Tautomerism is a special type of functional isomerism where the isomers exist in dynamic equilibrium.

• Requirement: For Keto-Enol tautomerism, the carbonyl compound must have at least one $\alpha$-hydrogen (hydrogen on the carbon next to the carbonyl group).
• Mechanism: The $\alpha$-hydrogen migrates to the oxygen, and the $C=O$ double bond shifts to $C=C$.
• Formaldehyde ($HCHO$) has no $\alpha$-carbon, so it cannot show tautomerism. Acetone and Acetaldehyde have $\alpha$-hydrogens and thus display it.