Question

Which of the following statement regarding the $S_{N}1$ reaction shown by alkyl halide is incorrect?

• A. The added nucleophile plays no kinetic role in $S_{N}1$ reaction.
• B. The $S_{N}1$ reaction involves the complete inversion of the configuration of the optically active substrate
• C. The $S_{N}1$ reaction on the chiral starting material ends up with racemisation of the product.
• D. The more stable the carbocation intermediate the faster the $S_{N}1$ reaction.

Answer 1

The correct option is B The $S_{N}1$ reaction involves the complete inversion of the configuration of the optically active substrate

In $S_{N}1$ reaction mechanism, formation of carbocation is the rate determining step.
Thus,
$\text{Rate of reaction}\propto \text{stability of carbocation}$

Carbocation stability increases as tertiary > secondary > primary.

The nucleophile is not involved in the RDS in $S_{N}1$ reaction. Thus it does not affect the rate of $S_{N}1$ reaction.

Rate of the reaction $\propto [R-X{]}^1[Nu{]}^0$

Formed carbocation is $s{p}^2$ hybridised and the nucleophile can attack from both the sides which leads to 50% inversion of configuration and 50% retention of configuration. This 50-50 mixture of d and l configuration leads to racemic mixture of products.
But practically, inversion of configuration is little bit higher than 50% and retention of configuration is little bit lower than 50% due to the hindrance of leaving group. So the product will be partial racemisation.

Hence, incorrect statement is (b).