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

Hint:
Carbocation ion is formed, and its formation is dependent on stabilization. The More the number of carbon bonds, the more it is stable. The move that is from a lower degree to a higher degree helps in further stabilization.

Complete step by step answer:
The benzyl carbocation acts more stable due to the resonance, which results in it being more reactive. In the case of ${S_N}1$ reactions, it is the stability that decides the reactivity of the carbocation. In the above question, the one which goes under the aliphatic ${S_N}1$ reaction.

 ${S_N}1$ mechanism takes place in the following steps:
->The carbocation is formed using the removal of the leaving group of the compound.
->The nucleophile attacks on the carbocation
-> To have the necessary product, the deprotonation of the protonated nucleophile takes place.

As for the second alternative, i.e. The carbocation formed by ethyl chloride is grade 1 due to hyperconjugation, which results in it being less stable than our first alternative, i.e. As the carbocation is in the resonance, benzyl chloride contributes to further stability.
In the third option, i.e. Chlorobenzene the carbocation formed would be highly unstable, and ${S_N}1$ reaction cannot take place.
The fourth choice now, i.e. The bond would be tertiary Isopropyl Chloride, but its stability would be comparatively less than that of hyper conjugation compared to benzyl carbocation.

So, the correct option is Option A

Note:
While solving the above question students always misunderstand the mechanism of electrons movement in their shells and make mistakes in it. The students should learn and remember the process of shifting and stabilization of carbocation.