Question

Which of the following reactions proceeds through concerted mechanism?

• A. Nucleophilic Substitution unimolecular
• B. Nucleophilic Substitution bimolecular
• C. Both (a) and (b)
• D. Neither (a) nor (b)

Answer 1

The correct option is B Nucleophilic Substitution bimolecular

In $S_{N}2$ reaction:
(1) Nucleophile back-side attacks the δ+ carbon center.
(2) Pentavalent unstable transition state forms in which nucleophile is forming bond with carbon while leaving group is breaking its bond simultaneously. This is known as concerted reaction mechanism.
(3) The leaving group leaves, forming the final product.
Example :


$S_{N}1$ reaction proceed in two step amd it doesnot following concerted mechanism.
Step 1: formation of carbocation by loss of leaving group.
Step 2 : attack of nucleophile to form the product.