
ii.
iii.
iv.
C*
CH3
H
d.
Asymmetric centre is denoted by asterisk (*) on it.
e.
Molecules containing asymmetric carbon atom always exists as a pair of isomers which are nonsuperimposable mirror images of each other.
Optical Activity of Lactic Acid:
Number of possible stereoisomers shown by a molecule is dependent upon the number of asymmetric
carbon atoms present.
Number of possible stereoisomer is given by a formula = 2n
where, n = number of asymmetric carbon atom(s) present in that molecule.
Lactic acid contains one asymmetric carbon atom which is attached to COOH, OH, CH3 groups and H
atom. Therefore, Number of stereoisomers = 2n = 21
Lactic acid shows 2 stereoisomers i.e., d and l form which are non-superimposable mirror images of each other.
COOH
COOH
C*
H
C*
OH
CH3
d-lactic acid
H
HO
CH3
l-lactic acid
Note:
i.
Stereoisomers which are non-superimposable mirror images of each other and rotate the plane of the plane
polarized light through the same angle but in opposite directions are known as enantiomers or enantiomorphs.
eg. d and l forms of lactic acid are called enantiomorphs of each other.
ii.
Enantiomers have identical physical properties (except the direction of rotation of plane polarized light,
though the amount of rotation is same) and chemical properties (except towards optically active reagents.)

i.
ii.
20
R, S Configuration:
R, S nomenclature system was devised by Cahn, Ingold and Prelog which indicates configuration i.e.,
arrangements of atoms or groups around chirality centre.
Rules for R, S nomenclature to determine the priority of groups attached to chiral centre are given as follows:
a.
Groups arranged around chiral centres are given a priority order.
Higher the atomic number of an atom directly attached to chiral centre, higher its priority.
b.
If two groups have identical atom directly attached to the chiral centre then the next atom in group is
considered to determine the priority.
Chapter 10: Haloalkanes and Haloarenes
eg.
Cl

H  C  CH3

C2H5
Order of priority: Cl > C2H5 > CH3 > H
2-Chlorobutane
c.
This is because carbon atom in methyl group is attached to 3 hydrogen atoms, whereas in ethyl group
it is attached to 2 hydrogen and 1 carbon. Hence ethyl group is given higher priority.
When group(s) having multiple bonds (CHO,  C  N, >C = C<) is/are attached to chiral centre,
then atoms attached to double or triple bond are considered as duplicate or triplicate.
Priority is given by considering triplicate and duplicate structure as shown below:
H
H
H H
H
H
 CC
 CO
C=C
; C=O
(C) (C)
(O) (C)
(N)
CN

C
 C  N  (C)
; C=O
(N) (C)
eg.
C

CO
(O) (C)
C
C
 C = O is given higher priority over  C = C  because in  C = O , carbon is attached to
H
H
2 oxygen atoms (one oxygen + one phantom oxygen) and in  C = C  carbon atom is attached to
other 2 C atoms (1 C-atom and other phantom C-atom).
iii.
iv.
d.
In the case of benzene ring, it is considered as one of the resonating structure.

(C)
(C)
C
 HC  CH


(C)
e.
The order of priority is as follows:
I > Br > Cl > SO3H > F > OCOCH3 > OH > NO2 > NH2 > COOCH3 > COOH > CONH2 > COCH3
> CHO > CH2OH > CN > C6H5 > C2H5 > CH3 > D > H
Tetrahedral structure of molecules can be drawn as follows:
Two bonds in the plane of paper (indicated by line), one bond above the plane of paper (indicated by thick
wedge) and one bond below the plane of paper (indicated by dotted line).
The group attached to the dotted line should have the least priority.
eg. 2-Chlorobutane (order of priority Cl > C2H5 > CH3 > H)
1
1
2
H5C2
Cl
Cl
C*
C*
3
CH3
(a)
4
H
4
H
3
CH3
(b)
2
C2H5
21
Chemistry Vol ‐ 2.2 (Med. and Engg.)
v.
It is like holding dotted line in a hand and looking at the structure from opposite side (as if viewing a
bouquet of flowers) then with this view, order of priority is 1  2  3  4.
vi. In structure (b), order of priority is in clockwise direction i.e., from right therefore it is
R-configuration (Latin word rectus meaning right).
vii. In structure (a), order of priority is in anticlockwise direction i.e., from left therefore it is S-configuration
(Latin word sinister meaning left).
viii. Two more examples:
1
1
a.
OH
OH
2
HOOC
C*
4
3
CH3
H
S –Lactic acid
b.
C*
4
H
3
CH3
R –Lactic acid
1
1
I
C*
2
H5C6
3
C(CH3)2
2
COOH
I
4
H
S –1-Iodo-2-methyl-1-phenylpropane
4
H
C*
2
C6H5
C(CH3)2
3
R-1-Iodo-2-methyl-1-phenylpropane
10.8 Nucleophilic substitution mechanism

