Class- XII Chemistry Chapter-10 Haloalkane and Haloarene Haloalkane The replacement of hydrogen atom(s) in an aliphatic hydrocarbon by halogen atom(s) results in the formation of alkyl halide (haloalkane). Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group. Haloarene The replacement of hydrogen atom(s) in an aromatic hydrocarbon by halogen atom(s) results in the formation of aryl halide (haloarene). Haloarenes contain halogen atom(s) attached to sp2hybridised carbon atom(s) of an aryl group. Classification of Haloalkanes and Haloarenes On theBasis ofNumber ofHalogenAtoms According to the hybridisation of the carbon atom to which the halogen is bonded. Compounds Containing sp3 C—X Bond (X= F,Cl,Br,I) (a) Alkyl halides or haloalkanes (R—X) In alkyl halides, the halogen atom is bonded to an alkyl group (R). They form a homologous series represented by CnH2n+1X. They are further classified as primary, secondary or tertiary.
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Class- XII
Chemistry
Chapter-10
Haloalkane and Haloarene
Haloalkane
The replacement of hydrogen atom(s) in an aliphatic hydrocarbon by halogen atom(s)
results in the formation of alkyl halide (haloalkane).
Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an
alkyl group.
Haloarene
The replacement of hydrogen atom(s) in an aromatic hydrocarbon by halogen atom(s)
results in the formation of aryl halide (haloarene).
Haloarenes contain halogen atom(s) attached to sp2hybridised carbon atom(s) of an
aryl group.
Classification of Haloalkanes and Haloarenes
On theBasis ofNumber ofHalogenAtoms
According to the hybridisation of the carbon atom to which the halogen is
bonded.
Compounds Containing sp3 C—X Bond (X= F,Cl,Br,I)
(a) Alkyl halides or haloalkanes (R—X)
In alkyl halides, the halogen atom is bonded to an alkyl group (R). They form a
homologous series represented by CnH2n+1X. They are further classified as primary,
secondary or tertiary.
(b) Allylic halides
These are the compounds in which the halogen atom is bonded to a sp3-hybridised
carbon atom adjacent to carbon-carbon double bond (C=C) i.e. to an allylic carbon.
(c) Benzylic halides
These are the compounds in which the halogen atom is bonded to a sp3-hybridised
carbon atom attached to an aromatic ring.
* Compounds Containing sp2 C—X Bond
(a) Vinylic halides
These are the compounds in which the halogen atom is bonded to a sp2-hybridised
carbon atom of a carbon-carbon double bond (C = C).
(b) Aryl halides
These are the compounds in which the halogen atom is directly bonded to the sp2
hybridised carbon atom of an aromatic ring.
a) From Alcohols
b) From Hydrocarbons
(I) From alkanes by free radical halogenation
are the compounds in which the halogen atom is directly bonded to the sp2
hybridised carbon atom of an aromatic ring.
Preparation of Haloalkanes
(I) From alkanes by free radical halogenation
are the compounds in which the halogen atom is directly bonded to the sp2-
Free radical chlorination or bromination of alkanes gives a complex mixture of isomeric
mono- and polyhaloalkanes, which is difficult to separate as pure compounds.
(II) From alkenes
i)Addition of hydrogen halides
by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide.
(II)Addition of halogens:
In the laboratory, addition of bromine in CCl
reddish-brown colour
of bromine constitutes an important method
molecule. The addition results in the synthesis of Vic
Vic-dibromide
chlorination or bromination of alkanes gives a complex mixture of isomeric
and polyhaloalkanes, which is difficult to separate as pure compounds.
Addition of hydrogen halides: An alkene is converted to corresponding alkyl halide
by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide.
In the laboratory, addition of bromine in CCl4 to an alkene resulting in discharge of
of bromine constitutes an important method for the detection of double bond in a
molecule. The addition results in the synthesis of Vic-dibromides, which are colourless.
chlorination or bromination of alkanes gives a complex mixture of isomeric
and polyhaloalkanes, which is difficult to separate as pure compounds.
: An alkene is converted to corresponding alkyl halide
by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide.
to an alkene resulting in discharge of
for the detection of double bond in a
dibromides, which are colourless.
c)From Halogen Exchange
Preparation of Haloarenes
I)From hydrocarbons by electrophilic substitution
Aryl chlorides and bromides can be easily prepared by electrophilic substitution of
arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts
like iron or iron (III) chloride. The ortho and para isomers can be easily separated due
to large difference in their melting points.
(ii) From amines by Sandmeyer’s reaction
or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium
salt is formed. Mixing the solution of freshly prepared diazonium salt with cuprous
chloride or cuprous bromide results in the r
or –Br.
Replacement of the diazonium group by iodine does not require the presence of
cuprous halide and is done simply by shaking the diazonium salt with potassium iodide.
Preparation of Haloarenes
I)From hydrocarbons by electrophilic substitution
ides and bromides can be easily prepared by electrophilic substitution of
arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts
like iron or iron (III) chloride. The ortho and para isomers can be easily separated due
e difference in their melting points.
(ii) From amines by Sandmeyer’s reactionWhen a primary aromatic amine, dissolved
or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium
salt is formed. Mixing the solution of freshly prepared diazonium salt with cuprous
chloride or cuprous bromide results in the replacement of the diazonium group by
Replacement of the diazonium group by iodine does not require the presence of
cuprous halide and is done simply by shaking the diazonium salt with potassium iodide.
ides and bromides can be easily prepared by electrophilic substitution of
arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts
like iron or iron (III) chloride. The ortho and para isomers can be easily separated due
When a primary aromatic amine, dissolved
or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium
salt is formed. Mixing the solution of freshly prepared diazonium salt with cuprous
eplacement of the diazonium group by –Cl
Replacement of the diazonium group by iodine does not require the presence of
cuprous halide and is done simply by shaking the diazonium salt with potassium iodide.
