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Hydrogenation – to treat with
between molecular hydrogen (H2) and another compound or element, usually in the presence of a
or . The process is commonly employed to
. Hydrogenation typically constitutes the addition of pairs of
to a molecule, generally an .
are required for the r non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces
bonds in .
Because of the importance of hydrogen, many related reactions have been developed for its use. Most hydrogenations use gaseous hydrogen (H2), but some involve the alternative sources of hydrogen, not H2: these processes are called . The reverse reaction, removal of hydrogen from a molecule, is called . A reaction where bonds are broken while hydrogen is added is called , a reaction that may occur to carbon-carbon and carbon-heteroatom (,
or ) bonds. Hydrogenation differs from
addition: in hydrogenation, the products have the same charge as the reactants.
Hydrogenation of
produces . In the case of partial hydrogenation,
may be generated as well.
Hydrogenation has three components, the
substrate, the hydrogen (or hydrogen source) and, invariably, a catalyst. The
reaction is carried out at different temperatures and pressures depending upon the substrate and the activity of the catalyst.
The addition of H2 to an alkene affords an
in the prototypical reaction:
RCH=CH2 + H2 → RCH2CH3 (R = , )
Hydrogenation is sensitive to
explaining the selectivity for reaction with the
double bond but not the internal double bond.
An illustrative example of a hydrogenation reaction is the addition of hydrogen to
to form . Numerous important applications of this
are found in pharmaceutical and food industries.
An important characteristic of alkene and alkyne hydrogenations, both the homogeneously and heterogeneously catalyzed versions, is that hydrogen addition occurs with "", with hydrogen entering from the least hindered side. Typical substrates are listed in the table
Substrates for and products of hydrogenation
, R2C=CR'2
alkane, R2CHCHR'2
many catalysts, one application is margarine
alkene, cis-RHC=CHR'
over-hydrogenation to alkane can be problematic
primary alcohol, RCH2OH
easy substrate
secondary alcohol, R2CHOH
more challenging than RCHO, prochiral for unsymmetrical ketones
two alcohols, RCH2OH + R'OH
challenging substrate
amine, RR'CHNHR"
easy substrate, often use transfer hydrogenation, actual precursor is N-protonated
, RC(O)NR'2
amine, RCH2NR'2
challenging substrate
primary amine, RCH2NH2
product amine reactive toward precursor nitrile in some cases
amine, RNH2
commercial applications use heterogeneous Ni and R major application is
With rare exceptions, no reaction below 480 °C (750 K or 900 °F) occurs between H2 and organic compounds in the absence of metal catalysts. The unsaturated substrate is
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. , , , and
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
and ) 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:
Two broad families of catalysts are known -
and . 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.
Illustrative homogeneous catalysts include the -based compound known as
and the -based . An example is the hydrogenation of :
Hydrogenation is sensitive to
explaining the selectivity for reaction with the
double bond but not the internal double bond.
The activity and selectivity of homogeneous catalysts is adjusted by changing the ligands. For
substrates, the selectivity of the catalyst can be adjusted such that one enantiomeric product is favored.
is also possible via heterogeneous catalysis on a metal that is modified by a chiral ligand.
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 . Different
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
to alkenes using . For example, when the catalyst
is placed on
and then treated with , the resulting catalyst reduces alkynes only as far as alkenes. The Lindlar catalyst has been applied to the conversion of
Asymmetric hydrogenation is also possible via heterogeneous catalysis on a metal that is modified by a chiral ligand.
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 .
Hydrogen also can be extracted ("transferred") from "hydrogen-donors" in place of H2 gas. Hydrogen donors, which often serve as
include , , dihydroanthracene, , and . In ,
is useful for the asymmetric reduction of polar unsaturated substrates, such as , , and .
substrates such as
can be hydrogenated , using
and reducing equivalents as the source of hydrogen.
Hydrogenation is a strongly
reaction. In the hydrogenation of vegetable oils and fatty acids, for example, the heat released is about 25 kcal per mole (105 kJ/mol), sufficient to raise the temperature of the oil by 1.6–1.7 °C per
of metal-catalyzed hydrogenation of alkenes and alkynes has been extensively studied. First of all
confirms the
of the addition:
RCH=CH2 + D2 → RCHDCH2D
Steps in the hydrogenation of a C=C double bond at a catalyst surface, for example Ni or Pt :
(1) The reactants are adsorbed on the catalyst surface and H2 dissociates.
(2) An H atom bonds to one C atom. The other C atom is still attached to the surface.
(3) A second C atom bonds to an H atom. The molecule leaves the surface.
On solids, the accepted mechanism is the Horiuti- mechanism:
Binding of the unsaturated bond, and hydrogen dissociation into atomic hydrogen onto the catalyst
Addition of
this step is reversible
Additio effectively irreversible under hydrogenating conditions.
