Question

Which of the following is/are correct regarding bimolecular nucleophilic substitution $\left(S_{N}2\right)$ reaction?

• A. It is a multistep reaction.
• B. Transition state formed in the reaction cannot be isolated.
• C. The attack of the reagent on the substrate and departure of leaving group occurs simultaneously.
• D. It occurs in a single step.

Answer 1

Answer: (B), (C), (D)

It occurs in a single step.

Mechanism for $S_{N}2$ reaction:


$S_{N}2$ reaction occurs in a single step through a transition state.

Transition State of $S_{N}2$:
The pentavalent structure, in which carbon is simultaneously attached to the incoming nucleophile and the outgoing leaving group.
It is unstable and it cannot be isolated.

$S_{N}2$ is a concerted reaction i.e. the attack of the reagent on the substrate and departure of leaving group occurs simultaneously.