i.
ii.
Nucleophilic substitution reaction:
A reaction in which one nucleophile is substituted by other nucleophile is known as “Nucleophilic
Substitution Reaction”.
Nucleophilic substitution reaction can proceed by two different paths depending on the nature of substrate,
the nucleophile, the leaving group and solvent. The two paths are:
a.
SN1
b.
SN2 mechanisms.
Note: i.
ii.
iii.
iv.
22
Mechanism of reaction:
Mechanism of reaction is a step by step description of exactly how the reactants are transformed into
product in as much details as possible.
Transition state:
During the course of reaction, reactants change from one form to other through certain state. This
state is known as Transition state.
The minimum energy necessary to fulfill all the conditions for the formation of transition state is
called as the energy of activation of the reaction (Eact).
Energy Profile Diagram:
The energy changes of chemical reaction are depicted by energy profile diagram which shows the
progress of the reaction along a path from reactants to the product.
Rate Determining Step (R. D. S):
a.
Slowest step in the reaction mechanism which determines the rate of reaction is known as Rate
Determining Step.
b.
The rate determining step involves breaking of bond which requires input of energy and hence
it is the slowest step in the course of the reaction.
Chapter 10: Haloalkanes and Haloarenes

i.
SN2 Mechanism:
When primary alkyl halide reacts with aqueous alkali, corresponding primary alcohol is formed. The
reaction is called as hydrolysis of an alkyl halide.
R  CH2  X

OH 
 R  CH2  OH
+
1 Alkyl halide
eg.
H3C  Br
(aq)
+ X
1 Alcohol

+ NaOH(aq) 
 CH3  OH + NaBr
Methyl bromide
Methyl alcohol

In this reaction, OH nucleophile substitutes halide ion of 1 alkyl halide to form 1 alcohol.

 H3C  OH + Br
H3C  Br + OH 
Methyl bromide
Methyl alcohol
ii.
In this reaction, the rate of formation of 1 alcohol is found to be proportional to the concentration of an
alkyl halide and also to that of base used.
Rate  [H3C  Br] [OH]
Rate = k [H3C  Br] [OH] (where k = proportionality constant)
The rate of reaction is dependent on the concentration of both the reactants. Therefore, it is second order
i.e., bimolecular reaction. Hence this reaction is known as “ Nucleophilic Substitution Bimolecular
Reaction” and denoted as “SN2”.
iii.
Mechanism:
a.
It is one step concerted mechanism in which the formation of C  OH bond and breaking of C  Br
bond takes place simultaneously.
b.
The formation of transition state is slow step i.e., rate determining step.
c.
Transition state (T. S.) is the highest energy state in the course of reaction.
d.
It may change into product or may go back to reactants.
e.
In the transition state, both incoming nucleophile (OH) and outgoing halide group (X) share the
negative charge and C-atom carry partial positive charge.
f.
When the C  OH bond forms completely, at the same instant C  Br bond breaks completely and the
reaction is completed.
iv.
Energy Profile Diagram:
R = reactants: CH3  Br + OH
T. S. = transition state
Eact = Activation energy
ΔH = Heat of reaction
P = Products: H3C  OH + Br
Potential energy 
T.S.
Eact
R
Reaction co-ordinate 
H
P
Energy profile diagram for SN2 mechanism
In SN2 mechanism, heat of reaction (ΔH) is negative, hence it is exothermic reaction. The product formed is
of much lower energy than reactant, therefore product is more stable.
v.
Stereochemistry:
a.
In SN2 mechanism, nucleophile (OH) attacks the C-atom of 1 alkyl halide from backside, this is due
to the following reasons:
1.
It is least hindered (crowded) site for attack of OH.
2.
Electrostatic attraction between carbon atom (with + charge) and OH (with  charge).
3.
Electrostatic repulsion between OH and Br is minimum.
23
Chemistry Vol ‐ 2.2 (Med. and Engg.)
H1
H1
C*
HO +
slow step