Melting and boiling points
For the same alkyl group, the boiling points of alkyl halides decrease in the order:
Thisis because with the increase in size and mass of halogen atom, the magnitude of
van der Waal forces increases.
The boiling points of isomeric haloalkanes decrease with increase in branching
The para-isomers are high melting as compared to their ortho
due to symmetry of para-isomers that fits in crystal lattice better as compared to
ortho- and meta-isomers.
Density
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The
density increases with increase in number of carbon atoms, halogen atoms and atomic
mass of the halogen atoms.
Solubility
The haloalkanes are very slightly soluble in
dissolve in organic solvents.
Chemical Properties of Haloalkanes
The reactions of haloalkanes may be divided into the following categories:
1. Nucleophilic substitution
2. Elimination reactions
3. Reaction with metals.
Nucleophiles are electron rich species. Therefore, they attack at that part of the
substrate molecule which is electron deficient. Examples:
Physical Properties
For the same alkyl group, the boiling points of alkyl halides decrease in the order:
RI> RBr> RCl> RF
Thisis because with the increase in size and mass of halogen atom, the magnitude of
van der Waal forces increases.
someric haloalkanes decrease with increase in branching
isomers are high melting as compared to their ortho-and meta
isomers that fits in crystal lattice better as compared to
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The
density increases with increase in number of carbon atoms, halogen atoms and atomic
The haloalkanes are very slightly soluble in water. However, haloalkanes tend to
Chemical Properties of Haloalkanes
The reactions of haloalkanes may be divided into the following categories:
ucleophiles are electron rich species. Therefore, they attack at that part of the
substrate molecule which is electron deficient. Examples:
For the same alkyl group, the boiling points of alkyl halides decrease in the order:
Thisis because with the increase in size and mass of halogen atom, the magnitude of
someric haloalkanes decrease with increase in branching.
and meta-isomers. It is
isomers that fits in crystal lattice better as compared to
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The
density increases with increase in number of carbon atoms, halogen atoms and atomic
, haloalkanes tend to
The reactions of haloalkanes may be divided into the following categories:
ucleophiles are electron rich species. Therefore, they attack at that part of the
Ambident nucleophiles:
Groups which possess two nucleophilic centres are called ambident nucleophiles.
Actually, cyanide group is a hybrid of two contributing structures and therefore can act
as a nucleophile in two different ways
[-C≡N ↔: C=N-], i.e., linking through carbon atom resulting in alkyl cyanides [RCN] and
through nitrogen atom leading to isocyanides [RNC
Similarly, nitrite ion also represents an ambident
nucleophile with two different points of linkage
. The linkage through oxygen results in alkyl nitrites
NO2].
(1) Nucleophilic substitution
The reaction in which a nucleophile replaces already existing nucleophile in a molecule
is called nucleophilicsubstitution reaction. In this type of reaction, a nucleophile reacts
with haloalkane (the substrate) having a partial positive charge
bonded to halogen. A substitution reaction takes place and halogen atom, called
leaving group departs as halide ion. Since the substitution reaction is initiated by a
nucleophile, it is called nucleophilic substitution reaction.
Groups which possess two nucleophilic centres are called ambident nucleophiles.
cyanide group is a hybrid of two contributing structures and therefore can act
as a nucleophile in two different ways
], i.e., linking through carbon atom resulting in alkyl cyanides [RCN] and
through nitrogen atom leading to isocyanides [RNC].
Similarly, nitrite ion also represents an ambident
nucleophile with two different points of linkage
. The linkage through oxygen results in alkyl nitrites
while through nitrogen atom, it leads to nitroalkanes [R
reactions:
The reaction in which a nucleophile replaces already existing nucleophile in a molecule
is called nucleophilicsubstitution reaction. In this type of reaction, a nucleophile reacts
with haloalkane (the substrate) having a partial positive charge on the carbon atom
bonded to halogen. A substitution reaction takes place and halogen atom, called
leaving group departs as halide ion. Since the substitution reaction is initiated by a
nucleophile, it is called nucleophilic substitution reaction.
Groups which possess two nucleophilic centres are called ambident nucleophiles.
cyanide group is a hybrid of two contributing structures and therefore can act
], i.e., linking through carbon atom resulting in alkyl cyanides [RCN] and
while through nitrogen atom, it leads to nitroalkanes [R
The reaction in which a nucleophile replaces already existing nucleophile in a molecule
is called nucleophilicsubstitution reaction. In this type of reaction, a nucleophile reacts
on the carbon atom
bonded to halogen. A substitution reaction takes place and halogen atom, called
leaving group departs as halide ion. Since the substitution reaction is initiated by a
Mechanism:
Substitution nucleophilic bimolecular (SN2)
The term ‘SN2’ stands for – Substitution Nucleophilic Bimolecular. This type of
reaction is also referred to as bimolecular nucleophilic substitution, associative
substitution, and interchange mechanism.
The SN2 reaction is a nucleophilic substitution reaction where a bond is broken and
another is formed simultaneously in a single step and no intermediate is formed. Two
reacting species are involved in the rate determining step of the reaction.
Reaction Kinetics: Since an SN2 Reaction is a second-order reaction, the rate-
determining step is dependent on the concentration of nucleophile as well as the
concentration of the substrate”.
SN2 reaction mechanism requires the attack of nucleophile from the back side of the
carbon atom. So, the product assumes a stereochemical position opposite to the
leaving group originally occupied. This is called inversion of configuration.
Of the simple alkyl halides, methyl halides react most rapidly in SN2 reactions because
there are only three small hydrogen atoms. Tertiary halides are the least reactive
because bulky groups hinder the approaching nucleophiles. Thus, the order of