Alkyl halide 3.1
CHAPTER
Alkyl halide
1.1
3
ALIPHATIC HALOGEN DERIVATIVES :
Compounds obtained by the replacement of one or more hydrogen atom(s) from hydrocarbons are known as
halogen derivatives. The halogen derivatives of alkanes, alkenes, alkynes and arenes are known as alkyl halide
(haloalkene), alkenyl halide (haloalkenes), alkynyl halides (haloalkynes) and aryl halides (halobenzenes) respectively.
Alkyl halides : Monohalogen derivatives of alkanes are known as alkyl halides
Structure of alkyl halides:
C
X
Classification of alkyl halides :
(i) Primary halide : If the halogen bearing carbon is bonded to one carbon atom or with no carbon atom
Example :
CH3 – X, R – CH2 – X
(ii) Secondary halide : If two carbon atoms are bonded to the halogen bearing carbon.
Example :
X
Cl
R – CH – R , CH3 – CH – CH3
(ii) Tertiary halide : Three other carbon atom bonded to the halogen bearing carbon atom.
Example :
R
CH3
R– C – X , CH3 – C – Cl
CH3
R
Halolkanes can be classified into following three categories.
(i) Monohaloalkanes
(ii) Dihaloalkanes
(iii) Polyhaloalkanes
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3.2 Theory and Exercise Book
1.2
IUPAC NOMENCLATURE OF ALKYL HALIDES
S.N.
Compound
IUPAC name
CH3
1.
2 – Chloro-2-methylpropane
CH3 – C – Cl
CH3
2.
CH3 – CH – CH2 – CH2
Br
3.
3-Bromo-1-chlorobutane
Cl
2-Bromo-1-chloro-4-fluoro-3-methylbutane
CH2 – CH – CH – CH2
F
CH3 Br
Cl
Cl
Br
4.
2-Bromo-1-chloro-3-iodocyclopentane
I
5.
CH3 – CH – CH2 – CH2 – CH2
OH
5-chloropentan-2-ol
Cl
F
6.
5-Fluoropent-1-ene
CH3Cl
7.
Chlorophenylmethane
CHCl2
8.
Dichlorophenylmethane
CCl3
9.
Trichlorophenylmethane
Cl
Cl
Cl
Cl
10.
2,2-Bis(chloromethyl)-1,3-dichloropropane
Br
H
11.
Cis-1,4-dibromocyclohexane
Br
H
CH3
12.
CH–CH–CH3
I
2-iodo-3-phenylbutane
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Alkyl halide 3.3
1.3
ISOMERISM IN HALOALKANES
S.N.
1.
Compound
IUPAC name
Structural Isomerism
(a) Chain
1. CH3 – CH2 – CH2 – CH2 – Cl
CH3 – CH – CH2 – Cl
1-Chlorobutane
1-Chloro-2-methylpropane
CH3
CH3 – CH2 – CH 2 – CH 2 – CH 2 – Cl 1-Chloropentane
CH3
2.
CH3 – CH – CH 2 – CH 2 – Cl 1-Chloro-3-methylbutane
CH3
1-Chloro-2,2-dimethylpropane
CH3 – CH – CH 2 – Cl
CH3
1. CH3 – CH2 – CH2 – Cl 1-Chloropropane
(b) Position
Cl
CH3 – CH – CH3 2-Chloropropane
2. CH3 – CH – CH2– Cl
CH3
1-Chloro-2-methylpropane
Cl
CH3 – CH – CH3 2-Chloro-2-methylpropane
CH3
2.
Stereoisomerism
CH3
*
H
Cl
*
Cl
H
(a) Optical isomerism
CH2–CH3
(2S)-2-Cholobutane
CH2–CH3
(2R)-2-Cholobutane
H
Cl
*
CH3
Cl
H
*
Cl
H
CH3
CH3
H
Cl
*
CH3
*
`
CH3
H
*
H
*
Cl
Cl
CH3
Meso-optically inactive
CH3
Enantiomers of
2,3-Dichlorobutane
Mirror
CH3
H
Cl
Mirror
CH 3
Cl
H
CH2CH 3
Cl
H
H
Cl
CH2CH3
Enantiomers
2,3-Dichloropentane
Plane of
symmetry
CH3
H
H
CH3
Cl
Cl
CH2CH3
Cl
Cl
Enantiomers
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H
H
CH2CH3
3.4 Theory and Exercise Book
1.4
BONDING IN ALKYL HALIDE :
Table : 1 Carbon halogen bond lengths
Bond
Bond length(Å)
CH3 – F
1.39
CH3 – Cl
1.78
CH3 – Br
1.93
CH3 – I
2.14
EXAMPLE
Classify the compound as a primary, secondary and tertiary halide
Br
H
(a) (CH3) 2CH–CH2Cl
(b)
H3C
CH3
H3C
F
(c)
Cl
H
(d) CH3 – CH – CH2Cl (e) Isopentyl bromide
(f) Neopentyl iodide
CH3
Sol.
(a) Primary
(e) Primary
1.5
PHYSICAL PROPERTIES OF ALKYL HALIDE :
(b) Secondary
(f) Primary
(c) Tertiary
(d) Primary
1.5.1 Dipole moment of the halogen derivatives:
 = 4.8 ×  × d
Where  is the charge and d is the bond length
These two effects e.g. charge and distance oppose each other, with the larger halogens having longer bond but
weaker electronegativity. The overall result is that the bond dipole moment increase in the order.
C – I < C – Br < C – F < C – Cl
:
1.29 D 1.48 D 1.51 D 1.56 D
The electronegativities of the halogen increase in the order:
I < Br < Cl < F
Table : 2 Molecular dipole moments of methylhalides
X
CH3X
CH2X2
CHX3
CX4
F
1.82 D
1.97 D
1.65 D
0
Cl
1.94 D
1.60 D
1.03 D
0
Br
1.79 D
1.45 D
1.02 D
0
I
1.64 D
1.11 D
1.00 D
0
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