Benzene is treated with CH3CH2CH2Cl and anhydrous AlCl3. The major hydrocarbon formed is oxidised with air (radical chain, sim 373 K) and then warmed…

The primary carbocation from 1-chloropropane/$\mathrm{AlCl_3}$ rearranges by a 1,2-hydride shift to the isopropyl cation, so the major product is cumene (isopropylbenzene), not propylbenzene. Autoxidation of cumene at the tertiary benzylic C-H gives cumene hydroperoxide, and acid-catalysed rearrangement (phenyl migration to oxygen) followed by hydrolysis gives phenol and propanone (cumene process). Propylbenzene, had it formed, would not give phenol, which is why the rearrangement is the key step.