Which one of the following will undergo Nucleophilic substitution, by S _ N ^1 mechanism, fastest?

Step-by-step solution

The rate of S _ N 1 reaction depends on the stability of the carbocation intermediate formed after the departure of the leaving group. For option (1), Br - CH ₂- CH = CH ₂ forms the allyl carbocation CH ₂= CH - CH ₂⁺ , which is resonance stabilized by the adjacent double bond. For option (2), C ₆ H ₅ Br forms a phenyl cation, which is highly unstable and does not undergo S _ N 1 reaction. For option (3), C ₆ H ₁₁ Br (cyclohexyl bromide) forms a secondary carbocation, which is less stable than the resonance-stabiliz