Hydrogenation
Definition and meaning of Hydrogenation in chemistry.
Hydrogenation is a chemical reaction in which molecular hydrogen (H2) is added across a carbon-carbon double or triple bond, converting an unsaturated compound into a saturated one.
In more detail
The reaction requires a metal catalyst, typically finely divided palladium, platinum, or nickel, because the strong H-H bond does not dissociate readily on its own. The catalyst surface adsorbs both H2 and the unsaturated substrate, weakening the H-H bond and allowing hydrogen atoms to add to each carbon of the double bond, converting the pi bond into two new sigma C-H bonds. Because it adds electrons (as bonded hydrogen) to the substrate, hydrogenation is classified as a reduction. It is used industrially to convert vegetable oils into solid or semi-solid fats (as in margarine production) and to saturate alkenes and alkynes in fuel and chemical synthesis.
Key facts
| General reaction | C=C + H2 (catalyst) → C-C (single bond) |
|---|---|
| Common catalysts | Pd, Pt, Ni (e.g., Raney nickel) |
| Reaction type | Addition reaction; classified as a reduction |
| Field | Organic Chemistry |
Ethene reacts with hydrogen gas over a nickel catalyst to form ethane: H2C=CH2 + H2 (Ni catalyst) -> H3C-CH3.
Frequently asked questions
Why is a catalyst needed for hydrogenation?
The H-H bond is strong (about 436 kJ/mol) and uncatalyzed addition to a double bond has a very high activation energy. A metal catalyst adsorbs and weakens the H2 molecule on its surface, allowing hydrogen atoms to transfer to the substrate much faster.
What is partial hydrogenation and why is it controversial in food?
Partial hydrogenation stops before full saturation, which can isomerize remaining double bonds from the natural cis form to the trans form. Trans fats produced this way are linked to cardiovascular disease, leading many countries to restrict them.