Question

Which compound undergoes faster $S_{N}1$ reaction?

• A.
• B.
• C. at equal rate
• D. cannot predict

Answer 1

The correct option is A

The first step of the $S_{N}1$ reaction is loss of a leaving group to give a carbocation, the rate of the reaction will be proportional to the stability of the carbocation.
$\text{Rate of reaction}\propto \text{stability of carbocation}$
Carbocation stability increases with increasing substitution of the carbon (tertiary > secondary > primary) due to +I and hyperconjugation effect.
(a) forms tertiary carbocation which is more stable than secondary formed by (b). Hence (a) will give faster $S_{N}1$ reaction.