R.D.S.
H2

+

C
HO
Br
H1
H3
H2
H3
Transition state
1alkyl halide
+ Br
HO
H3
H2
C*
Fast


Br
1 alcohol
(Inversion of configuration)
Backside attack of nucleophile
SN mechanism results in the inversion of configuration i.e., in the product, OH occupies position exactly
opposite to that of Br and positions of H2 and H3 atoms are exactly opposite in product to that in reactant.
2
b.
Note:
i.
Order of reactivity for halide atom is I > Br > Cl > F; because as size of atom increases, the bond
dissociation energy decreases.
ii.
Reactivity of an alkyl halide in SN2 mechanism is in the following order:
CH3X > 1 alkyl halide > 2 alkyl halide > 3 alkyl halide.
H
H
H
Nu
H
C
H
C
X
H
Methyl
halide
H
Nu
H
X
H
C
H
C
H
H
H
H
Ethyl
halide (1)
H
C
H
C
H
H
C
H
iii.
iv.
v.
In 1896, Paul Walden theoretically anticipated inversion of configuration.
In 1935, Ingold and Hughes gave experimental evidences for inversion of configuration.
The inversion is known as ‘Walden’ inversion.

i.
SN1 Mechanism:
When tertiary alkyl halide reacts with aqueous alkali, tertiary alcohol is formed.
R1
R1


R  C X + NaOH  R2  C  OH + NaX


(aq.)
R3
R3
eg.
CH3

H3C  C  Br +

CH3
tert-Butyl bromide
(3 Alkyl halide)
24
CH3

NaOH  H3C  C  OH

(aq.)
CH3
tert-Butyl alcohol
(3 Alcohol)
+
NaBr
H
tert-Butyl halide (3)
(Maximum steric
hindrance)
Steric hindrance increases
3 Alcohol
X
C
H
H
H
Isopropyl
halide (2)
3 Alkyl halide
C

X Nu
C
Nu
H
Chapter 10: Haloalkanes and Haloarenes
ii.

iii.
Study of reaction kinetics shows that, the rate of formation of 3 alcohol is proportional to the concentration
of only 3 alkyl halide.
Rate  [(H3C)3C  X]
Rate = k [(H3C)3C  X]
It is the first order reaction i.e., unimolecular reaction. Hence this reaction is known as “Nucleophilic
Substitution Unimolecular Reaction” and denoted as “SN1”.
Mechanism: A two step mechanism has been proposed for this type of substitution.
a.
The first step is a slow (rate determining) step, which involves heterolytic cleavage of C  X bond to
form a carbocation as an intermediate.
CH3
CH3

Slow step


+ X
C*
C+
R.D.S.
X
CH3
CH3
CH3
Carbocation
3 Alkyl halide
Second step is fast, in which nucleophile OH attacks highly reactive carbocation, to form a product.
CH3
CH3

Fast
C*

+ OH 
C+
b.
CH3
OH
CH3
CH3
CH3
Carbocation
iv.
CH3
3 Alcohol
Energy Profile Diagram:
R
R = reactant i.e., R  C  X
Potential energy
T.S.1
R
T.S.1 = Transition state of first step
R
= Carbonium ion/carbocation
T.S.2
E act2
E act1
R
R
+
C
C
(where R, R and R may
be same or different.)
R
R
T.S.2 = Transition state of second step
Eact1 = Activation energy of first step
Eact2 = Activation energy of second step
ΔH = heat of reaction
R
R
R
+
ΔH
P
Reaction co-ordinates
Energy profile diagram for SN1 mechanism
P = product i.e., R  C  OH
R
From energy profile diagram it is clear that, in SN mechanism ΔH is negative. Hence, it is an exothermic
reaction.
Stereochemistry:
a.
In this reaction, carbocation formed has planar structure (C-atom is sp2 hybridized) therefore
nucleophile OH can attack from both front and back side of carbocation.
b.
Back side attack of OH results in the inversion of configuration i.e., OH occupies position opposite
to halide ion and position of remaining group is opposite to that of reactant.
1
v.
25
Chemistry Vol ‐ 2.2 (Med. and Engg.)
c.
d.
Front side attack of OH results in the retention of configuration i.e., position of X is taken up by
OH and remaining groups are exactly at the same position as that of reactant.
In SN1 mechanism of optically active reactants, the two configurations formed are nonsuperimposable mirror images of each other i.e., enantiomers and they are formed in nearly equal
proportions. Therefore product formed is a racemic mixture () which is optically inactive.
R1
C*
R1

