keto-enol tautomerization
Summary
Keto-enol tautomerization is the rapid, acid- or base-catalyzed interconversion of a carbonyl compound and its enol by moving a proton and shifting the double bond. For simple aldehydes and ketones the keto form is usually favored at equilibrium.
Detail
Meaning: Keto-enol tautomerization is the interconversion of a carbonyl compound (the keto form, with an α-hydrogen) and an enol (C=C with an OH). A proton moves and the double bond shifts, so the forms are constitutional isomers in rapid equilibrium. Acid or base catalyzes it: base removes the α-hydrogen to give an enolate that is protonated on oxygen; acid protonates the carbonyl oxygen, then an α-hydrogen is lost. Simple aldehydes and ketones usually favor the keto form because C=O is stronger than C=C. Conjugation, intramolecular hydrogen bonding (β-diketones), or aromaticity (phenol) can favor the enol.
Equation: R–CO–CH₂R′ ⇌ R–C(OH)=CHR′, with Keq = [enol]/[keto]. Keq is dimensionless; concentrations are in mol/L.
On the MCAT: Hydration of an alkyne gives an enol that tautomerizes to a ketone (an aldehyde for a terminal alkyne by hydroboration–oxidation). Enolization at a chiral α-carbon racemizes it and swaps α-hydrogens for deuterium in D₂O. Enols and enolates are the nucleophiles in aldol and α-halogenation reactions.
Common confusion: Tautomers are not resonance structures. Resonance forms differ only in electron placement, while tautomers differ in the position of a hydrogen atom and are distinct compounds. Only carbonyls with an α-hydrogen can enolize.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.