Which of the following compounds will undergo the fastest ${\text{S}}_{\text{N}}1$ reaction in aqueous ethanol?

Solution

1. The rate-determining step of ${\text{S}}_{\text{N}}1$ is the formation of a carbocation.
2. Compound (B) forms a tertiary carbocation with five carbons attached to the cationic center, providing maximum hyperconjugation and inductive stabilization.
3. Compound (D) forms a tertiary carbocation with four carbons attached.
4. Compound (A) and (C) form primary carbocations which are highly unstable.
5. Hence the answer is (B).