hydrogenating

Hydrogenating, or hydrogenation, is the chemical reaction in which molecular hydrogen (H2) adds across unsaturated bonds in organic molecules, most commonly C=C or C≡C bonds, to give saturated products. The reaction is widely used in organic synthesis and industrial chemistry.

Most hydrogenations are catalyzed by transition metals. In heterogeneous hydrogenation, substrates and hydrogen gas adsorb on metal surfaces such as nickel, palladium, platinum, rhodium, ruthenium, or specialized forms like Raney nickel. Hydrogen atoms add to the substrate, typically in a syn fashion, producing saturated products. Reactions generally require elevated H2 pressure and moderate to high temperatures, and may be run in organic solvents or neat. Homogeneous catalysts, including certain organometallic complexes, enable milder conditions and greater selectivity for challenging substrates. Selectivity is achieved by choosing the catalyst, solvent, and reaction conditions.

Common transformations include hydrogenation of alkenes to alkanes, and selective hydrogenation of alkynes to alkenes (often using a poisoned catalyst to avoid over-reduction). Carbonyl compounds such as aldehydes and ketones can be reduced to alcohols, and more complex reductions include nitriles to amines. Some aromatic rings can be hydrogenated under forceful conditions, though these are less common industrially.

Industrial and practical applications include the hydrogenation of fats and oils in the food industry to produce solid fats; historically, partial hydrogenation produced trans fats, prompting reforms for healthier products. In refining and petrochemistry, hydrogenation is used to saturate olefins and to remove sulfur or other impurities in hydrodesulfurization. Safety considerations include the flammability of hydrogen and the need for controlled high-pressure equipment and proper catalyst handling.