enolate ion
/ EE-no-late /
When a base removes an alpha-hydrogen from a carbonyl compound, the anion left behind is the enolate ion. Its name combines 'ene' (a double bond) and 'olate' (a deprotonated alcohol, an alkoxide). And indeed, that captures its dual nature: an enolate is a single species with the negative charge delocalized over two atoms — the alpha-carbon and the carbonyl oxygen.
Draw it as two resonance structures. In one, the negative charge sits on carbon and the C=O double bond remains — this is the carbanion form. In the other, the negative charge sits on oxygen and the double bond has shifted to between the two carbons (C=C) — this is the enolate (oxygen-anion) form. The truth is a single hybrid: one ion in which both atoms share the charge, with oxygen carrying the larger share because it is more electronegative. These are not two molecules flipping back and forth; they are two drawings of one real thing.
This split personality is exactly why the enolate is so useful. It is a nucleophile with TWO reactive sites. The oxygen is more negative, but the carbon is the softer, more useful nucleophile for building carbon skeletons — so most synthetically valuable reactions (alkylation, aldol, Michael) happen at carbon, forging a new C-C bond. The enolate is the single most important carbon nucleophile in all of carbonyl chemistry.
Treat acetone, CH3-CO-CH3, with base. Remove an alpha-H and you get the acetone enolate: drawn as CH2=C(-O minus)-CH3 (charge on O) or as (minus)CH2-CO-CH3 (charge on C). The carbon end then attacks an electrophile.
One ion, two resonance forms; the carbon end does the synthetically useful work.
Do not confuse the enolate (a negatively charged ion, made by removing a proton with base) with the enol (a neutral molecule, the alcohol tautomer made by moving the proton to oxygen). They are related but distinct: enol is neutral, enolate is anionic.