Hydrogenation is defined as the reaction between molecular hydrogen and an organic or an inorganic substrate. Hydrogenation reactions are exothermic but do not proceed at ordinary temperatures, except at negligible rates.
Hydrogenation reactions involving typically a catalytic reaction between molecular hydrogen (H2) and another compound or element. A large number of homogeneous and heterogeneous hydrogenation catalysts are available, and they are optimized according to a specific application.
RCOOH + 2H2 → RCH2OH + H2O
An important aspect of hydrogenation reactions is its reversibility. Hydrogenation reactions are favored at low temperatures, while an increase in temperature will drive the reaction backward. Therefore a reaction in which hydrogen molecules are added to an alkene is called a hydrogenation reaction also called reduction reaction. Alkenes will undergo an addition reaction with hydrogen gas in the presence of a catalyst.