Question

The compound which undergoes $S_{N}1$ reaction most rapidly is:

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

In $S_{N}1$ reaction, the rate determing step is the formation of carbocation.
Thus,
$\text{Rate of}{S}_{N}1\text{reaction}\propto \text{Stability of carbocation}$
$\text{Carbocation stability order: Allylic}>3^o>2^o>1^o>\text{Methyl}$

In (d), carbocation forms at bridge head carbon which is most unstable.