Least reactive alkyl chloride in a $\mathrm{S}_{\mathrm{N}} 2$ reaction is
Least reactive alkyl chloride in a $\mathrm{S}_{\mathrm{N}} 2$ reaction is
1-Chloro butane
1-Chloro 2-Methyl propane
2-Chloro 2-Methyl propane
2-Chloro butane
Solution
An $\mathrm{S}_{\mathrm{N}} 2$ reaction involves a single-step concerted mechanism and the alkyl halide that is most substituted at the $\alpha$-position will be least reactive.
Thus, 2-chloro-2-methyl propane will be least reactive.