Which of the following undergoes nucleophilic substitution exclusively by S_N1 mechanism?

Benzyl chloride

The \(S_N1\) mechanism is a two-step process that involves the formation of a carbocation intermediate. The stability of the carbocation intermediate is important for the rate of the reaction. Benzyl chloride has a benzyl carbocation intermediate, which is stabilized by resonance. This makes benzyl chloride the most reactive of the three compounds, and it will undergo \(S_N1\) exclusively.

Chlorobenzene and isopropyl chloride do not have carbocation intermediates that are stabilized by resonance, so they can undergo \(S_N2\) reactions as well as \(S_N1\) reactions. Therefore, the answer is (2).

Here is a table showing the stability of the carbocation intermediates and the likelihood of each compound undergoing \(S_N1\):