enolate anion
Summary
An enolate anion is the resonance-stabilized conjugate base formed when a base removes an α hydrogen from a carbonyl compound; the negative charge is shared by the α carbon and the oxygen. It reacts as a carbon nucleophile, which is the basis of the aldol and Claisen reactions.
Detail
Meaning: An enolate anion is the conjugate base formed when a base removes a hydrogen from the α carbon (the carbon next to C=O) of an aldehyde, ketone, or ester. Resonance spreads the negative charge over the α carbon and the oxygen: C=C–O⁻ ↔ ⁻C–C=O. That resonance, plus the electron-withdrawing carbonyl, makes α hydrogens far more acidic (pKa near 20 for a typical ketone) than ordinary alkane C–H bonds. Protonating the oxygen gives the neutral enol.
On the MCAT: In the aldol reaction, an enolate attacks the carbonyl carbon of a second aldehyde or ketone, giving a β-hydroxy carbonyl, which can dehydrate to an α,β-unsaturated carbonyl. With esters, the Claisen condensation gives a β-keto ester. Enolates also react with alkyl halides by SN2 (alkylation). Questions ask which hydrogen is removed, what the product is, or why a hydrogen between two carbonyls is more acidic (the anion is stabilized by both).
Common confusion: The enolate is not the enol: the enolate is an anion, the enol is neutral. Only hydrogens on the α carbon are acidic this way. The aldehyde C–H and β hydrogens are not. Oxygen carries much of the charge, but the α carbon is the nucleophile in aldol reactions.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.