Engineering Chemistry S Datta

156 ENGINEERING CHEMISTRY
The mechanism of the substitution can be represented as follows:
–
OH + C X HO C X
HO C+X –
T.S.
Here the mechanism shows that C—X bond breaks as the nucleophile OH – forms a new
bond with carbon simultaneously.
Elimination reaction. This is a reaction that splits off a simple compound from a
molecule to form a double bond.
Br
Example 1. C C + KOH (alcoholic)
C C +KBr+HO
H
Elimination of a hydrogen halide from an alkyl halide produces alkene.
The elimination reaction is of two types and they are base catalysed elimination reactions:
(a) Unimolecular reaction (E 1 )
(b) Bimolecular reaction (E 2 ).
(a) E 1 reaction. This reaction occurs in two steps:
X
Step 1. C C
Slow
X – +
+ C C
2
H
H
A carbocation
Step 2.
C
C
Fast
C C +H:
B
H
: B
(b) E 2 reaction. This reaction occurs in one step process
X
C
C
–
X +
C C +H:
B
H
: B
Another example of elimination reaction is the removal of water from an alcohol to form
an alkene.
H OH H
H
⏐ ⏐ ⏐ Conc. H SO
⏐
2 4
H⎯
C ⎯C⎯
C ⎯ H ⎯⎯⎯⎯→ H⎯
C—
C⎯
C ⎯H
Heat (– H2O)
⏐ ⏐ ⏐
⏐ ⏐ ⏐
H H H
H H H