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
reactivity followed is:
Primary halide > Secondary halide > Tertiary halid
Substitution nucleophilic unimolecular (SN1)
SN1 reactions are generally carried out in polar protic solvents (like water, alcohol,
acetic acid, etc.). The reaction between tert-butyl bromide and hydroxide ion yields
tert-butyl alcohol and
follows the first order kinetics, i.e., the rate of reaction depends upon the
concentration of only one reactant, which is tert- butyl bromide.
It occurs in two steps. In step I, the polarised C—Br bond undergoes slow cleavage to
produce a carbocation and a bromide ion. The carbocation thus formed is then
attacked by nucleophile in step II to complete the substitution reaction.
Step 1 is the slowest and reversible. It involves the C–Br bond breaking for which the
energy is obtained through solvation of halide ion with the proton of protic solvent.
Since the rate of reaction depends upon the slowest step, the rate of reaction depends
only on the concentration of alkyl halide and not on the concentration of hydroxide
ion. Further, greater
the stability of carbocation, greater will be its ease of formation from alkyl halide and
faster
will be the rate of reaction. In case of alkyl halides, 30 alkyl halides undergo SN1
reaction
very fast because of the high stability of 30 carbocations.
For the same reasons, allylic and benzylic halides show high reactivity towards the SN1
reaction. The carbocation thus formed gets stabilised through resonance as shown
below:
Difference between SN1 and SN2
SN1 SN2
The rate of reaction is
unimolecular.
The rate of reaction is bimolecular
It follows the first order kinetics It follows the second order kinetics
It is a twostep mechanism It is only a one step mechanism
Carbocation is formed as an
intermediate part of the
reaction.
No carbocation is formed during the
reaction.
There is no partial bond formed
with the carbon during this
reaction.
Carbon forms a partial bond with the
nucleophile and the leaving group.
There are many steps in this
reaction which start with the
removal of the group while
attacking the nucleophile.
The process takes place in only one
cycle, with a single intermediate
stage.
the rate of reaction depends
only on the concentration of
substrate and not on the
concentration of nucleophile
the rate of reaction depends on the
concentration of both the substrate
and the nucleophile
Some basic stereochemical principles and notations
1)Optical Activity/Optically Active Substances: -
When a plane polarised light is made to pass through certain substances, the plane of
polarisation of the emergent light is not the same as that of incident light, but it has
been rotated through some angle. This phenomenon is known as optical activity. The
substances which rotate the plane of polarisation are said to be optically active.
Examples: quartz, sugar crystals, turpentine oil, sodium chloride etc.
Optically active substances are of two types,
Dextro-rotatory (right-handed)
Laevo - rotatory (left-handed)
Dextro-rotatory (right-handed) which rotate the plane of polarisation in the clock wise
direction on looking towards the source.
Laevo - rotatory (left-handed) which rotate the plane of polarisation in the
anti-clockwise direction on looking towards the source.
2) Chirality
Chirality is defined as “an object which is asymmetric and cannot be superimposed
over its mirror image is known as chiral or stereo enter”. This property is known as
chirality.
For example- our Hand, legs etc.
The object which is symmetric in nature and can be superimposed over its mirror
image is known as achiral.
For example- cube, cone etc.
chirality
Optically active substances are of two types,
handed)
handed)
handed) which rotate the plane of polarisation in the clock wise
direction on looking towards the source.
handed) which rotate the plane of polarisation in the
clockwise direction on looking towards the source.
“an object which is asymmetric and cannot be superimposed
is known as chiral or stereo enter”. This property is known as
our Hand, legs etc.
The object which is symmetric in nature and can be superimposed over its mirror
handed) which rotate the plane of polarisation in the clock wise
handed) which rotate the plane of polarisation in the
“an object which is asymmetric and cannot be superimposed
is known as chiral or stereo enter”. This property is known as
The object which is symmetric in nature and can be superimposed over its mirror
Polarimeter:
The angle by which the plane polarised light is rotated is measured by an instrument
called polarimeter.
Enantiomers:
Enantiomers are a pair of molecules that exist in two forms that are mirror images of
one another but cannot be superimposed one upon the other.
