JEE Main · 2025 · 28 Jan 2025 · Shift 2 · Hydrocarbons

Hot, alkaline $KMn{O}_4$ oxidises a benzylic alkyl side chain all the way to $-COOH$, but only when that chain carries at least one hydrogen on the carbon directly attached to the ring (a benzylic $\alpha$-hydrogen); without an $\alpha$-H there is nothing for the oxidant to abstract and the side chain survives intact, and a second aromatic ring attached directly to the first has no oxidisable side chain at all. Checking each of the seven given compounds against this rule, five carry a benzylic C-H and are oxidised through to benzoic acid, while the remaining two — one with no benzylic hydrogen and one a biaryl with no alkyl chain to oxidise — are unaffected. Five of the seven compounds give benzoic acid.