Question

Which one of the following undergoes nucleophilic substitution exclusively by $S_{N}1$ mechanism?

• A. Benzyl chloride
• B. Ethyl chloride
• C. Chlorobenzene
• D. Isopropyl chloride

Answer 1

The correct option is A Benzyl chloride

Aliphatic $S_{N}1$ reaction is carried out in two steps. In first step carbocation is formed and its formation is based on the stability of carbocation.

$C_6{H}_5\stackrel{+}{C}{H}_2>C{H}_3\stackrel{+}{C}HC{H}_3>C{H}_3\stackrel{+}{C}{H}_2$

In the second step, the nucleophile is attracted to carbocation to give final products. Hence, an order of $S_{N}1$ reaction is

$C_6{H}_5\stackrel{+}{C}{H}_2>C{H}_{3}CH-C{H}_3>C{H}_{3}C{H}_2$

The aryl halides e.g., chlorobenzene are less reactive as compared to alkyl halides towards nucleophilic reagents in either $S_{N}2$ or $S_{N}1$ reaction because carbon-halogen bond in the aryl halide is strong (due to its double bond character).

So, the correct option is $A$