Enantiomers possess identical physical properties namely, melting point, boiling point,
solubility, refractive index, etc. They only differ with respect to the rotation of plane
polarised light. If one of the enantiomers is dextro rotatory, the other will be laevo
rotatory.
Racemic mixture:
A mixture containing two enantiomers in equal proportions will have zero optical
rotation, as the rotation due to one isomer will be cancelled by the rotation due to t
other isomer. Such a mixture is known as racemic mixture or racemic modification. A
racemic mixture is represented by prefixing dl or (±) before the name, for example (±)
butan-2-ol. The process of conversion of enantiomer into a racemic mixture is know
as racemisation.
Retention: If during a reaction, no bond to the stereo centre is broken, the product will
have the same general configuration of groups around the stereo centre as that of
reactant. Such a reaction is said to proceed with retention of
The angle by which the plane polarised light is rotated is measured by an instrument
Enantiomers are a pair of molecules that exist in two forms that are mirror images of
er but cannot be superimposed one upon the other.
Enantiomers possess identical physical properties namely, melting point, boiling point,
solubility, refractive index, etc. They only differ with respect to the rotation of plane
the enantiomers is dextro rotatory, the other will be laevo
A mixture containing two enantiomers in equal proportions will have zero optical
rotation, as the rotation due to one isomer will be cancelled by the rotation due to t
other isomer. Such a mixture is known as racemic mixture or racemic modification. A
racemic mixture is represented by prefixing dl or (±) before the name, for example (±)
ol. The process of conversion of enantiomer into a racemic mixture is know
If during a reaction, no bond to the stereo centre is broken, the product will
have the same general configuration of groups around the stereo centre as that of
reactant. Such a reaction is said to proceed with retention of the configuration.
The angle by which the plane polarised light is rotated is measured by an instrument
Enantiomers are a pair of molecules that exist in two forms that are mirror images of
Enantiomers possess identical physical properties namely, melting point, boiling point,
solubility, refractive index, etc. They only differ with respect to the rotation of plane
the enantiomers is dextro rotatory, the other will be laevo
A mixture containing two enantiomers in equal proportions will have zero optical
rotation, as the rotation due to one isomer will be cancelled by the rotation due to the
other isomer. Such a mixture is known as racemic mixture or racemic modification. A
racemic mixture is represented by prefixing dl or (±) before the name, for example (±)
ol. The process of conversion of enantiomer into a racemic mixture is known
If during a reaction, no bond to the stereo centre is broken, the product will
have the same general configuration of groups around the stereo centre as that of
the configuration.
Inversion of configuration: A process in which the relative configuration of an atom is
changed. Since a molecule can form two enantiomers around a chiral centre, the
Walden inversion converts the configuration of the molecule from one enantiomeric
form to another.
If (A) is the only compound obtained, the process is called retention of configuration.
If (B) is the only compound obtained, the process is called inversion of configuration.
If a 50:50 mixture of the above two is obtained then the process is called racemisation
and the product is optically inactive, as one isomer will rotate light in the direction
opposite to another.
2. Elimination reactions:
When a haloalkane with β-hydrogen atom is heated with alcoholic solution of
potassium hydroxide, there is elimination of hydro
halogen atom from the α-carbon atom. As a result, an alkene is formed as a product.
Since β-hydrogen atom is involved in elimination, it is often called β
A process in which the relative configuration of an atom is
changed. Since a molecule can form two enantiomers around a chiral centre, the
converts the configuration of the molecule from one enantiomeric
nly compound obtained, the process is called retention of configuration.
If (B) is the only compound obtained, the process is called inversion of configuration.
If a 50:50 mixture of the above two is obtained then the process is called racemisation
he product is optically inactive, as one isomer will rotate light in the direction
hydrogen atom is heated with alcoholic solution of
potassium hydroxide, there is elimination of hydrogen atom from β
carbon atom. As a result, an alkene is formed as a product.
hydrogen atom is involved in elimination, it is often called β
A process in which the relative configuration of an atom is
changed. Since a molecule can form two enantiomers around a chiral centre, the
converts the configuration of the molecule from one enantiomeric
nly compound obtained, the process is called retention of configuration.
