Question

Which of the following structural isomer of compound $C_4{H}_{9}Br$, is most reactive towards $S_{N}1$ reaction?

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

The rate determining step of the $S_{N}1$ reaction is formation of carbocation.
Thus, 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. Thus the isomer of compound $C_4{H}_{9}Br$ which is most reactive towards $S_{N}1$ reaction is tert- butyl bromide. Hence option (a) is correct.