Identify the major products $P$, $Q$ and $R$ in the following sequence of reactions: $\mathrm{C}{}_6\mathrm{H}{}_6+\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Cl}\stackrel{\text{anhydrous AlCl3}}{\to }\mathrm{P}$ $\mathrm{P}\underset{\text{(ii) H3O+}}{\overset{\text{(i) O2}}{\to }}\mathrm{Q}+\mathrm{R}$

Solution & Explanation

\textbf{Answer:} (D) $\mathrm{P}=\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{CH}(\mathrm{CH}{}_3){}_2$ (cumene), $\mathrm{Q}=\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{OH}$ (phenol), $\mathrm{R}=\mathrm{CH}{}_3\mathrm{COCH}{}_3$ (acetone) \textbf{Solution:} Friedel-Crafts alkylation of benzene with n-propyl chloride/$\mathrm{AlCl}{}_3$ proceeds via an n-propyl carbocation that undergoes a 1,2-hydride shift to the more stable secondary isopropyl carbocation; hence $P$ = isopropylbenzene (cumene). Cumene then follows the Hock (cumene) process: aerial $\mathrm{O}{}_2$ gives cumene hydroperoxide $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{C}(\mathrm{CH}{}_3){}_2\mathrm{OOH}$, which on acid-catalysed rearrangement cleaves to phenol ($Q$) and acetone, $\mathrm{CH}{}_3\mathrm{COCH}{}_3$ ($R$).