If (B) is the only compound obtained, the process is called inversion of configuration.
If a 50:50 mixture of the above two is obtained then the process is called racemisation
he product is optically inactive, as one isomer will rotate light in the direction
hydrogen atom is heated with alcoholic solution of
gen atom from β-carbon and a
carbon atom. As a result, an alkene is formed as a product.
hydrogen atom is involved in elimination, it is often called β-elimination.
Alexander Zaitsev (also pronounced as
can be summarised as “in dehydrohalogenation reactions, the preferred product is
that alkene which has the greater number of alkyl groups attached to the doubly
bonded carbon atoms.”
3. Reaction with metals Most organic chlorides, bromides and iodides react with
certain metals to give compounds containing carbon
are known as organo-metallic compounds
Grignard Reagents: Important class of organo
Victor Grignard in 1900 is alkyl magnesium halide, RMgX, referred as
Reagents.
These reagents are obtained by the reaction of haloalkanes with magnesium metal in
dry ether.
Wurtz reaction
Alkyl halides react with sodium in dry ether to give hydro
the number of carbon atoms present in the halide. This reaction is
known as Wurtz reaction.
1. Nucleophilic substitution:
(also pronounced as Saytzeff) who in 1875 formulated a rule which
“in dehydrohalogenation reactions, the preferred product is
that alkene which has the greater number of alkyl groups attached to the doubly
Most organic chlorides, bromides and iodides react with
certain metals to give compounds containing carbon-metal bonds. Such compounds
metallic compounds.
Important class of organo-metallic compounds discovered by
ctor Grignard in 1900 is alkyl magnesium halide, RMgX, referred as
These reagents are obtained by the reaction of haloalkanes with magnesium metal in
Alkyl halides react with sodium in dry ether to give hydrocarbons containing double
the number of carbon atoms present in the halide. This reaction is
Reactions of Haloarenes
75 formulated a rule which
“in dehydrohalogenation reactions, the preferred product is
that alkene which has the greater number of alkyl groups attached to the doubly
Most organic chlorides, bromides and iodides react with
metal bonds. Such compounds
metallic compounds discovered by
ctor Grignard in 1900 is alkyl magnesium halide, RMgX, referred as Grignard
These reagents are obtained by the reaction of haloalkanes with magnesium metal in
carbons containing double
Aryl halides are extremely less reactive towards nucleophilic substitution reactions
due to the following reasons:
(i) Resonance effect: In haloarenes, the electron pairs on halogen atom are in
conjugation with π-electrons of the ring and the following resona
are possible. C—Cl bond acquires a partial double bond character due to
resonance. As a result, the bond cleavage in haloarene is difficult than
haloalkane and therefore, they are less reactive towards nucleophilic
substitution reaction.
(ii) Difference in hybridisation of carbon atom in C
In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of
haloarene, the carbon atom attached to halogen is sp2
The sp2 hybridised carbon with a greater s
hold the electron pair of C—X bond more tightly than sp3
haloalkane with less s-character. Thus, C
in haloarene is 169 pm. Since it is difficult to b
therefore, haloarenes are less reactive than haloalkanes towards nucleophilic
substitution reaction.
(iii) Instability of phenyl cation:
In case of haloarenes, the phenyl cation formed as a result of self
be stabilised by resonance and therefore, SN1 mechanism is ruled out.
(iv) Because of the possible repulsion
to approach electron rich arenes.
2. Electrophilic substitution reactions
(i) Halogenation
Aryl halides are extremely less reactive towards nucleophilic substitution reactions
due to the following reasons:
In haloarenes, the electron pairs on halogen atom are in
electrons of the ring and the following resona
Cl bond acquires a partial double bond character due to
resonance. As a result, the bond cleavage in haloarene is difficult than
haloalkane and therefore, they are less reactive towards nucleophilic
substitution reaction.
Difference in hybridisation of carbon atom in C—X bond:
In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of
haloarene, the carbon atom attached to halogen is sp2-hybridised.
