Transesterification: practice and mechanism
With excess methanol and acid the ester swaps its alkoxy group: the ethyl ester becomes the methyl ester. The excess shifts the equilibrium.
Example
Like the Fischer esterification, transesterification is an acid-catalysed equilibrium. Methanol in large excess displaces the ethyl group: the ethyl ester becomes the methyl ester and ethanol is released.
Mechanism step by step
- 1
The acid protonates the ester carbonyl and makes the carbon more electrophilic.
- 2
Methanol attacks; after proton shifts ethanol is released from the tetrahedral intermediate.
- 3
Deprotonation gives the methyl ester. The excess methanol puts the equilibrium on the product side.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Without water no carboxylic acid forms. Methanol is the nucleophile, so only the alcohol part is exchanged.
- The ethyl group does not become an ether. Methanol attacks the carbonyl carbon and ethanol is released.
Often confused with: Ester hydrolysis (saponification) · Fischer esterification
Frequently asked questions
Where is this used industrially?
Biodiesel: vegetable oils (triglycerides) are transesterified with methanol to fatty acid methyl esters and glycerol.
Does it work under basic conditions?
Yes, with sodium methoxide. The important thing is that no water is present, otherwise saponification occurs.