In the second step, the metallointermediate formed is a saturated compound that can rotate and then break down, again detaching the alkene from the catalyst. Consequently, contact with a hydrogenation catalyst necessarily causes cis-trans-isomerization, because the isomerization is thermodynamically favorable. This is a problem in partial hydrogenation, while in complete hydrogenation the produced trans-alkene is eventually hydrogenated.
For aromatic substrates, the first bond is hardest to hydrogenate because of the free energy penalty for breaking the aromatic system. The product of this is a cyclohexadiene, which is extremely active an in conditions reducing enough to break the aromatization, it is immediately reduced to a cyclohexene. The
is ordinarily reduced immediately to a fully saturated cyclohexane, but special modifications to the catalysts (such as the use of the anti-solvent water on ruthenium) can preserve some of the cyclohexene, if that is a desired product.
In many homogeneous hydrogenation processes, the metal binds to both components to give an intermediate alkene-metal(H)2 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 ("oxidative addition"):
LnM + H2 → LnMH2
binding of alkene:
LnM(η2H2) + CH2=CHR → Ln-1MH2(CH2=CHR) + L
transfer of one hydrogen atom from the metal to carbon (migratory insertion)
Ln-1MH2(CH2=CHR) → Ln-1M(H)(CH2-CH2R)
transfer of the second hydrogen atom from the metal to the alkyl group with simultaneous dissociation of the alkane ("reductive elimination")
Ln-1M(H)(CH2-CH2R) → Ln-1M + CH3-CH2R
Preceding the oxidative addition of H2 is the formation of a .
The hydrogenation of nitrogen to give ammonia is conducted on a vast scale by the
process, consuming an estimated 1% of the .
Oxygen can be partially hydrogenated to give , although this process has not been commercialized.
Catalytic hydrogenation has diverse industrial uses. Most frequently, industrial hydrogenation relies on heterogeneous catalysts.
In petrochemical processes, hydrogenation is used to convert alkenes and aromatics into saturated alkanes (paraffins) and cycloalkanes (naphthenes), which are less toxic and less reactive. For example,
is usually hydrogenated.
of heavy residues into diesel is another application. In
processes, some hydrogen pressure is maintained to
formed on the catalyst and prevent its accumulation.
Hydrogenation is a useful reaction for converting more oxidized oxygen and nitrogen compounds such as aldehydes, imines and nitriles into the corresponding saturated compounds, i.e. alcohols and amines. Primary alcohols can be synthesized from aldehydes by hydrogenation. Thus, alkyl aldehydes, which can be synthesized with the
and an alkene, can be converted to alcohols. E.g.
is produced from propionaldehyde, produced from ethene and carbon monoxide. , a , is produced by hydrogenation of the sugar , an aldehyde. Primary amines can be synthesized by , while nitriles are readily synthesized from cyanide and a suitable electrophile. For example, isophorone diamine, a precursor to the
monomer , is produced from isophorone nitrile by a tandem nitrile hydrogenation/reductive amination by ammonia, wherein hydrogenation converts both the nitrile into an amine and the imine formed from the aldehyde and ammonia into another amine.
The largest scale application of hydrogenation is for the processing of vegetable oils. Typical vegetable oils are derived from polyunsaturated fatty acids (containing more than one carbon-carbon ). Their partial hydrogenation reduces most but not all, of these carbon-carbon double bonds. The degree of hydrogenation is controlled by restricting the amount of hydrogen, reaction temperature and time, and the catalyst.
Partial hydrogenation of a typical plant oil to a typical component of margarine. Most of the C=C double bonds are removed in this process, which elevates the melting point of the product.
Hydrogenation converts liquid vegetable
into solid or semi-solid fats, such as those present in . Changing the degree of saturation of the fat changes some important physical properties such as the melting range, which is why liquid oils become semi-solid. Solid or semi-solid fats are preferred for baking because the way the fat mixes with flour produces a more desirable texture in the baked product. Because partially hydrogenated vegetable oils are cheaper than animal source fats, are available in a wide range of consistencies, and have other desirable characteristics (e.g., increased oxidative stability/longer shelf life), they are the predominant fats used as
in most commercial baked goods.
Main article:
A side effect of incomplete hydrogenation having implications for human health is the
of some of the remaining unsaturated carbon bonds. The
configuration of these
predominates in the unprocessed fats in most edible fat sources, but incomplete hydrogenation partially converts these molecules to , which have been implicated in circulatory diseases including . The conversion from cis to trans bonds is favored because the trans configuration has lower energy than the natural cis one. At equilibrium, the trans/cis isomer ratio is about 2:1. Food legislation in the US and codes of practice in EU have long required labels declaring the fat content of foods in retail trade and, more recently, have also required declaration of the trans fat content. The use of trans fats in human food products has been effectively banned in
(since 2003) and Switzerland (2008). In the US, local legislation banned trans fats from restaurants and public kitchens in
(since 2005) and . Other countries and regions have introduced mandatory labeling of trans fats on food products and appealed to the industry for voluntary reductions. Partially hydrogenated fats can be replaced, for instance by
or by formulation such as adding
to unsaturate-rich oils.
