tautomerize

Meaning: Tautomers are constitutional isomers that interconvert quickly when a hydrogen moves and a double bond shifts, usually catalyzed by acid or base. The classic case is keto-enol: a carbonyl compound with an α hydrogen moves that H to the oxygen, giving C=C–OH. Imine-enamine is the nitrogen version. For most simple aldehydes and ketones the keto form is strongly favored at equilibrium, because C=O is more stable than C=C plus an O–H.

On the MCAT: Expect questions on which tautomer is favored, or on why a compound with a chiral α carbon racemizes in acid or base (the enol or enolate intermediate is planar at that carbon, so stereochemistry is lost). The enol content rises when the enol is stabilized: phenol is an aromatic enol, and β-diketones gain conjugation and an intramolecular hydrogen bond. The idea also appears in biochemistry. Rare tautomers of nucleotide bases can mispair, and sugars can interconvert between aldose and ketose forms through an enediol.

Common confusion: Tautomers are not resonance structures. Resonance forms keep every atom in place and differ only in electron placement. Tautomers differ in the position of a hydrogen atom, so they are distinct compounds in equilibrium. Also, an enol is not an enolate; the enolate is the deprotonated anion. If an atom has moved, it is tautomerism.