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 (
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
 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
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.
 Typical substrates are listed in the table
Substrates for and products of hydrogenation
||heat of hydrogenation
|large application is production of margerine
||-90 to -130 kcal/mol
semihydrogenation gives cis-RHC=CHR'
||-300 kcal/mol for full hydrogenation
||-60 to -65 kcal/mol
||-60 to -65 kcal/mol
|RCH2OH + R'OH
|often applies to production of
||-25 to -105 kcal/mol
|applicable to fatty alcohols
||25 to -75 kcal/mol
|major application is
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. 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. Similarly, heterogeneous catalysts are affected by their supports, i.e. the material upon with the heterogeneous catalyst is bound.
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
Hydrogen also can be extracted ("transferred") from "hydrogen-donors" in place of H2 gas. Hydrogen donors, which often serve as
transfer hydrogenation is useful for the asymmetric reduction of polar unsaturated substrates, such as
imines. The hydrogenation of polar substrates such as ketones and aldehydes typically require
transfer hydrogenation, at least in homogeneous catalysis. These catalysts are readily generated in chiral forms, which is the basis of asymmetric hydrogenation of ketones.
Transfer hydrogenation catalyzed by transition metal complexes proceeds by an "outer sphere mechanism."
Polar substrates such as
nitriles can be hydrogenated
protic solvents and reducing equivalents as the source of hydrogen.