Hydrogenation
Definition and meaning of Hydrogenation in chemistry.
Hydrogenation is a reaction that adds hydrogen atoms across a carbon-carbon double or triple bond. This turns an unsaturated molecule, one with double or triple bonds, into a saturated molecule with only single bonds. It is a type of chemical reduction.
In more detail
The reaction almost always needs a metal catalyst, usually palladium, platinum, or nickel. This is because the bond holding two hydrogen atoms together (H-H) is strong and hard to break on its own. The metal catalyst holds both the hydrogen gas and the unsaturated molecule on its surface at the same time.
This weakens the H-H bond and lets single hydrogen atoms attach to each carbon of the double bond. The double bond becomes a single bond, and two new carbon-hydrogen bonds form. Hydrogenation is called a reduction because it adds hydrogen, which also adds electron density, to the molecule.
Factories use hydrogenation to turn liquid vegetable oils into solid or semi-solid fats, like margarine. Stopping the reaction partway is called partial hydrogenation. This can accidentally flip some remaining double bonds from their natural bent shape into a straighter trans shape.
Trans fats made this way are linked to heart disease, so many countries now limit them in food. Food scientists now often use a different process, called interesterification, to harden fats without creating trans fats. Hydrogenation is also used on an industrial scale to turn coal or biomass-derived gases into liquid fuels.
Beyond food, refineries use similar hydrogenation steps to remove sulfur and other impurities from crude oil fractions before they become finished fuels.
Key facts
| General reaction | C=C + H2 with a catalyst gives C-C (single bond) |
|---|---|
| Common catalysts | Palladium, platinum, nickel (e.g., Raney nickel) |
| Reaction type | Addition reaction, classified as a reduction |
| Field | Organic Chemistry |
| Industrial use | Turning liquid vegetable oils into solid fats like margarine |
| Health note | Partial hydrogenation can create trans fats |
Ethene gas reacts with hydrogen gas over a nickel catalyst to form ethane. The equation is written as H2C=CH2 + H2, with a nickel catalyst, producing H3C-CH3. The double bond in ethene becomes a single bond in ethane.
Frequently asked questions
Why is a catalyst needed for hydrogenation?
The H-H bond is strong, so an uncatalyzed reaction with a double bond would need a huge amount of energy to start. A metal catalyst weakens the H2 molecule on its surface, letting the reaction happen much faster.
What is partial hydrogenation and why is it controversial in food?
Partial hydrogenation stops before every double bond is saturated. This can flip some remaining double bonds into an unnatural trans shape, and trans fats made this way are linked to heart disease.
Is hydrogenation the same as combustion?
No. Hydrogenation adds hydrogen atoms across a double bond using a catalyst and no flame. Combustion burns a fuel with oxygen and releases carbon dioxide, water, and heat.