Question

Which one of the following halogen compounds is difficult to be hydrolysed by $S_{N}1$ mechanism?

• A. Chlorobenzene
• B. Isopropyl chloride
• C. Tertiary butyl chloride
• D. Benzyl chloride

Answer 1

The correct option is A Chlorobenzene

Since the rate determining step of $S_{N}1$ reaction involves Carbocation formation.
$\text{Rate of reaction}\propto \text{stability of carbocation}$

Carbocation at $s{p}^2$ hybridised carbon of chlorobenzene is highly unstable and will not be hydrolysed by $S_{N}1$ mechanism. Also, the aryl halide has a double bond character in the C-Cl bond which makes the cleavage more difficult.

The stability order of carbocation formed by the above halides will be c>d>b>a and hence the reactivity order toward $S_{N}1$ reaction will be c>d>b>a