Hydrogenation

Hydrogenation is a chemical reaction between molecular hydrogen 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 unsaturated substrate, the hydrogen 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. A reaction where bonds are broken while hydrogen is added is called hydrogenolysis, a reaction that may occur to carbon-carbon and carbon-heteroatom bonds. Some hydrogenations of polar bonds are accompanied by hydrogenolysis.

Hydrogen sources

For hydrogenation, the obvious source of hydrogen is 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, 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.

Substrates

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.

Substrate	Product	Comments	Heat of hydrogenation
		large application is production of margarine	−90 to −130
		semihydrogenation gives	−300
		often employs transfer hydrogenation	−60 to −65
		often employs transfer hydrogenation	−60 to −65
		often applies to production of fatty alcohols	−25 to −105
		applicable to fatty alcohols	−25 to −75
		major application is aniline	−550

Catalysts

With rare exceptions, 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 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. Non-precious metal catalysts, especially those based on nickel have also been developed as economical alternatives, but they are often slower or require higher temperatures. The trade-off is activity 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 and heterogeneous. 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.

Homogeneous catalysts

Some well known homogeneous catalysts are indicated below. These are coordination complexes that activate both the unsaturated substrate and the. 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. 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. 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

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 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.

Transfer hydrogenation

Transfer hydrogenation uses hydrogen-donor molecules other than molecular. These "sacrificial" hydrogen donors, which can also serve as solvents for the reaction, include hydrazine, formic acid, and alcohols such as isopropanol.
In organic synthesis, transfer hydrogenation is useful for the asymmetric hydrogenation of polar unsaturated substrates, such as ketones, aldehydes and imines, by employing chiral catalysts.

Electrolytic hydrogenation

substrates such as nitriles can be hydrogenated electrochemically, using protic solvents and reducing equivalents as the source of hydrogen.

Thermodynamics and mechanism

The addition of hydrogen to double or triple bonds in hydrocarbons is a type of redox reaction that can be thermodynamically favorable. For example, the addition of hydrogen to ethene has a Gibbs free energy change of -101 kJ·mol⁻¹, which is highly exothermic. In the hydrogenation of vegetable oils and fatty acids, for example, the heat released, about 25 kcal per mole, is sufficient to raise the temperature of the oil by 1.6–1.7 °C per iodine number drop.
However, the reaction rate for most hydrogenation reactions is negligible in the absence of catalysts. The mechanism of metal-catalyzed hydrogenation of alkenes and alkynes has been extensively studied. First of all isotope labeling using deuterium confirms the regiochemistry of the addition:

Heterogeneous catalysis

On solids, the accepted mechanism is the Horiuti-Polanyi mechanism:

Binding of the unsaturated bond
Dissociation of on the catalyst
Addition of one atom of hydrogen; this step is reversible
Addition of the second atom; effectively irreversible.

In the third step, the alkyl group can revert to alkene, which can detach from the catalyst. Consequently, contact with a hydrogenation catalyst allows cis-trans-isomerization. The trans-alkene can reassociate to the surface and undergo hydrogenation. These details are revealed in part using D₂, because recovered alkenes often contain deuterium.
For aromatic substrates, the first hydrogenation is slowest. The product of this step is a cyclohexadiene, which hydrogenate rapidly and are rarely detected. Similarly, the cyclohexene is ordinarily reduced to cyclohexane.

Homogeneous catalysis

In many homogeneous hydrogenation processes, the metal binds to both components to give an intermediate alkene-metal₂ complex. The general sequence of reactions is assumed to be as follows or a related sequence of steps:

binding of the hydrogen to give a dihydride complex via oxidative addition :
binding of alkene:
transfer of one hydrogen atom from the metal to carbon :
transfer of the second hydrogen atom from the metal to the alkyl group with simultaneous dissociation of the alkane

Alkene isomerization often accompanies hydrogenation. This important side reaction proceeds by beta-hydride elimination of the alkyl hydride intermediate:
Often the released olefin is trans.