Question

Which of the statement(s) is/are correct for $S_{N}2$ reaction?

• A. Carbon atom in the transition state of $S_{N}2$ reaction is pentavalent.
• B. $S_{N}2$ reaction proceeds via walden inversion in configuration of substrate.
• C. Lesser the steric hindrance in alkyl part, faster the $S_{N}2$ reaction.
• D. All of the above.

Answer 1

The correct option is D All of the above.

Mechanism of $S_{N}2$ reaction

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
process.
As shown above $S_{N}2$ reaction proceeds via walden inversion in configuration of substrate due to back attack of nucleophile.

More the bulky substituents present around the carbon, more difficulty for the nucleophile to approach the carbon having leaving group. Therefore, the order of reactivity for alkyl halides is :
$\text{Super primary>Primary>Secondary>Tertiary}$