Which of the following undergoes nucleophilic substitution exclusively by SN1 mechanism?

Nucleophilic substitution reactions can occur through either the SN1 or SN2 mechanism. The SN1 mechanism involves a two-step process, with the formation of a carbocation intermediate, while the SN2 mechanism involves a single step, with the nucleophile attacking the carbon atom from the backside. To determine which of the given compounds undergoes nucleophilic substitution exclusively by the SN1 mechanism, we need to consider the stability of the carbocation intermediate. Benzyl chloride can form a relatively stable carbocation intermediate, which makes it a good candidate for the SN1 mechanism. Ethyl chlorcide and isopropyl chloride can also undergo the SN1 mechanism, but they can also undergo the SN2 mechanism. Chlorobenzene, on the other hand, is an aryl halide, which typically undergoes the SN1 mechanism, but it is not very reactive due to the partial double bond character between the carbon and chlorine atoms. However, among the given options, benzyl chloride is the most likely to undergo nucleophilic substitution exclusively by the SN1 mechanism. Therefore, the correct answer is option (A) Benzyl chloride. Think of it this way: the stability of the carbocation intermediate plays a crucial role in determining the mechanism of nucleophilic substitution.