Saponification of fats: mechanism and explanation
A fat is a triester of glycerol and three fatty acids. Sodium hydroxide saponifies all three ester bonds: glycerol and three sodium salts of the fatty acids form, i.e. soap.
Example
The example is a short-chain fat (tributyrin, found in butter). One carboxylate is drawn standing for all three. Real soaps have 12 to 18 carbons: the long chain is non-polar, the carboxylate group polar.
Mechanism step by step
- 1
Hydroxide attacks the carbonyl carbon of one ester bond (tetrahedral intermediate).
- 2
The glycerol alkoxide leaves, giving the fatty acid, which is immediately deprotonated to the carboxylate. This makes saponification irreversible.
- 3
The same happens at the other two ester bonds: glycerol and three soap anions.
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 does soap clean?
The non-polar chains gather around grease and dirt while the carboxylate groups face the water (micelles). This lets grease be rinsed off with water.
What is the saponification number?
The mass of KOH in mg that saponifies 1 g of fat. The shorter the fatty acids, the more molecules per gram and the higher the number.