Transesterification to biodiesel: mechanism and explanation
Methoxide exchanges the glycerol of the fat triester for methanol: fatty acid methyl esters (biodiesel) and glycerol form.
Example
As in any transesterification the alkoxide attacks the carbonyl carbon and another alkoxy group leaves. The excess methanol shifts the equilibrium, and the heavy glycerol settles as a separate phase.
Mechanism step by step
- 1
Methoxide attacks the carbonyl carbon of one ester bond of the fat.
- 2
The glycerol alkoxide leaves, giving a fatty acid methyl ester; the alkoxide takes a proton from methanol (catalyst regenerated).
- 3
After three transesterifications the glycerol is completely free.
To read
This topic is part of the syllabus but not in the training: polymers consist of repeating units and biomolecules react through enzymes in the cell. There is no clear-cut task with exactly one product for it.
Go to the trainer with 157 reactions to practiseFrequently asked questions
Why must the oil be dry?
Water would generate hydroxide and saponify the fat. The soap disturbs phase separation and consumes the catalyst.
Why is biodiesel thinner than vegetable oil?
Instead of one large triester there are three smaller methyl esters. They move more freely, viscosity drops and the fuel suits diesel engines.