Predict the order of reactivity of the compounds in `S_(N)1` and `S_(N) 2` reactions .
The four isomeric bromobutanes

Explanation

`CH_(3) CH_2 CH_(2) CH_(2) Br lt (CH_(3))_(2) CH CH_(2) Br lt CH_(3) CH_(2) CH (Br) CH_(3) lt (CH_(3))_(3) C Br (S_(N) 1)`
` CH_(3) CH_(2) CH_(2) CH_(2) Br gt (CH_(3))_(2) CH CH_(2) Br gt CH_(3) CH_(2) CH (Br) CH_(3) gt (CH_(3))_(3) C Br (S_(N) 2)`
Of the two primary bromides , the carbocation intermediate derived from `(CH_(3))_(2) CH CH_(2) Br` is more stable than that dervied from `CH_(3) CH_(2) CH_(2) CH_(2) Br` because of greater electron donating inductive effect pf `(CH_(3))_(2)CH-` group . Therefore , `(CH_(3))_(2) CH CH_(2) Br` is more reactive than `CH_(3) CH_(2) CH_(2) Br` in `S_N 1` reactions . `CH_(3)CH_(2)CH (Br) CH_(3)` is a secondary bromide and `(CH_(3))_(3)` CBr is a tertiary bromide . Hence the above order is followed in `S_N 1` . The reactivity in `S_(N) 2` reactions follows the reverse order as the steric hinderance around the electrophilic carbon increases in that order .