Unit
Objectives
10
Haloalkanes and
Haloar enes
Haloarenes
After studying this Unit, you will be
able to
• name haloalkanes and haloarenes
according to the IUPAC system of
nomenclature from their given
structures; Halogenated compounds persist in the environment due to their
• describe the reactions involved in resistance to breakdown by soil bacteria.
the preparation of haloalkanes and
haloarenes and understand
various reactions that they The replacement of hydrogen atom(s) in an aliphatic
-
undergo; or aromatic hydrocarbon by halogen atom(s) results
-
• correlate the structures of in the formation of alkyl halide (haloalkane) and aryl
-
haloalkanes and haloarenes with halide (haloarene), respectively. Haloalkanes contain
-
various types of reactions; halogen atom(s) attached to the sp3 hybridised carbon
-
• use stereochemistry as a tool for atom of an alkyl group whereas haloarenes contain
understanding the reaction
halogen atom(s) attached to sp2 hybridised carbon
mechanism;
atom(s) of an aryl group. Many halogen containing
• appreciate the applications of
organo-metallic compounds;
-
organic compounds occur in nature and some of
-
CH43
these are clinically useful. These classes of compounds -
• highlight the environmental effects -
-
of polyhalogen compounds. find wide applications in industry as well as in day-
to-day life. They are used as solvents for relatively
-
non-polar compounds and as starting materials for
& -
-
the synthesis of wide range of organic compounds.
E
Chlorine containing antibiotic, chloramphenicol, *
c X 3 -
C
- -
-
-
2 produced by microorganisms is very effective for the
T -
treatment of typhoid fever. Our body produces iodine
MAIN containing
-
hormone, thyroxine, the deficiency of which
JEE
& Boards
-
causes a disease called goiter. Synthetic halogen
>
-
-
t compounds, viz. chloroquine is used for the treatment
-
of malaria; halothane is used as an anaesthetic
- -
during
-
surgery. Certain fully fluorinated compounds
-
are being considered as potential blood substitutes
-
in surgery.
-
In this Unit, you will study the important methods
of preparation, physical and chemical properties and
uses of organohalogen compounds.
2019-20
,10.1 Classification Haloalkanes and haloarenes may be classified as follows:
10.1.1 On the These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.)
Basis of compounds depending on whether they contain one, two or more halogen
Number of atoms in their structures. For example,
Halogen
=>
Atoms -
-
-
-
-
-
-
-
-
-
- -
Monohalocompounds may further be classified according to the
hybridisation of the carbon atom to which the halogen is bonded, as
discussed below.
10.1.2 Compounds This class includes
Containing (a) Alkyl halides or haloalkanes (R—X)
sp3 C—X
In alkyl halides, the halogen atom is bonded to an alkyl group (R).
E
Bond (X= F,
Cl, Br, I) They form a homologous series represented by CnH2n+1X. They are
further classified as primary, secondary or tertiary according to the
nature of carbon to which halogen is attached. If halogen is attached to
a primary carbon atom in an alkyl halide, the alkyl halide is called
primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached
to secondary or tertiary carbon atom, the alkyl halide is called
secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively.
>
-
R CHz -X
-
-
go Allgliz
Sos
-
↓
R-CH-X
120
(b) Allylic halides
-
Vinylic
These are the compounds in which the halogen atom is bonded to an
sp3-hybridised carbon atom adjacent to carbon-carbon double bond
Sp3
R
-
(C=C) i.e. to an allylic carbon.
Allylic carbon Sp2
Allylic carbon
(c) Benzylic halides
-
These are the compounds in which the halogen atom is bonded to an
-
sp3-hybridised carbon atom attached to an aromatic ring.
-
Chemistry 290
2019-20
,↳Be ↑gi -
H
↑
*
[
Beglic
-
This class includes:
Double Bond wal
10.1.3 Compounds
Containing
sp2 C—X
>
-
(a) Vinylic halides
-
exactly
Bond These are the compounds in which the halogen atom is bonded to
a sp2-hybridised carbon atom of a carbon-carbon double bond
O
(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.
C
10.2 Nomenclature Having learnt the classification of halogenated compounds, let us now learn
how these are named. The common names of alkyl halides are derived by
naming the alkyl group followed by the name of halide. In the IUPAC system
-a of nomenclature, alkyl halides are named as halosubstituted hydrocarbons.
For mono halogen substituted derivatives of benzene, common and IUPAC
names are the same. For dihalogen derivatives, the prefixes o-, m-, p- are
iso-propyl used in common system but in IUPAC system, as you have learnt in Class
CH2-c
XI, Unit 12, the numerals 1,2; 1,3 and 1,4 are used.
Ch
*
iso-butyechlorid
.