C+
Slow step


R.D.S.
X
R2
R2
R3
+ X
R3
Carbocation
3 Alkyl halide
R1
R1
C*
R2
HO
R1

C+
Back side


attack
R2
R3
Inversion of
configuration (50%)
C*
Front side


attack
OH
R2
R3
R3
HO
Retention of
configuration (50%)
Note:
i.
In SN1 mechanism, the reactivity of the halide, R-X, follows order: R–I > R–Br > R – Cl > R–F; because as
size of atom increases, the bond dissociation energy decreases.
ii.
Stability order for carbocation is 3 > 2 > 1.
CH3
eg.
a.


H 3C , H3C  CH2

H3C  CH

CH3
b.
1 Carbocation
(Least stable)
2 Carbocation
iii.
iv.
+
C
H3C
CH3
3 Carbocation
(Most stable)
3 alkyl halides prefer SN1 mechanism; 2 alkyl halides show mixed mechanism whereas 1 alkyl halides
prefer SN2 mechanism.
Halides in which halogen atom is bonded to a sp3 hybridized carbon atom next to an aromatic ring are
called benzylic halides.
CH2X
(where X = F, Cl, Br, I)
Benzylic halide
eg.
a.
CH2Br

Bromophenylmethane
(1 benzylic halide)
b.
CH3

H3C  C  Br

2-Bromo-2-phenylpropane
(3 benzylic halide)
26
c.
Chapter 10: Haloalkanes and Haloarenes
v.
Benzylic halides form carbocation which undergoes stabilization through resonance as follows:
+
+
CH2
CH2
CH2
CH2
+
+
(IV)
(III)
3
Halides in which halogen atom is bonded to a sp hybridized carbon atom next to a carbon carbon double
bond are called as allylic halides.
(I)
vi.
(II)
CH2X
3-Haloprop-1-ene
(Allyl halide)
eg.
a.
Cl

H2C = CH  CH2
b.
CH3

H3C  CH = CH  CH  I
4-Iodopent-2-ene
(2 allylic halide)
3-Chloroprop-1-ene
(1 allylic halide)
3-Haloprop-1-ene forms carbocation which undergoes stabilization through resonance as follows:
+
+
CH2 = CH  CH2  H2C  CH = CH2
vii.
Benzylic and allylic halides may be primary, secondary or tertiary in nature; but they undergo SN1 mechanism.

Comparison between SN2 and SN1:
SN2
SN1
No.
Factor
i.
Kinetics
ii. Molecularity
iii. Number of steps
iv. Bond making and
bond breaking
2 order
Bimolecular
One step
Simultaneous
v.
vi.
One step, one transition state
Only back side attack
1 order
Unimolecular
Two steps
First the bond in the reactant breaks
and then a new bond in product is
formed
Two steps, two transition state
Back side attack and front side attack
viii.
ix.
x.
xi.
Inversion of configuration
(If substrate is optically active)
Mainly 1 substrates
Non-polar solvent favourable
Strong Nucleophile favourable
No intermediate
Racemisation (If substrate is optically
active)
Mainly 3 substrates
Polar solvent favourable
Weak Nucleophile favourable
Intermediate involved
Transition state
Direction of attack
of nucleophile
vii. Stereochemistry
Type of substrate
Polarity of solvent
Nucleophile
Intermediate
nd
st
10.9 Haloarenes

i.
ii.
Haloarenes:
The halogen derivatives of aromatic hydrocarbons are called as haloarenes or aryl halides.
OR
Haloarenes are obtained by replacing one or more hydrogen atom(s) of an arene with corresponding
number of halogen atom(s).
They are obtained by substituting H-atom of aromatic ring with halogen atom.
Ar  H
X2

 Ar  X + HX
Lewis acid
(where X = F, Cl, Br, I)
310 K  320 K
Benzene
Haloarene
27
Chemistry Vol ‐ 2.2 (Med. and Engg.)

