In the following pairs of halogen compounds,which would undergo $S_{N}2$ reaction faster?
$(i)$ Cyclohexylmethyl chloride and Chlorocyclohexane
(ii) $n$-Butyl iodide and $n$-Butyl chloride

(N/A) The rate of $S_{N}2$ reaction depends on the steric hindrance around the electrophilic carbon and the nature of the leaving group.
$(i)$ Cyclohexylmethyl chloride is a primary $(1^{\circ})$ alkyl halide,whereas Chlorocyclohexane is a secondary $(2^{\circ})$ alkyl halide. Since $S_{N}2$ reactions are highly sensitive to steric hindrance,the less hindered primary halide reacts faster. Thus,Cyclohexylmethyl chloride reacts faster than Chlorocyclohexane.
(ii) Both $n$-Butyl iodide and $n$-Butyl chloride are primary $(1^{\circ})$ alkyl halides. The rate of $S_{N}2$ reaction depends on the ability of the leaving group to depart. Iodide $(I^-)$ is a better leaving group than chloride $(Cl^-)$ because it is a weaker base. Therefore,$n$-Butyl iodide reacts faster than $n$-Butyl chloride.