Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

Keto-enol tautomerism needs an alpha hydrogen. (A) 2-methylpropanal has one alpha H; (C) cyclopentanone has alpha $\mathrm{C}{\mathrm{H}}_2$ groups; (D) acetophenone has an alpha methyl. In (B), 2,2-dimethylpropanal (pivaldehyde), the only alpha carbon is quaternary - it carries no hydrogen at all, so no enol can form. Hence (B).