Classification:
Depending upon the number of halogen atom(s) attached to an aromatic ring, they are classified as follows:
Haloarenes
Dihaloarenes
Two halogen atoms
are attached to
benzene.
eg.
Cl
Cl
Monohaloarenes
One halogen atom is
attached to benzene.
eg.
Cl
Trihaloarenes
Three halogen atoms
are
attached
to
benzene.
eg.
Cl
Cl
Cl
Chlorobenzene

1,2-Dichlorobenzene
1,2,3-Trichlorobenzene
Polyhaloarenes
More than three halogen
atoms are attached to
benzene.
Cl
eg.
Cl
Cl
Cl
1,2,3,5-Tetrachlorobenzene
Nomenclature:
Common and IUPAC names of some of the haloarenes:
Sr. No.
Structure
Common Name
Chlorobenzene
IUPAC Name
Chlorobenzene
o-Chlorotoluene
1-Chloro-2-methylbenzene or
2-Chlorotoluene
m-Chlorotoluene
1-Chloro-3-methylbenzene or
3- Chlorotoluene
p-Chlorotoluene
1-Chloro-4-methylbenzene or
4-Chlorotoluene
Br
Bromobenzene
Bromobenzene
Br
o-Dibromobenzene
1,2-Dibromobenzene
m-Dibromobenzene
1,3-Dibromobenzene
p-Dibromobenzene
1,4-Dibromobenzene
Cl
i.
ii.
iii.
2
CH3
Cl
1
3
CH3
1
Cl
CH3
4
iv.
1
Cl
v.
vi.
vii.
viii.
1
Br
1
3 Br
Br
1
4
28
2 Br
Br
Chapter 10: Haloalkanes and Haloarenes
Br
1
ix.
m-Bromochlorobenzene
1-Bromo-3-chlorobenzene
sym-Tribromobenzene
1,3,5-Tribromobenzene
—
1,2,3,5-Tetrabromobenzene
3 Cl
Br
1
x.
Br
5
3 Br
Br
Br
xi.
Br
Br
10.10 Nature of C  X bond in haloarenes
i.
ii.
In haloarenes, halogen atom having p-orbital with unpaired electron overlaps with sp2 hybrid orbital of Catom of benzene ring to form C  X bond.
Lone pair of electrons from halogen atom is involved in  electron system of aromatic ring, showing
extended conjugation. As result of resonance, C  X bond shows following structures:
+
X
iii.
+
+
X
X
X
(where X = F, Cl, Br, I)
Thus, in aryl halides, C  X bond acquires partial double bond character making itself stronger and shorter
in length than in alkyl halides.
10.11 Preparation of haloarenes
Haloarenes can be prepared by the following methods:
i.
By Electrophilic Substitution:
a.
Chloroarenes and bromoarenes can be prepared from benzene or aromatic hydrocarbons by treatment
with Cl2 or Br2 in the presence of Lewis acid like iron, FeCl3, FeBr3, BCl3, AlCl3, etc. at ordinary
temperatures (310 K  320 K).
H
Lewis acid


+ X2 
310K  320K
dark
Benzene
b.
c.
+ HX
Halobenzene
This method is called as direct halogenation of aromatic compounds.
Lewis acid acts as catalyst and halogen carrier for electrophilic substitution.
H
Br
FeBr3
+ HBr


+ Br2 
310K  320K
dark
Benzene
d.
X
Bromobenzene
If excess of reagent is used, then second halogen atom is introduced at ortho or para position with
respect to the first halogen. This is because halogens are o-, p-directing groups.
Br
Br
Br
Br
FeBr3
+ HBr
+


+ Br2 
310K  320K
Bromobenzene
excess
dark
o-Dibromobenzene
Br
p-Dibromobenzene
The ortho and para isomers can be easily separated due to large difference in their melting points.
29
Chemistry Vol ‐ 2.2 (Med. and Engg.)
e.
Direct iodination of benzene ring is a reversible reaction due to the HI (strong reducing agent) which
is formed as a byproduct, hence reaction is carried out in the presence of strong oxidising agent (like
nitric acid or iodic acid or mercuric oxide).
I

+ I2 
+ HI
Benzene
Iodobenzene
The above reversible reaction can proceed in forward direction in presence of an oxidising agent.
eg.
1.
5HI
+

HIO3
3H2O + 3I2
Iodic acid
2.
2HI
+

HgO
Mercuric oxide
f.
ii.
HgI2 + H2O
Fluorine reacts violently and uncontrollably with benzene or other aromatic hydrocarbons. Thus,
fluoroarene compounds cannot be prepared by direct fluorination method.
Sandmeyer’s reaction:
a.
When primary aromatic amine (like aniline) is treated with sodium nitrite and dilute HCl at 273 K –
278 K, it results in the formation of benzene diazonium salt.
b.
Reaction of freshly prepared diazonium salt solution with cuprous (I) salt (cuprous chloride or
cuprous bromide dissolved in corresponding halogen acids) results in the formation of chloro or
bromobenzene respectively. This reaction is known as “Sandmeyer’s reaction”.


eg.
NH2
Cl
N  NCl
Cu 2 Cl2 / HCl
NaNO2 /dil.HCl




+ N2
1.
 HCl
273K  278K,
 H 2O
Aniline
Benzene
diazonium chloride
NH2
2.
Aniline
c.

