Question

Arrange the following halides in decreasing order of reactivity in $S_{N}1$ reaction.

• A. II > III > IV > I
• B. IV > III > II > I
• C. III > IV > II > I
• D. I > II > III > IV

Answer 1

The correct option is A II > III > IV > I

$S_{N}1$ reaction proceeds via carbocation intermediate, hence, reactivity follow the order of stability of carbocation.
Here, besides resonance stability of benzylic carbocations, hyperconjugation by alkyl group at para positon is also affecting stability. Comparing (II, III and IV), Methyl group from para position will stabilise, the most by hyperconjugation effect due to three $\alpha -H$ followed by, ethyl group with two $\alpha -H$ and then by isopropyl group with only one $\alpha -H.$
Compound (I) does not have this hyperconjugation effect, so it is the least stable.
Hence the correct order of reactivity in $S_{N}1$ is
II > III > IV > I