Question

Assertion: Walden inversion takes place in $S_{N}2$ reaction.

Reason: Half-life period of $S_{N}2$ reaction is inversely proportional to the concentration of the substrate or nucleophile or both.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct
• E. Both Assertion and Reason are incorrect

Answer 1

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

The reason for Walden inversion is the following:

The nucleophile attacks the carbon at $180$° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the $C–X$ σ* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed with inversion of the tetrahedral geometry at the central atom.

The reason is correct but it's not the correct explanation

The rate of a $S_{N}2$ reaction is second order, as the rate-determining step depends on the nucleophile concentration, $[Nu-]$ as well as the concentration of substrate,$\left[RX\right]$.

$r=k\left[RX\right][Nu-]$

$t_{1/2}$ of the reaction also depends on both substrate and nucleophile.

Thus, Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

Hence, the correct option is $\text{B}$