The sp2 hybridised carbon with a greater s-character is more electronegative and can
X bond more tightly than sp3-hybridised carbon in
character. Thus, C—Cl bond length in haloalkane is 177pm while
in haloarene is 169 pm. Since it is difficult to break a shorter bond than a longer bond,
therefore, haloarenes are less reactive than haloalkanes towards nucleophilic
Instability of phenyl cation:
In case of haloarenes, the phenyl cation formed as a result of self-ionisation will n
be stabilised by resonance and therefore, SN1 mechanism is ruled out.
(iv) Because of the possible repulsion, it is less likely for the electron rich nucleophile
to approach electron rich arenes.
2. Electrophilic substitution reactions
Aryl halides are extremely less reactive towards nucleophilic substitution reactions
In haloarenes, the electron pairs on halogen atom are in
electrons of the ring and the following resonating structures
Cl bond acquires a partial double bond character due to
resonance. As a result, the bond cleavage in haloarene is difficult than
haloalkane and therefore, they are less reactive towards nucleophilic
In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of
character is more electronegative and can
hybridised carbon in
Cl bond length in haloalkane is 177pm while
reak a shorter bond than a longer bond,
therefore, haloarenes are less reactive than haloalkanes towards nucleophilic
ionisation will not
be stabilised by resonance and therefore, SN1 mechanism is ruled out.
, it is less likely for the electron rich nucleophile
(ii) Nitration
(iii) Sulphonation
(iv) Friedel-Crafts reaction
3. Reaction with metals
Wurtz-Fittig reaction A mixture of an alkyl halide and aryl halide gives an alkylarene
when treated with sodium in dry ether and is
A mixture of an alkyl halide and aryl halide gives an alkylarene
when treated with sodium in dry ether and is
A mixture of an alkyl halide and aryl halide gives an alkylarene
called Wurtz-Fittig reaction.
Fittig reaction
Aryl halides also give analogous compounds when treated with sodium in dry ether, in
which two aryl groups are joined together. It is called Fittig reaction.
NCERT INTEXT QUESTIONS:
Page no. 285
Answer 10.1
Aryl halides also give analogous compounds when treated with sodium in dry ether, in
aryl groups are joined together. It is called Fittig reaction.
Aryl halides also give analogous compounds when treated with sodium in dry ether, in
aryl groups are joined together. It is called Fittig reaction.
Page no. 289
Answer: 10.2
H2SO4 is an oxidising agent. It oxidises HI produced during the reaction to I
prevents the reaction between an alcohol and HI to form alkyl iodide. To prevent this,
a nonoxidizing acid like H3PO3
Answer:10.3
Four isomers are possible. These are:
is an oxidising agent. It oxidises HI produced during the reaction to I
reaction between an alcohol and HI to form alkyl iodide. To prevent this,
a nonoxidizing acid like H3PO3 is used.
Four isomers are possible. These are:
is an oxidising agent. It oxidises HI produced during the reaction to I2 and thus
reaction between an alcohol and HI to form alkyl iodide. To prevent this,
Answer:10.4
Answer:10.5
Page No. 291
Answer:10.6
(i) Chloromethane < Bromomethane < Dibromo methane < Bromoform
The reason is:
(a)for same alkyl group, BoilingPoint increases with size of halogen atom.
(b)Boiling Point increases as number of halogen atoms increase.
(ii) Isopropyl chloride < 1 – Chloropropane < 1 – Chlorobutane
Reason:
(a)For same halogen, Boiling Point increases as size of alkyl group increases.
(b)Boiling Point decreases as branching increases.
Page No.307.
Answer: 10.7
In SN2 mechanism, reactivity depends upon the steric hindrance around the C-atom
carrying the halogen. Lesser the steric hindrance, faster the reaction.
