Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

(A) cyclopentanone is a simple ketone - trace enol. (D) cyclopentane-1,2,3,4,5-pentone has no alpha hydrogen at all - zero enol. (B) cyclopentane-1,3-dione has a doubly activated $\mathrm{C}{\mathrm{H}}_2$, so it enolises well. In (C), cyclopentane-1,2,3-trione, the single $\mathrm{C}{\mathrm{H}}_2$ is flanked by carbonyls on both sides and the enol formed is further stabilised by conjugation with the remaining two carbonyls and strong intramolecular hydrogen bonding - a quasi-aromatic, croconic-acid-like system - so it has the maximum enol content. Hence (C).