As steric hindrance (crowding) increases, the rate of \({S_{{N^2}}}\) reaction decreases.
The order of reactivity towards \({S_{{N^2}}}\) reaction for alkyl halides is
Primary halides \(>\) Secondary halides \(>\) Tertiary halides
$ 3 \mathrm{ROH}+\mathrm{PCl}_3 \rightarrow 3 \mathrm{RCl}+\mathrm{A} $
$ \mathrm{ROH}+\mathrm{PCl}_5 \rightarrow \mathrm{RCl}+\mathrm{HCl}+\mathrm{B}$




