Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Hydrogenation is widely used in the production of chemicals, such as in the production of margarine, soap, and synthetic rubber. It is also used in the refining of petroleum and in the production of pharmaceuticals.