Question

The mechanism of \(S_{N}{1}\) reaction is given as :

A student writes general characteristics based on the given mechanism as :
(a) The reaction is favoured by weak nucleophiles.
(b) \(R^+\) would be easily formed if the substituents are bulky.
(c) The reaction is accompanied by racemisation.
(d) The reaction is favoured by non-polar solvents.

Which observation are correct ?
\( R - X \underset{\substack{\operatorname{lon} \text { pair }}}{ R ^{\oplus}} X ^{\theta} \rightarrow R ^{\oplus} \| X ^{\ominus} \stackrel{ Y ^{\ominus}}{\longrightarrow} R - Y + X ^{\theta} \) Solvent Separated ion pair

• A. (a) and (c)
• B. (a), (b) and (c)
• C. (b) and (d)
• D. (a) and (b)

Answer 1

The correct option is B (a), (b) and (c)

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

Carbocations are stabilised by bulky group by +I effect.

Weak nucleophiles favours $S_{N}1$ reaction and strong nucleophile favour $S_{N}2$ reaction.

In polar-protic solvent, the leaving group gets solvated and favour $S_{N}1$ reaction.

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.

Hence, option (b) is correct.