JEE Main · 2026 · 21 Jan 2026 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

Compound P, C${}_6$H${}_{12}$O${}_3$, answers the iodoform test (it holds a CH${}_3$CO$-$ group) but fails Tollens' test (no free $-$CHO), which marks it as an acetal rather than a free aldehyde. The structure CH${}_3$COCH${}_2$CH(OCH${}_3$)${}_2$ meets this: the methyl ketone end gives the iodoform test while the acetal end is masked toward Tollens' reagent. On dilute acid the acetal hydrolyses to an aldehyde, giving Q $=$ CH${}_3$COCH${}_2$CHO, which now shows both a positive iodoform test (CH${}_3$CO$-$) and a positive Tollens' test ($-$CHO). The structure of P is CH${}_3$COCH${}_2$CH(OCH${}_3$)${}_2$.