(i) Which alkyl halide from the followng pair is chiral and undergoes faster `S_(N)2` reaction?

(ii) Out of `S_(N)1` and `S_(N)2` which reaction occurs with
(a) inversion of configuration (b) racemisation?

Explanation

(i) Reactivity in `S_(N)2` reactions decreases in the order: `1^(@) gt 2^(@) gt 3^(@)` alkyl halide. Sine, (a) is a `1^(@)` alkyl hailde while (bZ) is a `2^(@)` alkyl halide, therefore, (a) reacts at a faster rate than (b). Further, alyl halide (b) is chiral since it contains a chiral crbon, i.e., carbon atom attached to four different atoms/groups, Br,H,`CH_(3) and CH_(2)CH_(3)`.

(ii) (a) In `S_(N)2` reactions, attack of the nucleophile occurs from the back side nad hence inversion of configuration occurs is chiral compounds. (b) In `S_(N)1` reactions, carbocations are the intermediates and hence racemisation of optically active-compounds occurs.