JEE Main 2025 Jan 29 Shift 1, Chemistry Q25: Properties and Important Reactions of Aldehydes & Ketones

Track the transformations step by step on the cyclopentane core:

P: Excess $\mathrm{Cr}{\mathrm{O}}_3$ oxidises both the secondary alcohol on the ring and the primary $-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ side-chain. The primary alcohol is oxidised all the way to a carboxylic acid, while the ring secondary alcohol becomes a ketone. P is therefore a 2-oxocyclopentane-1-carboxylic acid type compound.

Q: Under acid catalysis, intramolecular condensation between the keto and acid functions yields a bicyclic keto-lactone (one mole acid catalyst, one elimination of water).

R: $\mathrm{C}{\mathrm{H}}_3\mathrm{MgBr}$/ether followed by ${\mathrm{H}}_3{\mathrm{O}}^{+}$ adds a methyl to the more reactive carbonyl, the lactone opening to give a hydroxy-ester / hydroxy-keto intermediate (with loss of $\mathrm{C}{\mathrm{H}}_4$ from any acidic OH proton on the Grignard's first equivalent).

S: $\mathrm{NaB}{\mathrm{H}}_4/{\mathrm{H}}_2\mathrm{O}$ reduces the remaining ketone to a secondary alcohol, leaving an open-chain or cyclopentane derivative bearing an $-\mathrm{OH}$, an $-\mathrm{OH}$ and an ester carbonyl as appropriate. S has molecular formula corresponding to a molar mass of $130\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$.

Mass of 0.1 mol of S $=0.1\times 130=13\mathbf{g}.$