CHAPTER
10
Haloalkanes and
Haloarenes
10.1 Classification 10.5 Methods of Preparation of Haloarenes
10.2 Nomenclature 10.6 Physical Properties
10.3 Nature of C—X Bond 10.7 Chemical Reactions
10.4 Methods of Preparation of Haloalkones
Topicwise Analysis of Last 10 Years’ CBSE Board Questions (2020-2011)
30
28
26 VSA
24 SA
LA I
22
→
LA II
20
Number of questions
18
16
14
12
10
8
6
4
2
0
10.1 10.2 10.3 10.4 10 .5 10.6 10.7
Topic →
8 Maximum total weightage is of Chemical 8 Maximum SA and LA I type questions were
Reactions. asked from Chemical Reactions.
8 Maximum VSA type questions were asked from
Chemical Reactions.
QUICK RECAP
8 Haloalkanes and haloarenes : The replacement at hydrogen atom(s) in a hydrocarbon, aliphatic or
aromatic, by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide
(haloarene) respectively.
,8 Classification :
HALOALKANES Pyridine
R OH + SOCl2
8 Alkyl halides : General formula is RX, where R Cl + SO2↑ + HCl↑
R = alkyl group. X Halogenation of alkanes :
8 General methods of preparation : R H + X2 RX + HX
X From alcohols : Alkyl halides can be
– Abstraction of hydrogen for a particular
prepared from alcohols in a number of ways :
halogen follows the order
– Action of hydrogen halides :
allylic > 3° > 2° > 1° > CH4
RCl + H2O – Reactivity of halogens for the above
reaction follows the order :
R – OH R – Br + H2O F2 > Cl2 > Br2 > I2
X Halide exchange method :
R – I + H2O dry acetone
R Cl + NaI R I + NaCl
• Order of reactivity of halogen acids This is known as Finkelstein reaction.
towards the above reaction is : – Fluoroalkanes can also be prepared by
HI > HBr > HCl halide exchange method by reacting alkyl
• Order of reactivity of different alcohols chloride with inorganic fluorides. This is
towards the above reaction is : known as Swart’s reaction.
tertiary > secondary > primary 2CH3CH2Cl + Hg2F2
– Action of phosphorus halides : Mercurous2CH3CH2F + Hg2Cl2
fluoride Mercurous
PCl5
R – Cl + POCl3 + HCl chloride
PCl3 X Action of hydrogen halides on ethers :
R – Cl + H3PO3
R O R + 2HX 2R – X + H2O
R – OH PBr3
(P + Br2)
R – Br + H3PO3 X Addition of hydrogen halides to alkene :
PI3
Alkenes add on a molecule of hydrogen
(P + I2)
R – I + H3PO3 halide to form alkyl halide.
– Action of thionyl chloride (Darzen’s C C + HX –C –C –
process) :
H X
, nucleophilic substitution reactions. Better
the leaving group, more facile is the
– Reaction follows electrophilic addition nucleophilic substitution reaction.
mechanism and takes place as per X SN1 and SN2 mechanisms : The nucleophilic
Markownikoff ’s rule. However, in presence substitution can proceed via SN1 mechanism
of peroxide, addition of HBr takes place or SN2 mechanism.
as per anti-Markownikoff ’s rule. Unimolecular (SN1) Bimolecular (SN2)
It is first order reaction. It is second order
reaction.
Generally carried out Carried out in polar
in polar protic solvents aprotic solvents like
like water, alcohol and acetone, DMSO,
acetic acid. acetonitrile or DMF.
Takes place in two steps Takes place in one step
Anti-Markownikoff ’s rule is also known as through carbocation as through transition state.
Peroxide effect or Kharasch effect. HCl and the intermediate.
HI do not show peroxide effect.