Answer:10.8
Answer: 10.9
NCERT EXRECISES
Answer:10.1
(i) 2-Chloro-3methylbutane, 2° alkyl halide
(ii) 3-Chloro-4methyl hexane, 2° alkyl halide
(iii) 1 -Iodo-2,2-dimethylbutane, 1 ° alkyl halide
(iv) l-Bromo-3, 3-dimethyl -1-phenylbutane, 2° benzylic halide
(v) 2-Bromo-3-methylbutane, 2° alkyl halide
(vi) 1-Bromo-2-ethyI-2-methylbutane, 1° alkyl halide
(vii)3-Chloro-3-methylpentane, 3° alkyl halide
(viii) 3-Chloro-5-methylhex-2-ene, vinylic halide
(ix)4-Bromo-4-methylpent-2-ene, allylic halide
(x)1-Chloro-4-(2-methylpropyl) benzene, aryl halide
(xi)1-Chloromethyl-3- (2,2-dimethylpropyl) benzene, 1 ° benzylic halide.
(xii)1-Bromo-2-(l-methyl propyl) benzene, aryl halide.
Answer:10.2
(i) 2-Bromo-3-chlorobutane
(ii) 1 Bromo-1 -chloro-1,2,2-trifluoroethane
(iii) l-Bromo-4-chlorobut-2-ene
(iv)2-(Trichloromethyl)-l, 1,1,2,3,3,3- heptachloropropane
(v)2-Bromo-3,3-bis-(4-chlorophenyl) butane
(vi)l-Chloro-l-(4-iodophenyl)-3,3- dimethyl but-l-ene.
Answer:10.3
Answer: 10.4
The three-dimensional structures of the three compounds along with the direction of
dipole moment in each of their bonds are given below:
CCl4 being symmetrical has zero dipole moment. In CHCl3, the resultant of two C – Cl
dipole moments is opposed by the resultant of C – H and C – Cl bonds. Since the dipole
moment of latter resultant is expected to be smaller than the former, CHCl3 has a finite
dipole (1.03 D) moment.
In CH2CI2, the resultant of two C – Cl dipole moments is reinforced by resultant of two
C – H dipoles, therefore, CH2CI2 (1 .62 D) has a dipole moment higher than that of
CHCl3. Thus, CH2CI2 has highest dipole moment.
Answer:10.5
The hydrocarbon with molecular formula C5H10 can either a cycloalkane or an alkene.
Since the compound does not react with Cl2 in the dark, therefore it cannot be an
alkene but must be a cycloalkane. Since the cycloalkane reacts with Cl2 in the presence
of bright sunlight to give a single monochloro compound, C5H9Cl, therefore, all the ten
hydrogen atoms of the cycloalkanes must be equivalent. Thus, the cycloalkane is
cyclopentane.
Answer:10.6
Answer: 10.7
Answer: 10.8
Nucleophiles which can attack through two different sites are called ambident
nucleophiles. For example, cyanide ion is a resonance hybrid of the following two
structures:
Nucleophiles which can attack through two different sites are called ambident
nucleophiles. For example, cyanide ion is a resonance hybrid of the following two
Nucleophiles which can attack through two different sites are called ambident
nucleophiles. For example, cyanide ion is a resonance hybrid of the following two
It can attack through carbon to form cyanide and through N to form is isocyanide.
Answer: 10.9
(i)Since I– ion is a better leaving group than Br
in SN2 reaction with OH– ion.
(ii)On steric grounds, 1° alkyl halides are more reactive than tert
reactions. Therefore, CH3CI will react at a faster rate than (CH
with OH– ion.
Answer:10.10
It can attack through carbon to form cyanide and through N to form is isocyanide.
ion is a better leaving group than Br- ion, therefore, CH3I reacts faster CH
(ii)On steric grounds, 1° alkyl halides are more reactive than tert-alkyl halides in SN
CI will react at a faster rate than (CH3)3CCl in a SN
It can attack through carbon to form cyanide and through N to form is isocyanide.
I reacts faster CH3Br
alkyl halides in SN2
CCl in a SN2 reaction
Answer: 10.11
Answer: 10.12
(i)
The Cl- atom in chlorobenzene is linked to sp2
cyclohexyl chloride it is linked to sp3
carbon has more s-character, it is more electronegative than sp3
atom. Hence, the density of electrons of C
chlorobenzene than in cyclohexyl chloride.
Moreover, the electron density is reduced by the
chlorobenzene. Hence, chlorobenzene has a decreased polarity of C
dipole moment is lower than that of cyclohexyl chloride.
