Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

Ordinary keto-enol tautomerism needs an sp3 alpha C-H. (A) cyclohexane-1,4-dione, (B) cyclohexane-1,3-dione and (D) cyclohex-4-en-1,2-dione all have sp3 CH₂ groups alpha to a carbonyl, so they enolise. (C) is cyclohexa-2,5-dien-1,4-dione (p-benzoquinone) : every alpha carbon is an sp2 vinyl carbon, and removing a vinyl proton would give an unstable vinyl anion; no ordinary keto-enol tautomerism is observed. Hence (C).