Which of the following compounds undergoes ${\text{S}}_{\text{N}}2$ reaction most rapidly?

Solution

1. The ${\text{S}}_{\text{N}}2$ reaction rate depends on the degree of steric hindrance at the electrophilic carbon.
2. Primary alkyl halides have the least steric hindrance compared to secondary or tertiary halides.
3. ${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2{\text{CH}}_2\text{Cl}$ is a primary alkyl halide.
4. Hence the answer is (A).