Bondline / Reactions

Keto–enol tautomerism (acid)

Protonation of the carbonyl oxygen, then loss of an α-proton, converts the keto form into the enol. The keto form is usually far more stable.

Enolates · 2 steps

Conditions

Trace acid (or base) catalyst. Explains α-halogenation and racemisation at α-carbons.

Mechanism in 2 steps

Red curly arrows show where each pair of electrons moves.

OHOH2+HOH+H2O++
Step 1The carbonyl oxygen is protonated.
HOH+H2OOHH3O+++
Step 2Water removes an α-proton; the C–H electrons form C=C and the C=O π electrons move onto oxygen.

About enolates reactions

A C–H next to a carbonyl is acidic, because the anion formed (an enolate) is stabilised by resonance. Enolates are carbon nucleophiles that make new C–C bonds.

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