Keto–enol tautomerism template

Propanone and its enol interconvert. Acid: protonate O, then remove an α-H. Base: remove an α-H (enolate), then protonate O.

Mechanism step by step

An α-hydrogen moves to oxygen, catalysed by acid or by base

1. Acid: O-protonation – The carbonyl O takes H⁺
2. Acid: α-H removed – Water takes an α-H; the C–H pair forms C=C
3. Base: enolate – Hydroxide removes an α-H
4. Base: O-protonation – Water protonates the enolate oxygen

Key points

• Tautomers are different compounds (an H and a σ bond move), not resonance forms.
• The keto form wins for simple ketones: propanone is less than 0.0001% enol, because a C=O π bond is much stronger than a C=C π bond.
• 1,3-Dicarbonyls such as pentane-2,4-dione are mostly enol: conjugation and an internal H bond.
• Enolisation racemises an α-stereocentre and exchanges α-H for D in D₂O.

Drawn with 52 atoms, 15 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 20 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 2 Textbook