Which of the following undergoes nucleophilic substitution exclusively by SN1 mechanism?
Answer & explanation
Correct answer: option 2
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 \(S_N1\):
|
Compound |
Carbocation intermediate |
Stability |
Likelihood of undergoing \(S_N1\) |
|
Benzyl chloride |
Benzyl carbocation |
Highly stabilized by resonance |
Exclusively \(S_N1\) |
|
Chlorobenzene |
Chlorophenyl carbocation |
Not stabilized by resonance |
\(S_N1\) and \(S_N2\) |
|
Isopropyl chloride |
Isopropyl carbocation |
Not stabilized by resonance |
\(S_N1\) and \(S_N2\) |