8 Physical properties : Rate of reaction : Rate of reaction :
X Alkyl halides being polar in nature are 3° > 2° > 1° > CH3 halides CH3 > 1° > 2° > 3° halides
(fastest) (slowest) (fastest) (slowest)
insoluble in water as they cannot break
Greater the stability of Less the steric
H-bonding already existing in water.
carbocation, faster will hindrance in transition
X They have higher melting and boiling
be the reaction. state, faster will be the
points, than those of the hydrocarbons of
reaction.
comparable molecular mass. For the same Tends to proceed with Tends to proceed with
alkyl group boiling point follows the order weak nucleophiles, strong nucleophiles,
– – –
as : e.g., CH3OH, H2O, e.g., CH3O , CN , OH ,
R I > R Br > R Cl > R F CH3CH2OH, etc. etc.
X Decreasing order of boiling points among Both retention Inversion of
the isomeric alkyl halides follows the order : and inversion of configuration takes
1° > 2° > 3° alkyl halides configuration takes place (Walden
X Decreasing order of density among the alkyl place. inversion).
halides is RI > RBr > RCl > RF. X Primary allylic and primary benzylic halides
For alkyl iodide decreasing order of density show higher reactivity in SN1 reactions
is as follows : than other primary alkyl halides due to the
CH3I > CH3CH2I > CH3CH2CH2I. greater stabilisation of allylic and benzylic
X Bond strength of C X bond follows the order carbocation intermediates by resonance.
CH3 F > CH3 Cl > CH3 Br > CH3 I X Vinylic and aryl halides are unreactive in
i.e., bond strength of C X bond decreases as nucleophilic substitution reactions. This is
the size of halogen atoms increases. because of double bond character of C X
X Correct stability order of RX is as follows : bond due to resonance.
R F > R Cl > R Br > R I
8 Optical isomerism/Enantiomerism :
8 Chemical properties : X Optical isomer is known as dextrorotatory
X Nucleophilic substitution reactions : The isomer (latin : dexter means right) (d-form
reactions in which a stronger nucleophile or +ve) if it rotates the plane polarised light
displaces a weaker nucleophile are called to the right (clockwise) and laevorotatory
, isomer (latin : laevo means left) (l-form or X Wurtz reaction :
–ve) if it rotates the plane polarised light to X + 2Na + X R Ether R
R R + 2NaX
the left (anticlockwise).
X An equimolar mixture of the d-form and 8 Reduction of alkyl halides :
l-form will be optically inactive and is called 2R X + 2[H]
Zn/HCl
2R H + ZnX2
racemic mixture (or dl- form or (±)-mixture).
X The process of conversion of an enantiomer HALOARENES
into racemic mixture is known as racemisation. 8 Aryl halides : General formula is ArX, where
X Chirality : The compound is said to have Ar = aryl group.
chirality if the central carbon atom is
8 General methods of preparation :
attached to four different groups and this
X By Raschig process :
centre is called chiral (asymmetric) centre or
CuCl2
stereogenic centre or stereocentre. 2C6H6 + 2HCl + O2 2C6H5Cl + 2H2O
500 K
X Achirality : The compound is said to have X From benzenediazonium salt :
achirality if the central carbon atom have
atleast two identical groups and this centre is
called achiral (symmetric).
X If molecule has a plane of symmetry it is
achiral (not chiral) and if molecule has no
plane of symmetry it is chiral.
X Enantiomers/d- and l-isomers : They are the
optical isomers which are non-superimposable
mirror images (or dissymmetric).
X Diastereomers : They are the optical isomers
which are not mirror images of each other.
They have different physical properties and
magnitude of specific rotation.
X Meso compounds : These compounds X By direct halogenation of benzene :
have two or more even number of chiral
carbon atoms and have an internal plane of
symmetry. They are optically inactive due to
internal compensation.
8 Elimination reactions: 4
X Dehydrohalogenation :
8 Physical properties :
X Aryl halides are colourless stable liquids with
pleasant odour.
H3C CH Cl + KOH (alc.) H3C CH CH2
X These are insoluble in water but readily
CH3 miscible with organic solvents.
8 Reaction with metals: X Most of them are steam volatile, heavier than
X Reaction with Mg metal : water.
