tautomerization

Meaning: Tautomers are constitutional isomers that interconvert by moving a proton and shifting a double bond. In keto-enol tautomerization, an α-hydrogen moves to the carbonyl oxygen, converting C=O with an α-C–H into C=C with an O–H. Base removes the α-H to form an enolate, which is then protonated on oxygen. Acid protonates the carbonyl oxygen first, then an α-H is lost. The keto form is usually favored for simple carbonyls because the C=O bond is stronger than C=C. The enol is favored more when it gains stabilization, such as conjugation, intramolecular hydrogen bonding (as in β-diketones), or aromaticity (phenol). Imine ⇌ enamine is a similar pair.

Equation: Keto ⇌ enol, with Keq = [enol]/[keto] (unitless).

On the MCAT: You may be asked to draw or pick the tautomer of a carbonyl compound, or to judge which form is favored. Tautomerization also explains why a chiral α-carbon can racemize under acid or base, how α-halogenation proceeds, and how aldoses and ketoses interconvert through an enediol. Questions on α-hydrogen acidity and enolate chemistry draw on the same idea.

Common confusion: Tautomers are not resonance structures. Resonance forms differ only in electron placement, with all atoms fixed, and are one real species. Tautomers differ in where a hydrogen sits, so they are distinct compounds in equilibrium.