Arrange the following compounds in the order of increasing `S_(N)2` reactivity and explain the order: `CH_(3)CH_(2)CH_(2)Br, CH_(3)CH_(3)Br, (CH_3)_(3)C CH_(2)Br, (CH_3)_(2)CHCH_(2)Br.

Explanation

Steric hindrance gets increased by increase in the number of alkyl groups on `beta`-carbon. As a result, attack of nucleophile from backside to `alpha`-carbon in `S_(N)2` reaction gets hampered. Thus, the order of increasing `S_(N)2` reactivity is-
`(CH_3)_(3)C CH_(2)Br < (CH_3)_(2)CHCH_(2)Br < CH_(3)CH_(2)CH_(2)Br < CH_(3)CH_(2)Br`.