Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

(A) cyclobutanone is a simple ketone - trace enol. (D) cyclobutanetetrone has no alpha hydrogen at all - zero enol. (B) cyclobutane-1,3-dione has a doubly activated CH₂, so it enolises well. In (C), cyclobutane-1,2,3-trione, the single CH₂ is flanked by carbonyls on both sides and the enol formed (1-hydroxycyclobut-1-en-2,3-dione) is further stabilised by conjugation with the remaining two carbonyls and strong intramolecular hydrogen bonding - a quasi-aromatic, squaric-acid-like system - so it has the maximum enol content. Hence (C).