Question

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

• A. Both I and II
• B. Both II and III
• C. both I and III
• D. Both III and IV

Answer 1

The correct option is D Both III and IV

Since the rate determining step of $S_{N}1$ reaction involves carbocation formation.
$\text{Rate of reaction}\propto \text{Stability of carbocation}$
III. Electron donating $-C{H}_3$ group stabilises benzylic carbocation, hence increases reactivity of corresponding substrate.
I is less reactive than III as carbocation formed by I is less stable than than formed by III.
IV. Electron withdrawing $-N{O}_2$ decreases stability of carbocation decreases reactivity of corresponding substrate in $S_{N}1$ reaction.
II. Electron withdrawing $-Br$ decreases stability of carbocation decreases reactivity of corresponding substrate in $S_{N}1$ reaction but this substrate is more reactive than IV.
Order of reactivity is III>I>II>IV. Hence III is most reactive and IV is least reactive.