8 -
-
-
· D
-
53
-
-
E
pentyl
~
-
↳
The dihaloalkanes having the same type of halogen atoms are named
-CHE as alkylidene or alkylene dihalides. The dihalo-compounds having both
the halogen atoms are further classified as geminal halides or gem-dihalides
when both the halogen atoms are present on the same carbon atom of the
chlocide
Neo-pentyl e 291 Haloalkanes and Haloarenes
⑳ E
test-butyl Brid
- Br
c-c-c-A c =
! 2019-20
I
, chain and vicinal halides or vic-dihalides when halogen atoms are present
on adjacent carbon atoms. In common name system, gem-dihalides are
named as alkylidene halides and vic-dihalides are named as alkylene
dihalides. In IUPAC system, they are named as dihaloalkanes.
Iso-butyl
- ( Some common examples of halocompounds are mentioned in Table 10.1.
② Table 10.1: Common and IUPAC Names of some Halides
Structure Common name IUPAC name
②
CHCHuscrodi
3
CH3CH2CH(Cl)CH3 sec-Butyl chloride 2-Chlorobutane
t (CH3)3CCH2Br
(CH3)3CBr . neo-Pentyl bromide
tert-Butyl bromide
1-Bromo-2,2-dimethylpropane
2-Bromo-2-methylpropane
-
CH2 = CHCl Vinyl chloride Chloroethene
sol
123
CH2 = CHCH2Br Allyl bromide 3-Bromopropene
- -
t
-
o-Chlorotoluene 1-Chloro-2-methylbenzene
---
or
2-Chlorotoluene
-
Benzyl chloride Chlorophenylmethane
d CH2Cl2
>
-
2H
- -
Methylene chloride Dichloromethane
CHCl3 CHX3 ~Chloroform Trichloromethane
o -
CHBr3 -Bromoform Tribromomethane
-
CCl4 ~
Carbon tetrachloride Tetrachloromethane
-
-
CH3CH2CH2F n-Propyl fluoride 1-Fluoropropane
-
Example 10.1 Draw the structures of all the eight structural isomers that have the
- -
molecular formula C5H11Br. Name each isomer according to IUPAC system
and classify them as primary, secondary or tertiary bromide.
-
- - -
Solution CH3CH2CH2CH2CH2Br 1-Bromopentane (1o)
-
CH3CH2CH2CH(Br)CH3
-
2-Bromopentane(2o)
CH3CH2CH(Br)CH2CH3 3-Bromopentane (2o)
-
(CH3)2CHCH2CH2Br 1-Bromo-3-methylbutane (1o)
Chemistry 292
2019-20
Objectives
10
Haloalkanes and
Haloar enes
Haloarenes
After studying this Unit, you will be
able to
• name haloalkanes and haloarenes
according to the IUPAC system of
nomenclature from their given
structures; Halogenated compounds persist in the environment due to their
• describe the reactions involved in resistance to breakdown by soil bacteria.
the preparation of haloalkanes and
haloarenes and understand
various reactions that they The replacement of hydrogen atom(s) in an aliphatic
-
undergo; or aromatic hydrocarbon by halogen atom(s) results
-
• correlate the structures of in the formation of alkyl halide (haloalkane) and aryl
-
haloalkanes and haloarenes with halide (haloarene), respectively. Haloalkanes contain
-
various types of reactions; halogen atom(s) attached to the sp3 hybridised carbon
-
• use stereochemistry as a tool for atom of an alkyl group whereas haloarenes contain
understanding the reaction
halogen atom(s) attached to sp2 hybridised carbon
mechanism;
atom(s) of an aryl group. Many halogen containing
• appreciate the applications of
organo-metallic compounds;
-
organic compounds occur in nature and some of
-
CH43
these are clinically useful. These classes of compounds -
• highlight the environmental effects -
-
of polyhalogen compounds. find wide applications in industry as well as in day-
to-day life. They are used as solvents for relatively
-
non-polar compounds and as starting materials for
& -
-
the synthesis of wide range of organic compounds.
E
Chlorine containing antibiotic, chloramphenicol, *
c X 3 -
C
- -
-
-
2 produced by microorganisms is very effective for the
T -
treatment of typhoid fever. Our body produces iodine
MAIN containing
-
hormone, thyroxine, the deficiency of which
JEE
& Boards
-
causes a disease called goiter. Synthetic halogen
>
-
-
t compounds, viz. chloroquine is used for the treatment
-
of malaria; halothane is used as an anaesthetic
- -
during
-
surgery. Certain fully fluorinated compounds
-
are being considered as potential blood substitutes
-
in surgery.
-
In this Unit, you will study the important methods
of preparation, physical and chemical properties and
uses of organohalogen compounds.
2019-20
,10.1 Classification Haloalkanes and haloarenes may be classified as follows:
10.1.1 On the These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.)
Basis of compounds depending on whether they contain one, two or more halogen
Number of atoms in their structures. For example,
Halogen
=>
Atoms -
-
-
-
-
-
-
-
-
-
- -
Monohalocompounds may further be classified according to the
hybridisation of the carbon atom to which the halogen is bonded, as
discussed below.
