Haloalkanes and Haloarenes - Result Question 43
####43. Which of the following compounds undergoes nucleophilic substitution reaction most easily?
[2011 M] (a)
(c)
(b)
(d)
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Solution:
- (a) To compare the rates of substitution, in chlorobenzene, electron-withdrawing group, and chlorobenzene having electron-releasing group, we compare the structures carbanion I (from chlorobenzene), II (from chlorobenzene containing electron-withdrawing group) and III (from chlorobenzene containing electronreleasing group).
$G$ withdraws electrons, neutralises (disperses)ve charge of the ring, stabilises carbanion, facilitates $S_N$ reaction (activation effect)
$G$ releases electrons, intensifies -ve charge, destabilizes carbanion, retards $S_N$ reaction (deactivation)
$NO_2$ is activating group and $CH_3$ and $OCH_3$ are deactiving group.
Hence, the correct order of nucleophilic substitution reaction is :
In $S_N$ reactions, a carbanion is formed as an intermediate, so any substituent that increases the stability of carbanion and hence the transition state leading to its formation will enhance the $S_N$ reactions.
