It 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.
Related or competing reactions
The same catalysts and conditions that are used for hydrogenation reactions can also lead to isomerization of the alkenes from cis to trans. This process is of great interest because hydrogenation technology generates most of the trans fat in foods (see below). A reaction where bonds are broken while hydrogen is added is called hydrogenolysis, a reaction that may occur to carbon-carbon and carbon-heteroatom (oxygen, nitrogen or halogen) bonds. Some hydrogenations of polar bonds are accompanied by hydrogenolysis.
For hydrogenation, the obvious source of hydrogen is H2 gas itself, which is typically available commercially within the storage medium of a pressurized cylinder. The hydrogenation process often uses greater than 1 atmosphere of H2, usually conveyed from the cylinders and sometimes augmented by "booster pumps". Gaseous hydrogen is produced industrially from hydrocarbons by the process known as steam reforming. For many applications, hydrogen is transferred from donor molecules such as formic acid, isopropanol, and dihydroanthracene. These hydrogen donors undergo dehydrogenation to, respectively, carbon dioxide, acetone, and anthracene. These processes are called transfer hydrogenations.
An important characteristic of alkene and alkyne hydrogenations, both the homogeneously and heterogeneously catalyzed versions, is that hydrogen addition occurs with "syn addition", with hydrogen entering from the least hindered side. This reaction can be performed on a variety of different functional groups.
With rare exceptions, H2 is unreactive toward organic compounds in the absence of metal catalysts. The unsaturated substrate is chemisorbed onto the catalyst, with most sites covered by the substrate. In heterogeneous catalysts, 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 H2. 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. The trade-off is activity (speed of reaction) vs. cost of the catalyst and cost of the apparatus required for use of high pressures. Notice that the Raney-nickel catalysed hydrogenations require high pressures:
Catalysts are usually classified into two broad classes: homogeneous catalysts and heterogeneous catalysts. Homogeneous catalysts dissolve in the solvent that contains the unsaturated substrate. Heterogeneous catalysts are solids that are suspended in the same solvent with the substrate or are treated with gaseous substrate.
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.
hydrogenation of propylene with Wilkinson's catalyst
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.
Heterogeneous catalysts for hydrogenation are more common industrially. In industry, precious metal hydrogenation catalysts are deposited from solution as a fine powder on the support, which is a cheap, bulky, porous, usually granular material, such as 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. Also, in the laboratory, unsupported (massive) precious metal catalysts such as platinum black are still used, despite the cost.
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.
The transition state
of two transfer-hydrogenation reactions from ruthenium-hydride complexes onto carbonyls
Transfer hydrogenation uses other hydrogen donor molecules in place of H2 itself. These reactants, which can also serve as solvents for the reaction, include hydrazine, dihydronaphthalene, dihydroanthracene, isopropanol, and formic acid. The reaction involves an outer-sphere mechanism.
In organic synthesis, transfer hydrogenation is useful for the asymmetric reduction of polar unsaturated substrates, such as ketones, aldehydes, and imines. The hydrogenation of polar substrates such as ketones and aldehydes typically requires transfer hydrogenation, at least reactions that use homogeneous catalysts. These catalysts are readily generated in chiral forms, which is the basis of asymmetric hydrogenation of ketones.
Polar substrates such as nitriles can be hydrogenated electrochemically, using protic solvents and reducing equivalents as the source of hydrogen.