Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

Keto-enol tautomerism requires an alpha C-H. Benzaldehyde (A) and benzophenone (B) have no alpha carbon bearing hydrogen (the carbonyl is attached directly to the ring). (C), 2,2,6,6-tetramethylcyclohexanone, has both alpha carbons fully substituted (no alpha H). (D), 1-phenylbutane-1,3-dione (benzoylacetone), has a CH₂ between two carbonyls, so it tautomerises readily. Hence (D).