keto-enol tautomerism

Meaning: Tautomers are constitutional isomers that interconvert quickly by moving a hydrogen and shifting a double bond. In keto-enol tautomerism, a carbonyl compound with an α-hydrogen (keto form, C=O) equilibrates with an enol (C=C–OH), catalyzed by acid or base. In base, loss of an α-hydrogen gives an enolate, which is protonated on oxygen. For simple aldehydes and ketones the keto form is usually strongly favored, since C=O is stronger than C=C. Enols are stabilized in phenol (aromaticity) and in 1,3-dicarbonyls (conjugation and intramolecular hydrogen bonding).

On the MCAT: It underlies α-hydrogen acidity, enolate chemistry, aldol reactions and α-halogenation. A chiral α-carbon can racemize through the planar enol or enolate, and α-hydrogens exchange with deuterium in D₂O. In biochemistry, phosphoenolpyruvate is held in the enol form by its phosphate group; once the phosphate is transferred, the product tautomerizes to pyruvate.

Common confusion: Tautomers are not resonance structures. Resonance forms differ only in electron placement, with every atom in the same position. Tautomers differ in where a hydrogen sits, are real, distinct compounds, and are drawn with equilibrium arrows (⇌), not a double-headed resonance arrow.