Ether X Their boiling points are higher than
CH3I + Mg CH3MgI
OH corresponding alkyl halides. The boiling
CH3MgI + H2O CH4 + Mg points rise gradually from fluoro to iodo
I compounds.
, 8 Chemical properties : – Sulphonation :
X Nucleophilic substitution reactions : Cl Cl
Cl OH conc. H2SO4
SO3H
(i) NaOH, 623 K, 300 atm
(ii) H+ 2-Chlorobenzene-
sulphonic acid (Minor)
The presence of an electron withdrawing Cl
group ( NO2) at ortho- and para-positions
increases the reactivity of haloarenes. +
Cl OH
SO3H
(i) NaOH, 443 K
4-Chlorobenzene-
(ii) H+
sulphonic acid (Major)
NO2 NO2 – Friedal–Crafts reaction :
Cl Cl
Cl OH Anhyd. AlCl3 CH3
NO2 NO2 + CH3Cl
(i) NaOH, 368 K
(ii) H+ 1-Chloro-2-methylbenzene
(Minor)
NO2 NO2
Cl
Cl OH
O2N NO2 O2N NO2 +
warm
H 2O
NO2 NO2 CH3
1-Chloro-4-methylbenzene
X Electrophilic substitution reactions : (Major)
– Halogenation :
Cl Cl Cl Cl
O
Cl Anhyd. AlCl3
Anhyd. + H3C C Cl
+ Cl2 FeCl3
+
1, 2-Dichlorobenzene Cl O Cl
Cl (Minor)
1, 4-Dichlorobenzene CH3
(Major) +
– Nitration :
2-Chloroacetophenone
Cl Cl (Minor) O CH3
conc. HNO3 NO2 4-Chloroacetophenone
(Major)
conc. H2SO4
X Wurtz Fittig reaction :
1-Chloro-2-nitrobenzene
Ether
(Minor) R X + 2Na + X
Cl
R + 2NaX
+
X Fittig reaction :
NO2 X
Ether
1-Chloro-4-nitrobenzene 2 + 2Na + 2NaX
(Major)
Diphenyl
10
Haloalkanes and
Haloarenes
10.1 Classification 10.5 Methods of Preparation of Haloarenes
10.2 Nomenclature 10.6 Physical Properties
10.3 Nature of C—X Bond 10.7 Chemical Reactions
10.4 Methods of Preparation of Haloalkones
Topicwise Analysis of Last 10 Years’ CBSE Board Questions (2020-2011)
30
28
26 VSA
24 SA
LA I
22
→
LA II
20
Number of questions
18
16
14
12
10
8
6
4
2
0
10.1 10.2 10.3 10.4 10 .5 10.6 10.7
Topic →
8 Maximum total weightage is of Chemical 8 Maximum SA and LA I type questions were
Reactions. asked from Chemical Reactions.
8 Maximum VSA type questions were asked from
Chemical Reactions.
QUICK RECAP
8 Haloalkanes and haloarenes : The replacement at hydrogen atom(s) in a hydrocarbon, aliphatic or
aromatic, by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide
(haloarene) respectively.
,8 Classification :
HALOALKANES Pyridine
R OH + SOCl2
8 Alkyl halides : General formula is RX, where R Cl + SO2↑ + HCl↑
R = alkyl group. X Halogenation of alkanes :
8 General methods of preparation : R H + X2 RX + HX
X From alcohols : Alkyl halides can be
– Abstraction of hydrogen for a particular
prepared from alcohols in a number of ways :
halogen follows the order
– Action of hydrogen halides :
allylic > 3° > 2° > 1° > CH4
RCl + H2O – Reactivity of halogens for the above
reaction follows the order :
R – OH R – Br + H2O F2 > Cl2 > Br2 > I2
X Halide exchange method :
R – I + H2O dry acetone
R Cl + NaI R I + NaCl
• Order of reactivity of halogen acids This is known as Finkelstein reaction.
towards the above reaction is : – Fluoroalkanes can also be prepared by
HI > HBr > HCl halide exchange method by reacting alkyl
• Order of reactivity of different alcohols chloride with inorganic fluorides. This is
towards the above reaction is : known as Swart’s reaction.
