enol
Summary
An enol has an OH group on a carbon of a C=C double bond (C=C–OH). It is a tautomer of a ketone or aldehyde. For most simple carbonyl compounds the keto form is strongly favored at equilibrium, though enol content rises with conjugation, hydrogen bonding or aromaticity.
Detail
Meaning: An enol (ene + ol) has an OH on an alkene carbon. It interconverts with the carbonyl (keto) form by keto-enol tautomerism: an α-hydrogen moves to oxygen and the double bond shifts from C=O to Cα=C. Acid or base catalyzes this, through an enolate in base or a protonated carbonyl in acid. The keto form usually dominates because C=O is stronger than C=C. Enol content is higher in β-dicarbonyls (conjugation, intramolecular hydrogen bonding) and in phenol (aromaticity).
Equation: keto ⇌ enol, with Keq = [enol]/[keto] (dimensionless). Keq is small for simple ketones and aldehydes.
On the MCAT: Expect to identify the tautomer and predict which form is favored. Alkyne hydration gives an enol that tautomerizes to a ketone (an aldehyde after hydroboration-oxidation of a terminal alkyne). Enols and enolates are nucleophilic at the α-carbon, which explains aldol reactions, α-halogenation and racemization of a chiral α-carbon.
Common confusion: Tautomers are not resonance forms. Tautomers are constitutional isomers in which an H changes position; they are separate species in equilibrium. Resonance structures differ only in electron placement. Both the enol and the enolate have resonance forms that place electron density on the α-carbon; the enolate, with a full negative charge, is the stronger nucleophile.
Sources
- Organic Chemistry by Clayden, Greeves and Warren
- Organic Chemistry by Wade
- Organic Chemistry by Klein
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.