Identify the major products P, Q and R in the following sequence of reactions: $\mathrm{C}{}_6\mathrm{H}{}_6+\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Cl}\stackrel{\text{anhyd. }\mathrm{AlCl}{}_3}{\to }\mathrm{P}$ $\mathrm{P}\underset{(\mathrm{ii})\mathrm{H}{}_3\mathrm{O}{}^{+}/\mathrm{\Delta }}{\overset{(\mathrm{i})\mathrm{O}{}_2}{\to }}\mathrm{Q}+\mathrm{R}$

Solution & Explanation

\textbf{Answer:} (D) P: cumene, Q: phenol, R: acetone \textbf{Solution:} In the Friedel-Crafts alkylation, $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Cl}$ with $\mathrm{AlCl}{}_3$ forms a $1^{\circ }$ propyl carbocation that rearranges by a 1,2-hydride shift to the more stable $2^{\circ }$ isopropyl cation. This attacks benzene to give cumene (isopropylbenzene), P: $\mathrm{C}{}_6\mathrm{H}{}_6+\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Cl}\stackrel{\mathrm{AlCl}{}_3}{\to }\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{CH}(\mathrm{CH}{}_3){}_2$ Cumene is then oxidised by air $\mathrm{O}{}_2$ to cumene hydroperoxide, which on treatment with $\mathrm{H}{}_3\mathrm{O}{}^{+}$/heat decomposes to phenol (Q) and acetone (R): $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{C}(\mathrm{CH}{}_3){}_2\mathrm{OOH}\stackrel{\mathrm{H}{}_3\mathrm{O}{}^{+}}{\to }\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{OH}+\mathrm{CH}{}_3\mathrm{COCH}{}_3$ This is the industrial cumene process.