Chlorobenzene

N  NCl

NaNO2 /dil.HCl
273K  278K,
 H 2O
Cu 2 Br2 / HBr


 HCl
Benzene
diazonium chloride
Br
+ N2
Bromobenzene
Diazonium salt on treatment with KI gives iodobenzene.


I
eg.
N  NCl
+ N2 + KCl
+ KI 
Benzene
diazonium chloride
Iodobenzene
10.12 Physical and chemical properties of haloarenes

Physical properties:
i.
Density of haloarenes:
ii.
a.
Bromo, iodo and polychloro derivatives of arenes are heavier than water.
b.
The density of haloarenes increases with increase in number of carbon atoms, halogen atoms and
atomic mass of halogen atoms.
Melting and boiling points of haloarenes:
Boiling points of isomeric dihalobenzenes are nearly the same. However, the melting point of para-isomer
is higher as compared to ortho- and meta-isomers. It is because of the symmetry of para-isomers which fit
in the crystal lattice better as compared to ortho- and meta isomers.
30
Chapter 10: Haloalkanes and Haloarenes
Cl
eg.
Cl
Cl
Cl
Cl
o-Dichlorobenzene
m-Dichlorobenzene
Cl
p-Dichlorobenzene
Boiling point (K)
Melting point (K)

i.

i.
ii.
453
256
446
249
448
323
Chemical properties:
The reactions of haloarenes include:
Substitution reactions
ii.
Reactions with metals
Substitution reactions:
Haloarenes undergo substitution reactions which can be either nucleophilic substitution or electrophilic
substitution.
Aryl halides are less reactive than alkyl halides and do not undergo nucleophilic substitution reactions
easily due to the following reasons:
a.
Resonance effect:
1.
In aryl halides, the C  X bond acquires partial double bond character due to resonating structures.
2.
This makes the C  X bond cleavage in aryl halide more difficult than the C  X bond cleavage in
akyl halide.
b.
Different hybridization states of C-atom in C  X bond:
Type of
compound
Alkyl halide
Aryl halide
eg.
1.
State of hybridization of
C-atom in a CX bond
sp3
sp2
% s-character
Less (25%)
More (33.33%)
Bond length of Strength of CX
bond
C  X bond
Longer
Weaker
Shorter
Stronger
X-ray analysis confirms that CCl bond length in chlorobenzene is 169 pm while that in methyl
chloride is 177 pm.
177 pm
H3C
Cl
Cl
169 pm
2.
c.
1.
2.
3.
Reduction in bond length imparts stability, making bond cleavage difficult in aryl halides.
Polarity of CX bond:
sp2 hybridised C-atom; Less tendency to release e s towards X-atom.
sp3 hybridised C-atom; More tendency to release e s towards X-atom.
Thus sp2 hybridized C-atom is more electronegative than sp3 hybridized C-atom.
eg.
Dipole moment
Compound
1.73 D
Chlorobenzene (Aryl halide)
2.05 D
Chloroethane (Alkyl halide)
4.

d.
Polarity  reactivity
Lesser polarity of aryl halides, lesser is their reactivity compared to alkyl halides.
Repulsion:
Electron rich arenes repel the attacking nucleophile (which is also electron rich); resulting in the
lesser reactivity towards substitution reactions.
Instability of phenyl cation:
Phenyl cation formed due to self ionization of aryl halide will not be stabilized by resonance.
e.
1.
31
Chemistry Vol ‐ 2.2 (Med. and Engg.)
Unstable phenyl cation (carbocation) cannot undergo SN1 reaction, thus ruling out the possibility of
SN1 mechanism.
3.
Pi () electrons of aromatic ring blocks the backside attack of nucleophile, thus ruling out the
possibility of SN2 mechanism.
However, under drastic conditions, aryl halides undergo nucleophilic substitution reactions.
Nucleophilic Substitution Reactions:
Under drastic conditions like high temperature and under pressure, halide group attached to arenes can be
replaced by OH, CN or NH2 group.
Dow’s Process: When aryl halide reacts with NaOH at 623 K under pressure of 200 atm300 atm, forms
sodium phenoxide which on acidification gives phenol. This process is known as “Dow’s process”.
Cl
ONa
OH
2.
iii.

