Hydrogenation

Hydrogen gas, H₂, can react with a molecule containing carbon-carbon double or triple bonds. In its simplest form, a molecule with one double bond would react with one molecule of hydrogen gas. An example is shown below.

Many carbon compounds have triple bonds, and in a case such as that, two molecules of hydrogen are necessary to completely saturate the carbon compound with hydrogen.

Hydrogenation of a double or triple carbon-carbon bond will not occur unless the catalyst is present. Scientists have developed many catalysts for this kind of reaction. Most of them include a heavy metal, such as platinum or palladium, in finely divided form. The catalyst adsorbs both the carbon compound and the hydrogen gas on its surface, in such a way that the molecules are arranged in just the right position for addition to occur. This allows the reaction to proceed at a fast enough rate to be useful.

Because at least one of the reagents (hydrogen) is a gas, often the reaction will occur at an even faster rate if it occurs in a pressurized container, at a pressure several times higher than atmospheric pressure.