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Which of the following undergoes nucleophilic substitution exclusively by $S_N1$ mechanism :
A
Ethyl chloride
B
Isopropyl chloride
C
Benzyl chloride
D
Chlorobenzene
Detailed Solution
The $S_N1$ reaction proceeds through a carbocation, so it is favoured by substrates that give a stable carbocation.
Benzyl chloride, $C_6H_5CH_2Cl$, ionises to give the benzyl carbocation $C_6H_5CH_2^+$.
The positive charge of the benzyl carbocation is delocalised over the benzene ring by resonance (to the ortho and para positions), which makes it very stable.
So benzyl chloride undergoes nucleophilic substitution by the $S_N1$ mechanism.
Ethyl chloride, a primary halide, reacts mainly by $S_N2$, and isopropyl chloride (secondary) can react by both mechanisms.
Chlorobenzene does not undergo nucleophilic substitution under ordinary conditions, because the C-Cl bond has partial double bond character.
So the answer is benzyl chloride.
Benzyl chloride, $C_6H_5CH_2Cl$, ionises to give the benzyl carbocation $C_6H_5CH_2^+$.
The positive charge of the benzyl carbocation is delocalised over the benzene ring by resonance (to the ortho and para positions), which makes it very stable.
So benzyl chloride undergoes nucleophilic substitution by the $S_N1$ mechanism.
Ethyl chloride, a primary halide, reacts mainly by $S_N2$, and isopropyl chloride (secondary) can react by both mechanisms.
Chlorobenzene does not undergo nucleophilic substitution under ordinary conditions, because the C-Cl bond has partial double bond character.
So the answer is benzyl chloride.