Which compound in each of the following pairs will react faster by `S_(N)1` reactions in aqueous KOH. Give reasons.
(a) `CH_(3)CH_(2)-Br` or `CH_(3)CH_(2)-Cl`
(b) `CH_(3)CH_(2)CH_(2)CH_(2)-X` or `(CH_(3))_(3)C-X`

Explanation

(a) `S_(N)1` reactions occur by carbocation intermediates. Since, `CH_(3)CH_(2)Br` or `CH_(3)CH_(2)Cl` give the same carbocation, `CH_(3)CH_(2)^(+)`, therefore, stability of the carbocation does not affect the rate of the reaction. Further, since, `Br^(-)` ion is a better leaving group than `Cl^(-)` ion, therefore, `CH_(3)CH_(2)-Br` will react faster than `CH_(3)CH_(2)-Cl` with aq. KOH by `S_(N)1` mechanism.
(b). `S_(N)1` reactions occur by carbocation intermediates

Since, `3^(@)` carbocation in more stable than `1^(@)` carbocation, therefore, `(CH_(3))_(3)C-X` will react faster than `CH_(3)CH_(2)CH_(2)CH_(2)-X` with aq. KOH by `S_(N)1` mechanism.