Question

For the given pair of $S_{N}2$ reactions, indicate which occurs with a larger rate constant.

(i) ${CH}_{3}Cl+I^{-}\stackrel{Acetone}{\to }$
(ii) ${CH}_{3}Br+I^{-}\stackrel{Acetone}{\to }$

• A. $\left(i\right)>\left(ii\right)$
• B. $\left(ii\right)>\left(i\right)$
• C. $\left(i\right)=\left(ii\right)$
• D. None of these

Answer 1

The correct option is B $\left(ii\right)>\left(i\right)$

In a Bimolecular Nucleophilic Substitution Reaction, better is the leaving group, faster is the reaction.

In the given two reactions,

1. $C{H}_3-Br$ bond is weaker than $C{H}_3-Cl$ bond because ${Br}^{-}$ ion is bigger than ${Cl}^{-}$.

2. ${Br}^{-}$ is a weaker base than ${Cl}^{-}$.

These two factors make ${Br}^{-}$ the better leaving group. So, the reaction is faster in case of it.

So, the correct option is B.