Hydrogenation

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 an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces double and triple bonds in hydrocarbons.

Process Hydrogenation has three components, the hydrogen source, the unsaturated substrate, and, generally, a catalyst. The conditions of the reaction - solvent (if any), temperatures, pressures - are determined by these same components. Some hydrogenations proceed at 1 atmosphere and at room temperature, while others proceed well at hundreds of degrees centigrade and hundreds of atmospheres pressure. Illustrative of a more demanding conditions is the hydrogenation of diethyl adipate, which is conducted at 2000-3000 psi H2 pressure and 255 °C. By contrast, hydrogenation of a terminal alkene using a homogeneous rhodium-based catalyst proceeds at 1 atm H2 and room temperature.

Hydrogen sources As illustrated by the preceding examples, hydrogenation reactions generally employ gaseous, elemental dihydrogen (H2), which is available commercially as pressurized cylinders. The hydrogenation process may require greater than one atmosphere of hydrogen. In some applications, a process termed transfer hydrogenation is used, wherein the hydrogen atoms added in the hydrogenation reaction are derived from compounds other than H2, e.g., formic acid, isopropanol, and dihydroanthracene (substrates dehydrogenated to yield, respectively, carbon dioxide, acetone, and anthracene.

Substrates

Catalysts Catalysts are usually classified into two broad classes: homogeneous and heterogeneous. Heterogeneous catalysts are solids that are suspended in the same solvent with the substrate or are treated with gaseous substrate. Homogeneous catalysts are dissolved in solutions containing the unsaturated substrate. Heterogeneous catalysts are more widely used, in part because they are more readily separated from the product.

Heterogeneous catalysts Heterogeneous catalysts vary widely in composition and are selected for the substrates. For the hydrogenation of alkenes and arenes, precious metals are often employed. A major exception being Raney nickel for hydrogenation of fats. For polar substrates like esters, copper catalysts are often favored. Heterogeneous catalyst vary not only in the identity of the active metal(s) but also in the catalyst supports, the material upon which most heterogeneous catalysts are deposited. Supports allow the catalysts to have high surface areas, which is especially important for precious metals. Typical supports are activated carbon, alumina, calcium carbonate or barium sulfate. For example, platinum on carbon is produced by reduction of chloroplatinic acid in situ in carbon. Examples of these catalysts are 5% ruthenium on activated carbon, or 1% platinum on alumina. Base metal catalysts, such as Raney nickel, are typically much cheaper and do not need a support. In the laboratory, unsupported (massive) precious metal catalysts such as platinum black are still used, despite the cost.

Homogeneous catalysts Some well known homogeneous catalysts are indicated below. These are coordination complexes that activate both the unsaturated substrate and the H2. Most typically, these complexes contain platinum group metals, especially Rh and Ir.

Homogeneous catalysts are also used in asymmetric synthesis by the hydrogenation of prochiral substrates. An early demonstration of this approach was the Rh-catalyzed hydrogenation of enamides as precursors to the drug L-DOPA. To achieve asymmetric reduction, these catalyst are made chiral by use of chiral diphosphine ligands. Rhodium catalyzed hydrogenation has also been used in the herbicide production of S-metolachlor, which uses a Josiphos type ligand (called Xyliphos). In principle asymmetric hydrogenation can be catalyzed by chiral heterogeneous catalysts, but this approach remains more of a curiosity than a useful technology. With rare exceptions, molecular hydrogen is unreactive toward organic compounds in the absence of metal catalysts. In a hydrogenation reaction involving a metal catalyst, unsaturated substrate is chemisorbed onto the catalyst, with most sites covered by the substrate. In heterogeneous catalysis, hydrogen forms surface hydrides (M-H) from which hydrogens can be transferred to the chemisorbed substrate. Platinum, palladium, rhodium, and ruthenium form highly active catalysts, which operate at lower temperatures and lower pressures of dihydrogen. Non-precious metal catalysts, especially those based on nickel (such as Raney nickel and Urushibara nickel) have also been developed as economical alternatives, but they are often slower or require higher temperatures. In hydrogenations, a trade-off can be the speed of the reaction vs. cost of the catalyst and apparatus required for use of high pressures. As in homogeneous catalysts, the activity is adjusted through changes in the environment around the metal, i.e. the coordination sphere. Different faces of a crystalline heterogeneous catalyst display distinct activities, for example. This can be modified by mixing metals or using different preparation techniques. Similarly, heterogeneous catalysts are affected by their supports. In many cases, highly empirical modifications involve selective "poisons". Thus, a carefully chosen catalyst can be used to hydrogenate some functional groups without affecting others, such as the hydrogenation of alkenes without touching aromatic rings, or the selective hydrogenation of alkynes to alkenes using Lindlar's catalyst. For example, when the catalyst palladium is placed on barium sulfate and then treated with quinoline, the resulting catalyst reduces alkynes only as far as alkenes. The Lindlar catalyst has been applied to the conversion of phenylacetylene to styrene.

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