Which alkyl halide from the following pairs would you expect to react more rapidly by an `S_(N)2` mechanism ? Explain your answer.
(i) `CH_(3)CH_(2)CH_(2)CH_(2)Br` OR `CH_(3)CH_(2)underset("Br")underset("| ")("C ")HCH_(3)`
(ii) `CH_(3)CH_(2)underset("Br")underset("| ")("C ")HCH_(3)` OR `H_(3)C-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")("C ")-Br`
(iii) `CH_(3)underset(CH_(3))underset("| ")("C ")HCH_(2)CH_(2)Br` OR `CH_(3)CH_(2)underset(CH_(3))underset("| ")("C ")HCH_(2)Br`

Explanation

The `S_(N)2` reaction procceeds through transition state where the leaving group and the nucleophile are bonded to carbon. The nucleophile always attacks from the opposite side of the leaving group and the attack is favoured if there is no crowding near the carbon. Hence `1^(@)` - halide is most reactive where `3^(@)` is least reactive.
`1^(@)R-X gt 2^(@)R-X gt 3^(@)-R-X`
(i) `CH_(3)-CH_(2)-CH_(2)-CH_(2)-Br` will be more reactive.
(ii) `CH_(3)-CH_(2)-underset("Br ")underset("| ")("C ")H-CH_(3)` will be more reactive.
(iii) `CH_(3)underset(CH_(3))underset("| ")(C ")HCH_(2)CH_(2)Br` will be more reactive, because `-CH_(3)` group is at greater distance from halogen.