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)-undersetunderset(CH_(3))(|)(CH)-CH_(2)Br`
C. `CH_(3)-undersetunderset(CH_(3))(|)overset(CH_(3))overset(|)(C)-CH_(2)Br`
D. `CH_(3)CH_(2)Br`

Answer

D

Explanation

The relative reactivity of alkyl halides towards `S_(N)2` reactions is as follows: `1^(@)gt2^(@)gt3^(@)`
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_(N)2` reaction.