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Hydrogenation is a chemical reaction in which unsaturated bonds between carbon atoms are reduced by attachment of a hydrogen atom to each carbon. The process thus results in the saturation of the carbon atoms, meaning that each carbon atom has 4 other atoms attached to it. When the process is carried to completion it converts unsaturated fatty acids to saturated ones. In practice the process is not usually carried to completion. Since the original oils typically contain more than one double bond per molecule (that is, they are poly-unsaturated), the result is usually described as partially hydrogenated vegetable oil, that is some, but usually not all, of the double bonds in each molecule have been reduced.

Hydrogenation is the process through which liquid vegetable oils are converted to solid or semi-solid fats, such as margarine. Changing the degree of saturation of the fat changes important physical properties such as melting point, which is why the liquids become semi-solid. Semi-solid fats are preferred for baking because the way the fat mixes with flour produces a more desirable texture in the baked product. Since partially hydrogenated vegetable oils are much less expensive than most other fats with similar characteristicts, and because they have other desirable characteristics leading to longer shelf life, they are the predominant fat used in most commercial baked goods.

Processes accomplishing the reverse are called "dehydrogenation" or "partial dehydrogenation."

A side effect of incomplete hydrogenation, which has implications for human health, is the isomerization of the remaining unsaturated carbon bonds. The cis configuration of these double bonds predominates in the unprocessed fats of most foods. But hydrogenation often converts these molecules to trans isomers, which in fats have been implicated in heart disease ( see trans fats ). The catalytic hydrogenation process favors the conversion from cis to trans bonds because the trans conformation is lower energy than the natural cis conformation.

Hydrogenation typically uses hydrogen gas as a reactant and an undissolved (or "heterogeneous") metal catalyst, such as nickel, palladium or platinum. Otherwise, the "homogeneous" rhodium-based catalyst known as Wilkinson's catalyst is often used. The reaction is usually carried out at elevated temperature and pressure. Such reactions belong to organic chemistry.

See also: hydrogenolysis, hydrodeoxygenation.