Keto-Enol Tautomers(2) Tautomerism is also catalyzed by acid. ■In acid: Step 1:Protonation on O Step 2:Deprotonation on C H H,0: HO: H,O+ C=C+H,0 keto form protonated carbonyl enol form Copyright 2005 Pearson Prentice Hall,Inc. Keto-Enol Tautomers (2) ◼Tautomerism is also catalyzed by acid. ◼In acid: