Hydrogenation
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Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. 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 to an unsaturated organic compound, like alkene.
Contents
General
Hydrogenation has three components: the unsaturated substrate, the hydrogen (or hydrogen source) and invariably, a catalyst. The reduction reaction is carried out at different temperatures and pressures depending upon the substrate and the activity of the catalyst. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures.
The steps in the hydrogenation of a C=C double bond at a catalyst surface, for example Ni or Pt, are:
- (1) The reactants are adsorbed on the catalyst surface and H2 dissociates.
- (2) An H atom bonds to one C atom. The other C atom is still attached to the surface.
- (3) A second C atom bonds to an H atom. The molecule leaves the surface.
The addition of hydrogen to double or triple bonds in hydrocarbons is a type of redox reaction that can be thermodynamically favorable. For example, the addition of hydrogen to ethene has a Gibbs free energy change of -101 kJ/mol, which is highly exothermic. In the hydrogenation of vegetable oils and fatty acids, for example, the heat released, about 25 kcal per mole (105 kJ/mol), is sufficient to raise the temperature of the oil by 1.6–1.7 °C per iodine number drop. However, the reaction rate for most hydrogenation reactions is negligible in the absence of catalysts.
Metal-free hydrogenation
For most practical purposes, hydrogenation requires a metal catalyst. Hydrogenation can, however, proceed from some hydrogen donors without catalysts, illustrative hydrogen donors being diimide and aluminium isopropoxide, the latter illustrated by the Meerwein–Ponndorf–Verley reduction. Some metal-free catalytic systems have been investigated in academic research. One such system for reduction of ketones consists of tert-butanol and potassium tert-butoxide and very high temperatures.[1]
