hydrogenation

Hydrogenation is a chemical reaction in which molecular hydrogen (H2) adds across unsaturated bonds, typically in the presence of a catalyst. It encompasses the reduction of carbon–carbon multiple bonds (alkenes, alkynes, and aromatics) and certain carbonyl compounds, leading to saturated products such as alkanes or alcohols.

Most hydrogenations are carried out with heterogeneous catalysts, commonly nickel, palladium, platinum, or rhodium on a support such as carbon or alumina, at elevated hydrogen pressure and moderate temperatures. Hydrogenation can also be carried out with homogeneous catalysts, including metal complexes that enable enantioselective reductions.

Industrially important examples include the saturation of vegetable oils to produce solid fats (hydrogenated oils), which historically produced trans fats under partial hydrogenation. Hydrogenation is also used to convert alkenes to alkanes, to reduce alkynes to alkenes or alkanes, and to reduce carbonyl groups in organic synthesis, pharmaceuticals, and petrochemicals.

The mechanism in heterogeneous hydrogenation typically involves adsorption of H2 and the substrate on a metal surface, dissociation of H2 into atomic hydrogen, and subsequent transfer of hydrogen to the substrate via surface intermediates (Langmuir–Hinshelwood mechanism). Reaction conditions and catalyst choice influence selectivity and rate.

Hydrogenation was developed in the early 20th century, with pioneering work by Paul Sabatier and others, and remains a fundamental transformation in organic synthesis, materials processing, and energy-related chemistry.