Hydrogenation of fats: mechanism and explanation
The C=C double bonds of unsaturated fatty acids are hydrogenated catalytically. Liquid oleic acid becomes solid stearic acid; oil thus becomes solid fat (margarine).
Example
The cis double bond kinks the chain and prevents close packing, which is why unsaturated fats are liquid. Without the kink the chains pack closely and the melting point rises.
Mechanism step by step
- 1
H₂ is split into H atoms on the nickel surface.
- 2
The double bond binds to the surface; the H atoms are transferred one after the other (syn addition).
- 3
The saturated chain leaves the surface. If the reaction is stopped early, a half-hydrogenated intermediate can also be left as a trans double bond.
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
What are trans fats?
During partial hydrogenation some cis double bonds isomerise to trans. Trans fats raise cardiovascular risk; today fats are usually fully hydrogenated or interesterified instead.
How is the number of double bonds determined?
With the iodine value: how many grams of iodine 100 g of fat add. The more double bonds, the higher the iodine value.