Hydrogenation

Hydrogenation is a fundamental chemical reaction used to add hydrogen to other compounds or elements. This process is most often used to reduce or saturate organic compounds, such as alkenes. In these reactions, pairs of hydrogen atoms are added to a molecule, which can reduce double or triple bonds within hydrocarbons. Because these reactions are often difficult to start, they almost always require a catalyst. A catalyst is a substance that enables the reaction to occur more efficiently. Without a catalyst, hydrogenation typically only takes place at extremely high temperatures.

The mechanism of hydrogenation depends on whether the catalyst is heterogeneous or homogeneous. In heterogeneous catalysis, which uses a solid catalyst, the process often follows the Horiuti-Polanyi mechanism. First, the unsaturated substrate undergoes chemisorption, meaning it sticks to the catalyst surface. Next, the molecular hydrogen (H2) dissociates into individual hydrogen atoms on the surface. An H atom then bonds to one carbon atom of the substrate, while the other carbon remains attached to the surface. Finally, a second H atom bonds to the second carbon atom, and the completed molecule leaves the surface.

Scientists classify catalysts into two broad categories: homogeneous and heterogeneous. Homogeneous catalysts are those that dissolve in the same solvent as the unsaturated substrate. These are often coordination complexes containing platinum group metals, such as rhodium or iridium. Heterogeneous catalysts are solids that are either suspended in a solvent or treated with a gaseous substrate. In industrial settings, heterogeneous catalysts are more common. Often, precious metals like platinum or ruthenium are deposited as a fine powder onto a porous support. Common supports include activated carbon, alumina, calcium carbonate, or barium sulfate.

Different metals offer different advantages regarding cost and activity. Precious metals like platinum, palladium, rhodium, and ruthenium are highly active. They can operate effectively at lower temperatures and lower pressures of hydrogen. However, these metals are expensive. Non-precious metals, such as nickel, serve as more economical alternatives. Examples include Raney nickel and Urushibara nickel. While cheaper, these base metal catalysts are often slower or require much higher temperatures and pressures to function.

Hydrogenation is not always performed using pure hydrogen gas. In a process called transfer hydrogenation, hydrogen is moved from donor molecules to the substrate. These donor molecules, such as formic acid, isopropanol, or dihydroanthracene, are often called "sacrificial" because they undergo dehydrogenation during the process. For instance, formic acid becomes carbon dioxide, and isopropanol becomes acetone. This method is particularly useful in organic synthesis for the asymmetric hydrogenation of polar substrates like ketones and aldehydes.

Industrial applications of hydrogenation are vast and impact many sectors. One major use is in the food industry, where the hydrogenation of vegetable oils produces margarine. However, this process can lead to isomerization, where alkenes change from a cis configuration to a trans configuration. This reaction is the primary way trans fats are generated in foods. On a much larger scale, the Haber-Bosch process uses a catalyst to hydrogenate nitrogen into ammonia. This single industrial process is so massive that it consumes an estimated 1% of the entire world's energy supply.

Understanding the energy changes in these reactions is also vital. Many hydrogenation reactions are exothermic, meaning they release heat. For example, adding hydrogen to ethene has a Gibbs free energy change of -101 kJ·mol−1. In the production of margarine, the heat released is enough to raise the temperature of the oil by 1.6 to 1.7 °C for every drop in the iodine number. This thermal energy release is a key characteristic of the chemical transformation occurring at the molecular level.