Question

The most and least reactive electrophiles in $S_{N}1$ reaction are respectively :

• A. $I$ and $II$
• B. $II$ and $III$
• C. $I$ and $III$
• D. $III$ and $IV$

Answer 1

The correct option is D $III$ and $IV$

Since the rate determining step of $S_{N}1$ reaction involves carbocation formation.
$\text{Rate of reaction}\propto \text{stability of carbocation}$
Electron donating $-C{H}_3$ group stabilizes benzylic carbocation and hence increases the reactivity of corresponding substrate. So $III$ is most reactive followed by I.
Comparing II and IV, strong electron withdrawing $-N{O}_2$ decreases the stability of carbocation and hence decreases the reactivity of corresponding substrate in $S_{N}1$ reaction as compared to Br. So $IV$ is least reactive than II.
The overall reactivity order is III>I>II>IV