10.1.2 Compounds This class includes
Containing (a) Alkyl halides or haloalkanes (R—X)
sp3 C—X
In alkyl halides, the halogen atom is bonded to an alkyl group (R).
E
Bond (X= F,
Cl, Br, I) They form a homologous series represented by CnH2n+1X. They are
further classified as primary, secondary or tertiary according to the
nature of carbon to which halogen is attached. If halogen is attached to
a primary carbon atom in an alkyl halide, the alkyl halide is called
primary alkyl halide or 1° alkyl halide. Similarly, if halogen is attached
to secondary or tertiary carbon atom, the alkyl halide is called
secondary alkyl halide (2°) and tertiary (3°) alkyl halide, respectively.
>
-
R CHz -X
-
-
go Allgliz
Sos
-
↓
R-CH-X
120
(b) Allylic halides
-
Vinylic
These are the compounds in which the halogen atom is bonded to an
sp3-hybridised carbon atom adjacent to carbon-carbon double bond
Sp3
R
-
(C=C) i.e. to an allylic carbon.
Allylic carbon Sp2
Allylic carbon
(c) Benzylic halides
-
These are the compounds in which the halogen atom is bonded to an
-
sp3-hybridised carbon atom attached to an aromatic ring.
-
Chemistry 290
2019-20
,↳Be ↑gi -
H
↑
*
[
Beglic
-
This class includes:
Double Bond wal
10.1.3 Compounds
Containing
sp2 C—X
>
-
(a) Vinylic halides
-
exactly
Bond These are the compounds in which the halogen atom is bonded to
a sp2-hybridised carbon atom of a carbon-carbon double bond
O
(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.
C
10.2 Nomenclature Having learnt the classification of halogenated compounds, let us now learn
how these are named. The common names of alkyl halides are derived by
naming the alkyl group followed by the name of halide. In the IUPAC system
-a of nomenclature, alkyl halides are named as halosubstituted hydrocarbons.
For mono halogen substituted derivatives of benzene, common and IUPAC
names are the same. For dihalogen derivatives, the prefixes o-, m-, p- are
iso-propyl used in common system but in IUPAC system, as you have learnt in Class
CH2-c
XI, Unit 12, the numerals 1,2; 1,3 and 1,4 are used.
Ch
*
iso-butyechlorid
.
8 -
-
-
· D
-
53
-
-
E
pentyl
~
-
↳
The dihaloalkanes having the same type of halogen atoms are named
-CHE as alkylidene or alkylene dihalides. The dihalo-compounds having both
the halogen atoms are further classified as geminal halides or gem-dihalides
when both the halogen atoms are present on the same carbon atom of the
chlocide
Neo-pentyl e 291 Haloalkanes and Haloarenes
⑳ E
test-butyl Brid
- Br
c-c-c-A c =
! 2019-20
I
, chain and vicinal halides or vic-dihalides when halogen atoms are present
on adjacent carbon atoms. In common name system, gem-dihalides are
named as alkylidene halides and vic-dihalides are named as alkylene
dihalides. In IUPAC system, they are named as dihaloalkanes.
Iso-butyl
- ( Some common examples of halocompounds are mentioned in Table 10.1.
② Table 10.1: Common and IUPAC Names of some Halides
Structure Common name IUPAC name
②
CHCHuscrodi
3
CH3CH2CH(Cl)CH3 sec-Butyl chloride 2-Chlorobutane
t (CH3)3CCH2Br
(CH3)3CBr . neo-Pentyl bromide
tert-Butyl bromide
1-Bromo-2,2-dimethylpropane
2-Bromo-2-methylpropane
-
CH2 = CHCl Vinyl chloride Chloroethene
sol
123
CH2 = CHCH2Br Allyl bromide 3-Bromopropene
- -
t
-
o-Chlorotoluene 1-Chloro-2-methylbenzene
---
or
2-Chlorotoluene
-
Benzyl chloride Chlorophenylmethane
d CH2Cl2
>
-
2H
- -
Methylene chloride Dichloromethane
CHCl3 CHX3 ~Chloroform Trichloromethane
o -
CHBr3 -Bromoform Tribromomethane
-
CCl4 ~
Carbon tetrachloride Tetrachloromethane
-
-
CH3CH2CH2F n-Propyl fluoride 1-Fluoropropane
-
Example 10.1 Draw the structures of all the eight structural isomers that have the
- -
molecular formula C5H11Br. Name each isomer according to IUPAC system
and classify them as primary, secondary or tertiary bromide.
-
- - -
Solution CH3CH2CH2CH2CH2Br 1-Bromopentane (1o)
-
CH3CH2CH2CH(Br)CH3
-
2-Bromopentane(2o)
CH3CH2CH(Br)CH2CH3 3-Bromopentane (2o)
-
(CH3)2CHCH2CH2Br 1-Bromo-3-methylbutane (1o)
Chemistry 292
2019-20