i.
dil.HCl


 NaCl
623K,  HCl
+ NaOH 
200atm  300atm
Chlorobenzene
ii.
Sodium phenoxide
Phenol
Chlorobenzene on heating with anhydrous copper cyanide and sodium cyanide at 473 K under pressure
gives cyanobenzene.
Cl
CN
anhydrous CuCN  NaCN

+ CN 
473 K, pressure
Chlorobenzene
iii.
+ Cl
Cyanobenzene
Chlorobenzene on heating with aqueous NH3 in the presence of catalyst cuprous oxide, at 473 K under
pressure gives aniline.
Cl
NH2
2
Chlorobenzene
iv.
Aniline
Effect of substituents on the reactivity of haloarenes:
a.
It has been found that presence of electron withdrawing groups like NO2, COOH, CN at o –
and/or p – position with respect to halogen atom greatly activates haloarenes to undergo nucleophilic
displacement reactions.
eg. o  and p – Nitrochlorobenzene easily undergo nucleophilic attack of OH to give o  and p –
Nitrophenol.
1.
 NO2 group is at ortho position with respect to halogen atom:
Cl

HO
+
O
N O
HO


slow step
Cl + O
N
O

Cl + O
N 
O
HO
HO


Resonating structures
o-Nitrochlorobenzene
HO

Cl + O
N

O
fast step



Resonance hybrid
32
+ 2CuCl + H2O
473K
+ 2NH3 + Cu2O 
 2
under pressure
OH
o-Nitrophenol
+
N
O

O
+ Cl

Cl
+
N
O
O
Chapter 10: Haloalkanes and Haloarenes
2.
NO2 group is at para position with respect to halogen atom:
Cl
HO

HO
Cl
Cl
HO


slow step



+
N
O
+
N
O

O
p-Nitrochlorobenzene

Cl

O

O
N
N

+
O
O
Cl
+
fast step


Resonance hybrid
e.
O
OH
+
c.
d.

O
N
Resonating structures
HO
b.
+
+
N
O
O
Cl
HO

O
p-Nitrophenol
From the above mechanism it is clear that, carbanion formed by the attack of OH gets stabilized
because of  electrons of benzene ring as well as negative charge on C-atom attached to electron
withdrawing NO2 group.
Hence, o- and p-substituted aryl halides show greater reactivity towards nucleophilic attack.
But in the case of m-substituted aryl halide, there is no negative charge at m-position in the
resonating structures; due to this the presence of electron withdrawing group at m-position has no
effect on reactivity.
It is observed that as number of electron withdrawing groups at ortho - and para-position (with
respect to halogen atom) increases, the reactivity of haloarenes also increases.
eg.
OH
Cl
1.
NO2
NO2
(i) NaOH
+ OH 


+ Cl
(ii) H ,368K
NO2
NO2
1-Chloro-2,4-dinitrobenzene
2,4-Dinitrophenol
(55% yield)
Cl
2.
OH
NO2
O2N
NO2
O2N
warm
+ OH 

H2O
NO2
1-Chloro-2,4,6-trinitrobenzene
+ Cl
NO2
2,4,6-Trinitrophenol
(Picric acid)
(93% yield)

i.
Electrophilic Substitution Reactions:
In the case of chlorobenzene, following resonating structures are obtained.
Cl
Cl+
Cl+
Cl+
ii.
In chlorobenzene, electron density is more at o  and p – position (since chlorine is o  and p  directing).
33
Chemistry Vol ‐ 2.2 (Med. and Engg.)
iii.
iv.
v.
It is observed that, halogen atoms are highly electronegative, they pull  electrons of benzene ring towards
themselves due to – I effect and hence aryl halides show reactivity towards electrophilic attack.
Hence, weaker resonating structures control o , p  orientations and stronger inductive effect controls
reactivity of aryl halides.
When an electrophile (E) attacks on ortho and/or para positions of aryl halide; more stable chloronium ion
is formed as follows:
Cl+
Cl+
H
E
Carbocation at
o-position/
chloronium ion
vi.
vii.
H E
Carbocation at
p-position/
chloronium ion
Attack of an electrophile at meta position forms comparitively less stable chloronium ion.
Thus, electrophilic substitution reaction in aryl halide (i.e., chlorobenzene) occurs slowly and under drastic
conditions compared to benzene.
viii. Halogenation: Chlorobenzene reacts with Cl2 in the presence of anhydrous FeCl3 or sunlight to give
o-dichlorobenzene or p-dichlorobenzene.
Cl
Cl
Cl
eg.
Cl
anhydrous FeCl3
+ Cl2 