Main article:
The earliest hydrogenation is that of
addition of hydrogen to oxygen in the , a device commercialized as early as 1823. The French chemist
is considered the father of the hydrogenation process. In 1897, building on the earlier work of , an American chemist working in the manufacture of soap products, he discovered that the introduction of a trace of nickel as a catalyst facilitated the addition of hydrogen to molecules of gaseous hydrocarbons in what is now known as the . For this work Sabatier shared the 1912 .
was awarded a patent in Germany in 1902 and in Britain in 1903 for the hydrogenation of liquid oils, which was the beginning of what is now a world wide industry. The commercially important , first described in 1905, involves hydrogenation of nitrogen. In the , reported in 1922 carbon monoxide, which is easily derived from coal, is hydrogenated to liquid fuels.
Also in 1922, Voorhees and Adams described an apparatus for performing hydrogenation under pressures above one atmosphere. The Parr shaker, the first product to allow hydrogenation using elevated pressures and temperatures, was commercialized in 1926 based on Voorhees and Adams’ research and remains in widespread use. In 1924
developed a nickel fine powder catalyst named after him which is still widely used in hydrogenation reactions such as conversion of nitriles to amines or the production of margarine.
The history of homogeneous hydrogenation has been assigned to the development the
of ketones using aluminium alkoxides. In the 1930s, Calvin discovered that copper(II) complexes oxidized H2. The 1960s witnessed the development of well defined
using transition metal complexes, e.g.,
(RhCl(PPh3)3). Soon thereafter Schrock and Obsorne's discovered that cationic Rh and Ir catalyze the hydrogenation of alkenes and carbonyls. In the 1970s, asymmetric hydrogenation was demonstrated in the synthesis of
and the 1990s saw the invention of . The development of homogeneous hydrogenation was influenced by work started in the 1930s and 1940s on the
and . More recently, homogeneous hydrogenation has been used to promote reductive C-C bond formation.
For most practical purposes, hydrogenation requires a metal catalyst. Hydrogenation can, however, proceed from some hydrogen donors without catalysts, illustrative hydrogen donors being
and . Some metal-free catalytic systems have been investigated in academic research. One such system for reduction of
consists of
and very high temperatures. The reaction depicted below describes the hydrogenation of :
study found this reaction is
in all three reactants suggesting a cyclic 6-membered .
Another system for metal-free hydrogenation is based on the -, compound 1, which has been called a . It reversibly accepts dihydrogen at relatively low temperatures to form the
2 which can reduce simple hindered .
The reduction of
has been reported to be catalysed by , its mono-anion, atmospheric hydrogen and UV light.
Today's bench chemist has three main choices of hydrogenation equipment:
Batch hydrogenation under atmospheric conditions
Batch hydrogenation at elevated temperature and/or pressure
Flow hydrogenation
The original and still a commonly practised form of hydrogenation in teaching laboratories, this process is usually effected by adding solid catalyst to a
of dissolved reactant which has been evacuated using
gas and sealing the mixture with a penetrable rubber seal. Hydrogen gas is then supplied from a H2-filled . The resulting three phase mixture is agitated to promote mixing. Hydrogen uptake can be monitored, which can be useful for monitoring progress of a hydrogenation. This is achieved by either using a graduated tube containing a coloured liquid, usually aqueous
for each reaction vessel.
Since many hydrogenation reactions such as
and the reduction of
systems proceed extremely sluggishly at atmospheric temperature and pressure, pressurised systems are popular. In these cases, catalyst is added to a solution of reactant under an inert atmosphere in a . Hydrogen is added directly from a cylinder or built in laboratory hydrogen source, and the pressurized slurry is mechanically rocked to provide agitation, or a spinning basket is used. Heat may also be used, as the pressure compensates for the associated reduction in gas solubility.
Flow hydrogenation has become a popular technique at the bench and increasingly the process scale. This technique involves continuously flowing a dilute stream of dissolved reactant over a fixed bed catalyst in the presence of hydrogen. Using established
technology, this technique allows the application of pressures from atmospheric to 1,450 psi (100 bar). Elevated temperatures may also be used. At the bench scale, systems use a range of pre-packed catalysts which eliminates the need for weighing and filtering
catalysts.
Catalytic hydrogenation is done in a
(PFR) packed with a supported catalyst. The pressures and temperatures are typically high, although this depends on the catalyst. Catalyst loading is typically much lower than in laboratory batch hydrogenation, and various promoters are added to the metal, or mixed metals are used, to improve activity, selectivity and catalyst stability. The use of nickel is common despite its low activity, due to its low cost compared to precious metals.
Gas Liquid Induction Reactors (Hydrogenator) are also used for carrying out catalytic hydrogenation.
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– early article for the general public on hydrogenation of oil produces in the 1930s