Question

Which 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

Answer: (A)

Benzyl chloride

Answer 2

Aliphatic S$_N$1 reaction is carried out in two steps. In form of slow step
Step (i) carbonium (carbocation) ion is formed and its formation is based upon the stability
Stability order of carbocation benzylic carbocation (resonating stable)
$C_6 H_5 \, _{CH_2}^{+} > CH_3 - \, _{CH}^{+} - CH_3 > CH_3 - \, _{CH_3}^{+}$
\hspace35mm 2^{\circ}carbocation \, \, \, \, \, \, \, \, 1^{\circ} carbocation
and in step (ii) nucleophile is attracted towards the carbonium ion in form of fast step to give final product.
Hence, in benzyl chloride, ethyl chloride and isopropyl chloride order of S$_N$1 reaction is benzyl chloride > isopropyl chloride > ethyl chloride In chlorobenzene, mechanism of SN1 reaction differ to aliphatic alkyl halide. The aryl halides are much less reactive as compared to alkyl halides, towards nucleophilic reagents in either S$_N$1 or S$_N$ 2 reaction.
The carbon-halogen bond in the aryl halide is quite strong and only forcing conditions can break up this bond.
Hence,
Step (i)
$C_6H_5CH_2 -CI \underrightarrow{Slow} \, \, C_6H_5 \, _{CH_2}^{+}+ CI^{-}$
\hspace50mm Rate ? _{(First \, order \, kinetics)}^{{[C_6H_5CH_2CI]}}
Step (ii)
$C_6H_5-_{CH_2}^{+}+Nu^- \underrightarrow{Fast} \, \, C_6H_5CH_2Cu$