hydrogenation, chemical reaction of a substance with molecular hydrogen, usually in the presence of a catalyst. A common hydrogenation is the hardening of animal fats or vegetable oils to make them solid at room temperature and improve their stability. Hydrogen is added (in the presence of a nickel catalyst) to carbon-carbon double bonds in the unsaturated fatty acid portion of the fat or oil molecule:Another hydrogenation is the synthesis of methanol from carbon monoxide. Hydrogenation reactions are important in petroleum refining; production of gasoline by cracking involves destructive hydrogenation (hydrogenolysis), in which large molecules are broken down to smaller ones and reacted with hydrogen. Most hydrogenation reactions are reversible and proceed to favorable equilibria at high pressure and moderate temperature.

Chemical reaction between molecular hydrogen (H₂) and another element or a compound, usually in the presence of a catalyst. It may involve adding hydrogen at the sites of double or triple bonds (see bonding) to make them single bonds (i.e., to saturate an unsaturated compound; see saturation), or to aromatic compounds to make them cyclic hydrocarbons. Edible oils with unsaturated fatty acids are liquid at room temperature; food manufacturers use hydrogenation to convert a fraction to saturated fatty acids to make the product more solid. A second type of hydrogenation involves breaking up a compound (hydrogenolysis, or destructive hydrogenation) and is of great importance in the petroleum industry. Numerous processes in gasoline and petrochemical manufacturing are based on it.

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