Nucleophilic acyl substitution
Base hydrolysis (saponification) of an ester
How the Base hydrolysis (saponification) of an ester works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: NaOH (aq), methyl acetate. Conditions: H₂O/MeOH, reflux. Bonds formed: C–O (tetrahedral); carboxylate; O–H of MeOH; bonds broken: C=O π (temporarily); C–OMe σ. Stereochemistry: Retention at the acyl carbon (planar); no stereo here. Rate and selectivity: Rate = k[ester][HO⁻]; irreversible because the acid is deprotonated.
At a glance
Step 1: Hydroxide adds to the carbonyl
Hydroxide's lone pair attacks the electrophilic carbonyl carbon of the ester. The C=O π pair shifts onto oxygen, forming a tetrahedral alkoxide intermediate — unlike carbonyl addition, this carbon has a leaving group attached.
Step 2: Collapse expels methoxide
The tetrahedral intermediate collapses: the alkoxide oxygen re-forms the C=O π bond, pushing the methoxide group off as a leaving group.
Step 3: Irreversible proton transfer
Methoxide immediately deprotonates the resulting carboxylic acid (an essentially irreversible proton transfer), giving the carboxylate salt and methanol — why saponification runs to completion.
Abbreviations and terms
- OMe
- Methoxy. –OCH₃.
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
How it compares with related reactions
| Base hydrolysis (saponification) of an ester | Acid-catalyzed amide hydrolysis | Amide synthesis from an acid chloride and amine | |
|---|---|---|---|
| Reagents | NaOH (aq), methyl acetate | Amide, H₃O⁺ (HCl or H₂SO₄), H₂O | Benzoyl chloride, methylamine, aqueous NaOH |
| Conditions | H₂O/MeOH, reflux | 6 M acid, reflux (hours) | CH₂Cl₂ / H₂O (biphasic), 0–25 °C |
| Stereochemistry | Retention at the acyl carbon (planar); no stereo here | No stereocentre created; α-stereocentres retained | No stereocentre; planar amide |
| Rate / selectivity | Rate = k[ester][HO⁻]; irreversible because the acid is deprotonated | Water attack / C–N cleavage rate-limiting; far slower than ester hydrolysis | Very fast; base neutralises HCl so the amine is not lost as its salt |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Base hydrolysis (saponification) of an ester?
NaOH (aq), methyl acetate. Conditions: H₂O/MeOH, reflux.
Which bonds are formed and which are broken?
Formed: C–O (tetrahedral); carboxylate; O–H of MeOH. Broken: C=O π (temporarily); C–OMe σ.
What is the stereochemical outcome?
Retention at the acyl carbon (planar); no stereo here
What controls the rate or the selectivity?
Rate = k[ester][HO⁻]; irreversible because the acid is deprotonated
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go