Base hydrolysis of an ester (saponification): ethyl ethanoate

Esters are hydrolysed by hot aqueous sodium hydroxide. Hydroxide adds to the carbonyl carbon, the alkoxide group is pushed out, and a final acid–base step makes the reaction go to completion, unlike acid hydrolysis, which is reversible.

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Nucleophilic acyl substitution (saponification)

Hydroxide attacks the ester's carbonyl carbon to form a tetrahedral intermediate. The C=O reforms and ethoxide leaves. Ethoxide then removes the acidic proton from ethanoic acid, giving ethanoate and ethanol. This last step is effectively irreversible.

Overall reaction

1. 1Hydroxide adds to the carbonylnucleophilic addition · reversible

   hydroxide ion

   +

   ethyl ethanoate

   tetrahedral intermediate

   Electron movement

   1. A lone pair on hydroxide attacks the δ+ carbonyl carbon.
   2. The C=O π electrons move onto oxygen.

   The carbonyl carbon changes from planar (sp²) to tetrahedral (sp³).

2. 2C=O reforms and ethoxide leaveselimination · reversible

   tetrahedral intermediate

   ethanoic acid

   +

   ethoxide ion

   Electron movement

   1. A lone pair on the O⁻ reforms the C=O bond.
   2. The C–OEt bond breaks and ethoxide leaves.

   Reforming the strong C=O bond drives out the ethoxide group.

3. 3Ethoxide deprotonates the acidproton transfer

   ethanoic acid

   +

   ethoxide ion

   ethanoate ion

   +

   ethanol

   Electron movement

   1. Ethoxide removes the acidic proton from ethanoic acid.
   2. The O–H electrons stay on oxygen, giving the ethanoate ion.

   A carboxylic acid is far more acidic than ethanol, so this proton transfer lies completely to the right. That is why base hydrolysis goes to completion.