Question

Which of the following undergoes nucleophilic substitution exclusively by ${S_N}1$ mechanism?
A.Ethyl chloride
B.Isopropyl chloride
C.Benzyl chloride
D.Chloro benzene

Answer 1

Hint: We must know that ${S_N}1$ reactions are nucleophilic substitution reactions. Tertiary halides are considerably faster in chemical reactivity than secondary alkyl halides, which are faster in reactivity than primary halides.

Complete answer:
We have to remember that the ${S_N}1$ stands for Nucleophilic substitution unimolecular reaction. The ${S_N}1$ reaction is a nucleophilic Substitution reaction in which the rate determining step involves only one component.
We know that the ${S_N}1$ reaction mechanism follows a two-step process. First, the carbocation is formed by the removal of the leaving group (bond breaking mechanism). Then the carbocation is attacked by the nucleophile (bond forming mechanism). The protonated nucleophile then undergoes deprotonation to give the required product. And the first step is rate determining step.
We have to know ${S_N}1$ reaction is favored by bulky groups present on the carbocation bonded to halogens. It is also favored by the nature of carbonium ion present in the substrate and the general order of the stability of carbocation that undergoes ${S_N}1$ reaction is given as,
${\text{Benzyl > Allyl > Tertiary > Secondary > Primary > Methyl}}\,{\text{halides}}$
Now coming back to the question, we have to know that benzyl chloride proceeds via ${S_N}1$ mechanism. The formation of carbocation is based on its stability. We can give the stability of carbocation as,
${C_6}{H_5}\mathop C\limits^ + {H_2} > C{H_3} - \mathop C\limits^ + H - C{H_3} > C{H_3} - \mathop C\limits^ + {H_2}$
We have to know that nucleophiles are attracted towards the carbonium ion which is carbocation. This takes place as a fast step and the final product is formed.
Though ethyl chloride, isopropyl chloride, benzyl chloride undergoes ${S_N}1$ mechanism. Due to the higher stability of benzyl carbocation, benzyl chloride undergoes ${S_N}1$ mechanism more exclusively.
Therefore,option (C) is correct.

Note:
We must know that the ${S_N}1$ reactions are important as they describe a mechanism of chemical reactivity and also it describes a bond breaking process with respect to the rate determining step.