Acid-catalysed ester hydrolysis template

Ethyl acetate and water with an acid catalyst: protonation of C=O, attack by water, proton transfer, loss of ethanol and deprotonation give acetic acid; every step is reversible.

Mechanism step by step

The Fischer esterification run backwards: excess water drives the equilibrium

1. Protonation – Acid activates the C=O
2. Water adds – Water attacks the carbonyl carbon
3. Proton transfer – H⁺ moves to the OEt oxygen (via the solvent)
4. Ethanol leaves – An OH lone pair pushes ethanol out
5. Deprotonation – Water takes back H⁺: the catalyst is regenerated

Key points

• Every step is the reverse of a Fischer esterification step (microscopic reversibility); a large excess of water pushes the equilibrium to the acid.
• Saponification (NaOH) differs: hydroxide is the nucleophile, a full equivalent is used up, and the final carboxylate makes it irreversible.
• With H₂¹⁸O the label ends up in the acid, not the alcohol: the acyl–oxygen bond breaks (AAC2).
• tert-Butyl esters instead lose the stable tert-butyl cation (alkyl–oxygen cleavage, AAL1), so they come off with mild acid.

Drawn with 83 atoms, 16 arrows, 11 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 10 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 16 Textbook