keto-enol tautomerism

A molecule with an alpha-hydrogen actually exists as a rapidly interconverting pair of forms. The keto form is the familiar carbonyl: C=O with an ordinary C-H next door. The enol form is its alter ego: the alpha-hydrogen has migrated onto the oxygen to make an O-H (an alcohol), and the double bond has slid over to sit between the two carbons (C=C). 'Enol' literally means ene plus ol — a double bond bearing a hydroxyl. The two forms are constitutional isomers called tautomers, and they trade places constantly.

Crucially, tautomerism is NOT resonance. Resonance structures differ only in where electrons are drawn; the atoms never move. Tautomers are genuinely different molecules — a real hydrogen atom actually moves from carbon to oxygen (and back). The interconversion needs a trace of acid or base to shuttle that proton, and it happens fast, so you usually cannot bottle one tautomer pure. For a simple ketone the equilibrium lies overwhelmingly toward keto: acetone is something like 99.9999% keto at any instant.

Even though the enol is a tiny minority, it matters enormously, because the enol is a carbon nucleophile. The C=C of an enol, pushed by the lone pairs on the adjacent oxygen, can attack electrophiles — this is how acid-catalyzed alpha-halogenation and acid-catalyzed aldol reactions work, with no strong base needed. Some compounds buck the trend: phenol is essentially 100% enol (its enol form is aromatic), and 1,3-dicarbonyls hold substantial enol because that form can hydrogen-bond and conjugate.