Transesterification
Transesterification converts an ester into a different ester by exchanging its –OR group. Step-by-step mechanism, reagents and practice.
What happens
Transesterification converts an ester into a different ester by exchanging its –OR group. The carbonyl stays; the alcohol part is swapped. It can happen under acidic conditions (R²OH + H⁺) or basic conditions (R²O⁻), which give the same overall result: OR¹ OUT → OR² IN. It is a nucleophilic acyl substitution, and the carbon count depends on the alcohol exchanged.
Reaction summary
Ester 1 + R²OH (acid or alkoxide) ⇌ Ester 2 + R¹OH (OR¹ OUT → OR² IN)
Reagents and conditions
- Starting material: Methyl acetate (ester)
- Reagents: Ethanol (the new –OR group); H⁺ (acidic) or EtO⁻ (basic)
- Conditions: Acidic: R²OH + H⁺; Basic: R²O⁻ (in R²OH)
- Products: Ethyl acetate (new ester); Methanol (old alcohol)
Mechanism (Acidic conditions)
- Protonate the Carbonyl: H⁺ protonates the carbonyl oxygen. This makes the carbonyl carbon more electrophilic and easier to attack.
- New Alcohol Attacks: The new alcohol, R²OH, attacks the carbonyl carbon, and a tetrahedral intermediate forms.
- Proton Transfers: Protons move around so the old –OR¹ group can leave as R¹OH.
- Old Alcohol Leaves: The intermediate collapses: R¹OH leaves and the C=O reforms.
- Deprotonation: A proton is removed. The new ester forms, R–COOR², and the acid catalyst is regenerated.
Mechanism (Basic conditions)
- Alkoxide Attacks: The new alkoxide, R²O⁻, attacks the carbonyl carbon of the ester. The C=O opens and a tetrahedral intermediate forms.
- Carbonyl Reforms: The intermediate collapses. The C=O reforms while R¹O⁻ leaves.
- New Ester Forms: The final ester is R–COOR². Again: OR¹ OUT → OR² IN.
Common mistakes
- Match your alkoxide. Under basic conditions, the alkoxide normally corresponds to the alcohol you want to introduce: CH₃O⁻ / CH₃OH for –OCH₃, and CH₃CH₂O⁻ / CH₃CH₂OH for –OCH₂CH₃. This helps avoid unwanted mixtures and exchange reactions.
- It's an equilibrium. Transesterification is generally an equilibrium process: push it toward the desired ester with an excess of the new alcohol, or by removing the alcohol product.
Memory trick
TRANSesterification = TRANSport a new OR group in.