Which of the following undergoes nucleophilic substitution exclusively by S _ N 1 mechanism?

Step-by-step solution

More stable the carbocation, more the tendency to undergo S _ N 1 reaction. Thus, order of carbocation to undergo S _ N 1 reaction is: Benzyl > Allyl >3^ >2^ >1^ >- CH ₃ Related Theory The rate of reaction depends on the concentration of the substrate in S_N l reaction While In S_N , the rate of reaction depends on the concentration of both the substrate and the nucleophile. ! Caution Benzyl carbocation is stabilised by resonance so it will undergo S_N I reaction.