(ii)Alkyl halides are polar molecules; therefore, their molecules are held together by
dipole-dipole attraction. The molecules of H2O are hold together by H
new forces of attraction between water and alkyl halide molecules are weaker than
the forces of attraction already existing between alkyl halide
and water-water molecules, therefore, alkyl halides are immiscible (not soluble) in
water. Alkyl halide are neither able to form H
the H-bounding network of water.
(iii)Grignard reagents are very reactive. They react with moistu
apparatus to form alkanes
Thus, Grignard reagents must be prepared under anhydrous conditions.
Answer: 10.14
atom in chlorobenzene is linked to sp2 hybridized carbon atom, whereas in
cyclohexyl chloride it is linked to sp3 hybridized carbon atom. Now, as sp2
character, it is more electronegative than sp3
atom. Hence, the density of electrons of C-Cl bond near the Cl atom is less in
chlorobenzene than in cyclohexyl chloride.
Moreover, the electron density is reduced by the –R effect of the b
chlorobenzene. Hence, chlorobenzene has a decreased polarity of C
dipole moment is lower than that of cyclohexyl chloride.
(ii)Alkyl halides are polar molecules; therefore, their molecules are held together by
attraction. The molecules of H2O are hold together by H
new forces of attraction between water and alkyl halide molecules are weaker than
the forces of attraction already existing between alkyl halide – alkyl halide molecules
ter molecules, therefore, alkyl halides are immiscible (not soluble) in
water. Alkyl halide are neither able to form H- bonds with water nor are able to break
bounding network of water.
(iii)Grignard reagents are very reactive. They react with moistu
apparatus to form alkanes
Thus, Grignard reagents must be prepared under anhydrous conditions.
hybridized carbon atom, whereas in
hybridized carbon atom. Now, as sp2 hybridized
character, it is more electronegative than sp3 hybridized carbon
Cl bond near the Cl atom is less in
R effect of the benzene ring of
chlorobenzene. Hence, chlorobenzene has a decreased polarity of C-Cl bond, also the
(ii)Alkyl halides are polar molecules; therefore, their molecules are held together by
attraction. The molecules of H2O are hold together by H-bonds. Since the
new forces of attraction between water and alkyl halide molecules are weaker than
alkyl halide molecules
ter molecules, therefore, alkyl halides are immiscible (not soluble) in
bonds with water nor are able to break
(iii)Grignard reagents are very reactive. They react with moisture present in the
apparatus to form alkanes
Thus, Grignard reagents must be prepared under anhydrous conditions.
Answer: 10.15
KCN is a resonance hybrid of the following two contributing structures:
Thus, CN– ion is an ambident nucleophile. Therefore, it can attack the “carbon atom of
C-Br bond in n-BuBr either through C or N. Since C – C bond is stronger than C – N
bond, therefore, attack occurs through C to form n
Answer:10.16
The SN2 reactions reactivity depends upon
slower the reaction. Thus, the order of reactivity will be 1°> 2° >3°
Since in case of 1° alkyl halides steric hindrance increases in the order) n
alkyl halides with a substituent at any positio
substituent at the β-position, two substituents at the β
reactivity decreases in the same order. Thus, the reactivity of the given alkyl bromides
decreases in the order:
1-Bromobutane > l-Bromo-3-me
dimethyl propane.
bond, therefore, attack occurs through C to form n-butyl cyanide.
The SN2 reactions reactivity depends upon steric hindrance. More the steric hindrance
slower the reaction. Thus, the order of reactivity will be 1°> 2° >3°
Since in case of 1° alkyl halides steric hindrance increases in the order) n
alkyl halides with a substituent at any position other than the β-position, one
position, two substituents at the β-position, therefore, the
reactivity decreases in the same order. Thus, the reactivity of the given alkyl bromides
methylbutane > l-Bromo-2-methyjbutane> 1
steric hindrance. More the steric hindrance
Since in case of 1° alkyl halides steric hindrance increases in the order) n-alkyl halides,
position, one
position, therefore, the
reactivity decreases in the same order. Thus, the reactivity of the given alkyl bromides
methyjbutane> 1-Bromo-2,2-
Answer: 10.17
C6H5CH2Cl is 10 aryl halides while C6H5CH(CI)C6H5 is a 2° aryl halide. In SN1 reactions, the
reactivity depends upon the stability of carbocations.
