Which of the following haloalkanes will undergo the fastest ${\text{S}}_{\text{N}}2$ reaction with $\text{NaI}$ in acetone?

Solution

1. The ${\text{S}}_{\text{N}}2$ reaction rate depends on the steric hindrance at the electrophilic carbon atom.
2. Primary alkyl halides are significantly more reactive than secondary or sterically hindered primary halides.
3. Among the choices, ${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2{\text{CH}}_2\text{Cl}$ is a primary halide with the least steric hindrance.
4. Hence the answer is (A).