Question

Out of (ref. image), which one is more reactive towards $S_{N^2}$ reaction and why?

Answer 1

So in case of A Cl is attached to secondary carbon and in Case of B Cl is attached to primary carbon.

So $S{N}^2$ reaction is reaction in which attack and removal of leaving group takes place at a same time.

So, more the bculkier groups less is $S{N}^2$ rate.

As in B, chloride attached to primary carbon $S{N}^2$ rate is more.