Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

Ordinary keto-enol tautomerism needs an sp3 alpha C-H. (A) cyclopentane-1,3-dione, (C) heptane-2,6-dione and (D) cyclohex-3-en-1-one all have sp3 $\mathrm{C}{\mathrm{H}}_2$ groups alpha to a carbonyl, so they enolise. (B) 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 (B).