In a `S_(N)2` substitution reaction of the type `R-Br+Cl^(-)overset(DMF)rarr R-Cl+Br^(-)` which one of the following has the highest relative rate ?

Options

A. `CH_(3)-CH_(2)-CH_(2)Br`
B. `CH_(3)-underset(CH_(3))underset("| ")("C ")H-CH_(2)Br`
C. `CH_(3)-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")("C ")-CH_(2)Br`
D. `CH_(3)CH_(2)Br`

Answer

D

Explanation

For such a reaction the rate of `SN^(2)` substitution reaction is maximum in case of `CH_(3)CH_(2)Br` because `SN^(2)` mechanism is followed in case of primary and secondary halides i.e., `SN^(2)` reaction is favored by small groups on the carbon atom attached to halogens so `CH_(3)CH_(2)Br gt CH_(3)CH_(2)CH_(2)Br gt CH_(3)-underset(CH_(3))underset("| ")("C ")H-Br gt CH_(3)-overset(CH_(3))overset("| ")underset(CH_(3))underset("| ")("C ")-Br` i.e., option (D) is correct.