+ HCl
+
or sunlight
Chlorobenzene
o-Dichlorobenzene
(Minor product)
Cl
p-Dichlorobenzene
(Major product)
Note: a.
Benzene when treated with chlorine in the presence of bright sunlight or ultraviolet light, adds up
three molecules of chlorine to give benzene hexachloride/BHC (C6H6Cl6).
H
H
Cl
|
C
HC
HC
CH
+
C
CH
bright sunlight
3Cl2 

or UV light
H
Cl
|
H
H
ix.
H
C
C
C
C
C
Cl
Benzene
b.
c.
d.
e.
C
Cl
Cl
H
Cl
H
Benzene hexachloride (BHC)
This is an addition type of reaction.
Benzene hexachloride is commercially known as BHC.
It exists in eight isomeric forms.
The gamma () isomer is called “Gammexane” or “Lindane” which is used as an insecticide.
Nitration: Chlorobenzene reacts with nitrating mixture i.e., conc.HNO3 and conc. H2SO4 to give 1-Chloro4-nitrobenzene (major product) and 1-Chloro-2-nitrobenzene (minor product). This reaction is known as
“Nitration”.
Cl
Cl
Cl
NO2
conc.H 2SO 4
+ HNO3 

+
+ H2O

(conc.)
Chlorobenzene
NO2
1-Chloro-4-nitrobenzene
(Major product)
34
1-Chloro-2-nitrobenzene
(Minor product)
Chapter 10: Haloalkanes and Haloarenes
x.
Sulphonation: Chlorobenzene on heating with conc. H2SO4 yields 4-chlorobenzene sulphonic acid (major
product) and 2-chlorobenzene sulphonic acid (minor product).
Cl
Cl
Cl
+

H2SO4 

SO3H
+
+ H2O
(conc.)
Chlorobenzene
xi.
SO3H
4-Chlorobenzene
sulphonic acid
(Major product)
2-Chlorobenzene
sulphonic acid
(Minor product)
Friedel-Craft’s Reaction:
a.
Introduction of an alkyl or acyl group in the haloarene ring or in the substituted benzene ring in the
presence of anhydrous aluminium trichloride is known as “Friedel-Craft’s Reaction”.
b.
The reaction can be carried out by reacting aryl halide (i) with alkyl chloride (Friedel-Craft’s
alkylation reaction) or (ii) with acyl chloride (Friedel-Craft’s acylation reaction).
eg.
Cl
Cl
CH3
CH3Cl
Chloromethane
+
+ HCl
Cl
1-Chloro-4-methylbenzene
(Major product)
O
Chlorobenzene
1-Chloro-2-methylbenzene
(Minor product)
CH3
anhydrous
AlCl3
Cl
H3C  C  Cl
Cl
O
C  CH3
Ethanoyl chloride
(Acetyl chloride)
+ HCl
+
C  CH3
2-Chloroacetophenone
(Minor product)
O
4-Chloroacetophenone
(Major product)

i.
Reaction with Sodium Metal (Wurtz Fittig Reaction):
When an aryl halide is heated with alkyl halide, it undergoes coupling reaction in the presence of sodium
metal and dry ether to give alkyl benzene. This reaction is known as “Wurtz Fittig Reaction”.
eg.
dry ether

CH3 + 2NaCl
Cl + 2Na + Cl  CH3 
Chlorobenzene
ii.
Methyl
chloride
Methyl benzene
(Toluene)
In the above reaction, along with toluene, ethane (obtained by coupling of two methyl groups) and diphenyl
(obtained by coupling of two phenyl groups) are also produced as byproducts.
dry ether

 CH3  CH3 + 2NaCl
a.
2H3C  Cl + 2Na 
Methyl
chloride
Ethane
Cl
b.
2
Chlorobenzene
dry ether

+ 2Na 
+ 2NaCl
Diphenyl
Note: Reaction of haloarenes with sodium metal is called as “Fittig reaction”.
35