Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

In (A), cyclopent-4-en-1,3-dione, enolisation of the central CH₂ would give 3-hydroxycyclopenta-2,4-dien-1-one : the ring would then be a fully conjugated cyclopentadienone system with 4 π electrons - anti-aromatic and very unstable, so the enol content is negligible (essentially 100% keto). (B) cyclobutane-1,3-dione, (C) cyclopentane-1,2-dione and (D) pentane-2,4-dione all give ordinarily stabilised enols. Hence (A).