In an `S_(N)2` substitution reaction of the type `R-Br+Cl^(-)overset("DMF")rarrR-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(|)(CH) - CH_2 Br`
C. `CH_3 - underset(CH_3) underset(|)overset(CH_3)overset(|) C - CH_2 Br`
D. `CH_3CH_2Br`

Answer

D

Explanation

The relative reactivity of alkyl halides towards `S_N 2` reactions is as follows: `1^@ gt 2^@gt 3^@`.
However, if the primary alkyl halide or the nucleophile/base is sterically hindered the nucleophile will have difficulty to getting the back side of the `alpha`-carbon as a result of this, the elimination product will be predominant. Here, `CH_3 CH_2 Br` is the least hindered, hence it has the highest relative rate towards `S_N2` reaction.