Which alkyl halide from the following pairs would you expect to react more rapidly by ' SN2' mechanism? Explain your answer

Explanation

CH_3 CH_2 CH_2 CH_2 Br Being primary halide,
there won.t be any steric hinderance .
FIGURE
Secondary alkyl halide would react faster than tertiary halide.
Figure

The presence of tmethyl group closer to the halide group will increase the steric hinderance and decrease the rate of the reaction.