tertiary > secondary > primary 2CH3CH2Cl + Hg2F2
– Action of phosphorus halides : Mercurous2CH3CH2F + Hg2Cl2
fluoride Mercurous
PCl5
R – Cl + POCl3 + HCl chloride
PCl3 X Action of hydrogen halides on ethers :
R – Cl + H3PO3
R O R + 2HX 2R – X + H2O
R – OH PBr3
(P + Br2)
R – Br + H3PO3 X Addition of hydrogen halides to alkene :
PI3
Alkenes add on a molecule of hydrogen
(P + I2)
R – I + H3PO3 halide to form alkyl halide.
– Action of thionyl chloride (Darzen’s C C + HX –C –C –
process) :
H X
, nucleophilic substitution reactions. Better
the leaving group, more facile is the
– Reaction follows electrophilic addition nucleophilic substitution reaction.
mechanism and takes place as per X SN1 and SN2 mechanisms : The nucleophilic
Markownikoff ’s rule. However, in presence substitution can proceed via SN1 mechanism
of peroxide, addition of HBr takes place or SN2 mechanism.
as per anti-Markownikoff ’s rule. Unimolecular (SN1) Bimolecular (SN2)
It is first order reaction. It is second order
reaction.
Generally carried out Carried out in polar
in polar protic solvents aprotic solvents like
like water, alcohol and acetone, DMSO,
acetic acid. acetonitrile or DMF.
Takes place in two steps Takes place in one step
Anti-Markownikoff ’s rule is also known as through carbocation as through transition state.
Peroxide effect or Kharasch effect. HCl and the intermediate.
HI do not show peroxide effect.
8 Physical properties : Rate of reaction : Rate of reaction :
X Alkyl halides being polar in nature are 3° > 2° > 1° > CH3 halides CH3 > 1° > 2° > 3° halides
(fastest) (slowest) (fastest) (slowest)
insoluble in water as they cannot break
Greater the stability of Less the steric
H-bonding already existing in water.
carbocation, faster will hindrance in transition
X They have higher melting and boiling
be the reaction. state, faster will be the
points, than those of the hydrocarbons of
reaction.
comparable molecular mass. For the same Tends to proceed with Tends to proceed with
alkyl group boiling point follows the order weak nucleophiles, strong nucleophiles,
– – –
as : e.g., CH3OH, H2O, e.g., CH3O , CN , OH ,
R I > R Br > R Cl > R F CH3CH2OH, etc. etc.
X Decreasing order of boiling points among Both retention Inversion of
the isomeric alkyl halides follows the order : and inversion of configuration takes
1° > 2° > 3° alkyl halides configuration takes place (Walden
X Decreasing order of density among the alkyl place. inversion).
halides is RI > RBr > RCl > RF. X Primary allylic and primary benzylic halides
For alkyl iodide decreasing order of density show higher reactivity in SN1 reactions
is as follows : than other primary alkyl halides due to the
CH3I > CH3CH2I > CH3CH2CH2I. greater stabilisation of allylic and benzylic
X Bond strength of C X bond follows the order carbocation intermediates by resonance.
CH3 F > CH3 Cl > CH3 Br > CH3 I X Vinylic and aryl halides are unreactive in
i.e., bond strength of C X bond decreases as nucleophilic substitution reactions. This is
the size of halogen atoms increases. because of double bond character of C X
X Correct stability order of RX is as follows : bond due to resonance.
R F > R Cl > R Br > R I
8 Optical isomerism/Enantiomerism :
8 Chemical properties : X Optical isomer is known as dextrorotatory
X Nucleophilic substitution reactions : The isomer (latin : dexter means right) (d-form
reactions in which a stronger nucleophile or +ve) if it rotates the plane polarised light
displaces a weaker nucleophile are called to the right (clockwise) and laevorotatory
, isomer (latin : laevo means left) (l-form or X Wurtz reaction :
–ve) if it rotates the plane polarised light to X + 2Na + X R Ether R
R R + 2NaX
the left (anticlockwise).
