The Hydrogenation Method

Hydrogenation is the chemical reaction resulting from the addition of hydrogen to a molecule. An example is the reaction between hydrogen and ethylene (a flammable, colorless, gaseous organic hydrocarbon obtained from petroleum and natural gas) to produce ethane. Hydrogenation is normally used to saturate or reduce organic compounds. The process typically involves the addition of hydrogen to a double or triple bond in hydrocarbons, and the bonds in the product are stronger than in the reactants. Most hydrogenation methods involve usable catalysts because noncatalytic hydrogenation is very slow.

Process

• The hydrogenation method has three parts:

  1. Unsaturated substrate
  2. Catalyst
  3. Hydrogen or hydrogen source

  Hydrogenation is accomplished at various temperatures and pressure conditions based on the nature of the substrate and the catalyst used. In general, hydrogenation in absence of a catalyst must have a very high activation energy (the minimum energy that an atomic system must acquire before a chemical reaction can occur), so the majority of noncatalytic hydrogenation is carried out at a very high temperature. The catalyst accelerates the rate of hydrogenation by providing an alternative pathway that involves a sequence of several low-activation steps.

Substrate

• Typical substrates used in hydrogenation include aldehydes and ketones, alkadienes, alkenes, alkenylbenzenes, alkyl azides, alkynes, benzene, carbohydrates, carbon monoxide, esters, imines, nitriles, nitroarenes and vegetable oils.

Catalysts

• The rate of hydrogenation is increased significantly in the presence of certain catalysts. A catalyst creates a chemical bonding between the hydrogen and the substrate and facilitates their union. Platinum group metals such as platinum, rhodium, and palladium are the hydrogenation catalysts most often used. Nonprecious metal catalysts like palladium, nickel and rhodium are also effective and economical alternatives, but they are often slow at room temperature and require higher temperatures.

Hydrogen Sources

• The obvious source of hydrogen for the process of hydrogenation is the hydrogen gas itself. Hydrogen gas is commercially available within a pressurized hydrogen cylinder. In general, hydrogenation involves more than 1 atmosphere of hydrogen. Hydrogen gas also can be generated directly from "booster pumps." However, the bulk of gaseous hydrogen gas is produced industrially from hydrocarbon materials using a steam reforming process. In some cases, hydrogen is extracted from hydrogen donors instead of hydrogen gas. Common hydrogen donors include hydrazine, formic acid and isopropanol.

Heats of Hydrogenation

• Hydrogenation is an exothermic, or heat-releasing, chemical reaction. The heat released during the process of hydrogenation is called its "heat of hydrogenation." For example, in the hydrogenation of fatty acids or vegetable oils, the heat given off is approximately 25 Kcal/mole.