Since the C6H5CHC6H5carbocation is more stable than C6H5CH2 carbocation, therefore,
C6H5CHCIC6H5 gets hydrolysed more easily than C6H5CH2Cl under SN1 conditions.
However, under SN2 conditions, the reactivity depends on steric hindrance, therefore,
under SN2conditions, C6H5CH2Cl gets hydrolysed more easily than C6H5CHClC6H5.
Answer: 10.18
The p-isomer being more symmetrical fits closely in the crystal lattice and thus has
stronger inter-
molecular forces of attraction than o- and m-isomers. Since during melting or
dissolution, the crystal lattice breaks, therefore, a large amount of energy is needed to
melt or dissolve the p-isomer than the corresponding o-and m-isomers. In other
words, the melting point of the p-isomer is higher and its solubility lower than the
corresponding o-and m-isomers.
Answer:10.19
Answer: 10.20
If aqueous solution, KOH is almost completely ionized to give OH– ions which being a
strong nucleophile brings about a substitution reaction on alkyl halides to form
alcohols. Further in the aqueous solution, OH– ions are highly solvated (hydrated). This
solvation reduces the basic character of OH
hydrogen from the P-carbon of the alkyl chloride to form alkenes. In contrast, an
alcoholic solution of KOH contai
base than OH– ions preferentially eliminate a molecule of HCl from an alkyl chloride to
form alkenes.
Answer: 10.21
There are two primary alkyl halides having the molecular formula, C4H9Br.
(ii) Since compound (a) when reacted with Na metal gave a compound (d) with
molecular formula C8H18 which was different from die compound obtained when n
butyl bromide was reacted with Na metal, therefore, (a) must be isobutyl bromide and
compound (d) must be 2,3-dimeth
(iii) If compound (a) is isobutyl bromide, than the compound (b) which it gives on
treatment with alcoholic KOH must be 2
(iv) The compound (b) on treatment with HBr gives compound (c) in accordance with
Markownikov’s rule. Therefore, compound (c) is tert
of compound (a) ,i.e., isobutyl ‘ bromide
solvation reduces the basic character of OH– ions which, therefore, fails to abstract a
carbon of the alkyl chloride to form alkenes. In contrast, an
alcoholic solution of KOH contains alkoxide (RO–) ion which being a much stronger
ions preferentially eliminate a molecule of HCl from an alkyl chloride to
There are two primary alkyl halides having the molecular formula, C4H9Br.
pound (a) when reacted with Na metal gave a compound (d) with
which was different from die compound obtained when n
butyl bromide was reacted with Na metal, therefore, (a) must be isobutyl bromide and
dimethylhexane.
(iii) If compound (a) is isobutyl bromide, than the compound (b) which it gives on
treatment with alcoholic KOH must be 2-methyl-1-propane.
(iv) The compound (b) on treatment with HBr gives compound (c) in accordance with
rule. Therefore, compound (c) is tert-butyl bromide which is an isomer
of compound (a) ,i.e., isobutyl ‘ bromide.
ions which, therefore, fails to abstract a
carbon of the alkyl chloride to form alkenes. In contrast, an
) ion which being a much stronger
ions preferentially eliminate a molecule of HCl from an alkyl chloride to
There are two primary alkyl halides having the molecular formula, C4H9Br.
pound (a) when reacted with Na metal gave a compound (d) with
which was different from die compound obtained when n-
butyl bromide was reacted with Na metal, therefore, (a) must be isobutyl bromide and
(iii) If compound (a) is isobutyl bromide, than the compound (b) which it gives on
(iv) The compound (b) on treatment with HBr gives compound (c) in accordance with
butyl bromide which is an isomer
Thus
(a)is isobutyl bromide,
(b)is 2-methyl-1 -propane,
(c)is tert-butyl bromide, and
(d)is 2,5-dimethylhexane.
Answer: 10.22