X An equimolar mixture of the d-form and 8 Reduction of alkyl halides :
l-form will be optically inactive and is called 2R X + 2[H]
Zn/HCl
2R H + ZnX2
racemic mixture (or dl- form or (±)-mixture).
X The process of conversion of an enantiomer HALOARENES
into racemic mixture is known as racemisation. 8 Aryl halides : General formula is ArX, where
X Chirality : The compound is said to have Ar = aryl group.
chirality if the central carbon atom is
8 General methods of preparation :
attached to four different groups and this
X By Raschig process :
centre is called chiral (asymmetric) centre or
CuCl2
stereogenic centre or stereocentre. 2C6H6 + 2HCl + O2 2C6H5Cl + 2H2O
500 K
X Achirality : The compound is said to have X From benzenediazonium salt :
achirality if the central carbon atom have
atleast two identical groups and this centre is
called achiral (symmetric).
X If molecule has a plane of symmetry it is
achiral (not chiral) and if molecule has no
plane of symmetry it is chiral.
X Enantiomers/d- and l-isomers : They are the
optical isomers which are non-superimposable
mirror images (or dissymmetric).
X Diastereomers : They are the optical isomers
which are not mirror images of each other.
They have different physical properties and
magnitude of specific rotation.
X Meso compounds : These compounds X By direct halogenation of benzene :
have two or more even number of chiral
carbon atoms and have an internal plane of
symmetry. They are optically inactive due to
internal compensation.
8 Elimination reactions: 4
X Dehydrohalogenation :
8 Physical properties :
X Aryl halides are colourless stable liquids with
pleasant odour.
H3C CH Cl + KOH (alc.) H3C CH CH2
X These are insoluble in water but readily
CH3 miscible with organic solvents.
8 Reaction with metals: X Most of them are steam volatile, heavier than
X Reaction with Mg metal : water.
Ether X Their boiling points are higher than
CH3I + Mg CH3MgI
OH corresponding alkyl halides. The boiling
CH3MgI + H2O CH4 + Mg points rise gradually from fluoro to iodo
I compounds.
, 8 Chemical properties : – Sulphonation :
X Nucleophilic substitution reactions : Cl Cl
Cl OH conc. H2SO4
SO3H
(i) NaOH, 623 K, 300 atm
(ii) H+ 2-Chlorobenzene-
sulphonic acid (Minor)
The presence of an electron withdrawing Cl
group ( NO2) at ortho- and para-positions
increases the reactivity of haloarenes. +
Cl OH
SO3H
(i) NaOH, 443 K
4-Chlorobenzene-
(ii) H+
sulphonic acid (Major)
NO2 NO2 – Friedal–Crafts reaction :
Cl Cl
Cl OH Anhyd. AlCl3 CH3
NO2 NO2 + CH3Cl
(i) NaOH, 368 K
(ii) H+ 1-Chloro-2-methylbenzene
(Minor)
NO2 NO2
Cl
Cl OH
O2N NO2 O2N NO2 +
warm
H 2O
NO2 NO2 CH3
1-Chloro-4-methylbenzene
X Electrophilic substitution reactions : (Major)
– Halogenation :
Cl Cl Cl Cl
O
Cl Anhyd. AlCl3
Anhyd. + H3C C Cl
+ Cl2 FeCl3
+
1, 2-Dichlorobenzene Cl O Cl
Cl (Minor)
1, 4-Dichlorobenzene CH3
(Major) +
– Nitration :
2-Chloroacetophenone
Cl Cl (Minor) O CH3
conc. HNO3 NO2 4-Chloroacetophenone
(Major)
conc. H2SO4
X Wurtz Fittig reaction :
1-Chloro-2-nitrobenzene
Ether
(Minor) R X + 2Na + X
Cl
R + 2NaX
+
X Fittig reaction :
NO2 X
Ether
1-Chloro-4-nitrobenzene 2 + 2Na + 2NaX
(Major)
Diphenyl
