Hydrogenation

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For trans fatty acid, see Trans fat

Hydrogenation is a class of chemical reactions in which the net result is an addition of hydrogen (H2). The usual targets of hydrogenation are unsaturated organic compounds, such as alkenes, alkynes, ketones, nitriles, and imine.[1] Most hydrogenations involve the direct addition of diatomic hydrogen under pressure in the presence of catalysts. Some hydrogenations involve the indirect addition of hydrogen, these are called transfer hydrogenations.

The classical example of a hydrogenation is the addition of hydrogen on unsaturated bonds between carbon atoms, converting alkenes to alkanes. Numerous important applications are found in the pharmaceutical and petrochemical industries.

The reverse of hydrogenation is dehydrogenation.

Contents

[edit] History

The French chemist Paul Sabatier is considered the father of the hydrogenation process. In 1897 he discovered that the introduction of a trace of nickel as a catalyst facilitated the addition of hydrogen to molecules of gaseous carbon compounds. Wilhelm Normann was awarded a patent in Germany in 1902 and in Britain in 1903 for the hydrogenation of liquid oils using hydrogen gas, which was the beginning of what is now a very large industry world wide.

[edit] The hydrogenation process

The largest scale technological uses of H2 are the hydrogenation and hydrogenolysis reactions associated with both heavy and fine chemicals industries. Hydrogenation is the addition of H2 to unsaturated organic compounds such as alkenes to give alkanes and aldehydes to give alcohols. Hydrogenolysis is the cleavage of C-X (X = O, S, N) bonds by H2 to give C-H and H-X bonds. Large-scale applications of hydrogenolysis reactions are associated with the upgrading of fossil fuels. Hydrogenation and hydrogenolysis reactions require metal catalysts, often those composed of platinum or similar precious metals. It is a curious fact that under mild conditions, H2 reacts directly with no organic compound in the absence of such catalysts.

The addition of H2 to an alkene affords an alkane is the protypical reaction:

RCH=CH2 + H2 → RCH2CH3 (R = alkyl, aryl)

[edit] The catalysts

All reactions between organic compounds and H2 require metal catalysts. With rare exception, no reaction below 480 °C occurs between H2 and organic compounds in the absence of metal catalysts. The catalyst adsorbs both the hydrogen gas and the organic molecule and facilitates their interaction. Platinum group metals, particularly platinum, palladium, rhodium, and ruthenium, are highly effective as catalysts. Non-precious metal catalysts, especially those based on nickel (such as Raney nickel and Urushibara nickel) have also been developed as economical alternatives. The trade-off is activity (speed of reaction) vs. cost of the catalyst and cost of the apparatus required for use of high pressures.

The activity and selectivity of catalysts can be adjusted by combining the metals with various other elements and ligands. Such modifications entail changing the coordination sphere of the metal center that is responsible of the transfer of the hydrogen to the substrate. 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 prochiral substrates, the selectivity of the catalyst can be adjusted such that one enantiomeric product is produced.

[edit] Hydrogenolysis

The catalytic hydrogenation of organic sulfur compounds to form gaseous hydrogen sulfide (H2S) is very widely used in petroleum refineries, petrochemical plants and other industries to desulfurize various final products, intermediate products and process feedstocks by converting sulfur compounds to gaseous hydrogen sulfide which is then easily removed by distillation. The gaseous hydrogen sulfide is subsequently recovered in an amine treater and finally converted to elemental sulfur in a Claus process unit. In those industries, desulfurization process units are often referred to as hydrodesulfurizers (HDS) or hydrotreaters (HDT).

In the petroleum refining and petrochemical industries, cobalt-molybdenum or nickel-molybdenum catalysts are commonly used for hydrogenation and hydrogenolysis catalysts.

In the pharmaceutical industry and for special chemical applications, soluble ""homogeneous"" catalyst are sometimes employed, such as the rhodium-based compound known as Wilkinson's catalyst, or the iridium-based Crabtree's catalyst.

[edit] Mechanism of reaction

First of all deuterium can be used to determine the regiochemistry of the addition:

RCH=CH2 + D2 → RCHDCH2D

Because of its technological relevance, metal-catalyzed “activation” of H2, has been the subject of considerable study, focusing on the mechanisms of by which metals mediate these reactions.[2] The general sequence of reactions is:

formation of unsaturated metal center by dissociation of a ligand:
LnM → LnM + L
binding of the hydrogen to give an dihydrogen complex:
Ln-1M + H2 → Ln-1M(η2H2)
oxidative addition of the hydrogen to give an dihydride complex:
Ln-1M(η2H2) → Ln-1MH2

The resulting metal dihydride then transfers the hydrogen atoms stepwise to the alkene.

The chemical details of the large variety of reactions called hydrogenation are generally similar. The mechanism by which the metals accelerate the combination of the alkene substrate and the hydrogen has been the subject of intense research. Essentially, the metal binds to both components to give an intermediate alkene-metal(H)2 complex. Preceding this step is the formation of metal-η2-H2 complexes.[3]

[edit] Hydrogen sources

Obviously source of H2 is the gas itself, often under pressure. Hydrogen can also be transferred from hydrogen-donor molecules, such as hydrazine[4][5], dihydronaphthalene, dihydroanthracene, isopropanol, and formic acid[6][7]. Transfer hydrogenation can be metal catalysed. But the reaction also proceeds well with some donors without catalysts, examples being diimide and aluminium isopropoxide.

[edit] Temperatures

The reaction is carried out at different temperatures and pressures depending upon the substrate. Hydrogenation is a strongly exothermic 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 iodine number drop.

[edit] Hydrogenation in food industry

Hydrogenation is widely applied to the processing of vegetable oils and fats. Complete hydrogenation converts unsaturated fatty acids to saturated ones. In practice the process is not usually carried to completion. Since the original oils usually contain more than one double bond per molecule (that is, they are poly-unsaturated), the result is usually described as partially hydrogenated vegetable oil; that is some, but usually not all, of the double bonds in each molecule have been reduced.

Hydrogenation results in the conversion of liquid vegetable oils to solid or semi-solid fats, such as those present in margarine. Changing the degree of saturation of the fat changes some important physical properties such as the melting point, which is why liquid oils become semi-solid. Semi-solid fats are preferred for baking because the way the fat mixes with flour produces a more desirable texture in the baked product. Since partially hydrogenated vegetable oils are cheaper than animal source fats, they are available in a wide range of consistencies, and have other desirable characteristics (eg, increased oxidative stability (longer shelf life)), they are the predominant fats used in most commercial baked goods. Fat blends formulated for this purpose are called shortenings.

[edit] Health implications

A side effect of incomplete hydrogenation having implications for human health is the isomerization of the remaining unsaturated carbon bonds. The cis configuration of these double bonds predominates in the unprocessed fats in most edible fat sources, but incomplete hydrogenation partially converts these molecules to trans isomers, which have been implicated in circulatory diseases including heart disease (see trans fats). The catalytic hydrogenation process favors the conversion from cis to trans bonds 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 has long required labels declaring the fat content of foods in retail trade, and more recently, have also required declaration of the trans fat content.

In 2006, New York City adopted the USA’s first major municipal ban on all but tiny amounts of artificial trans fats in restaurant cooking. The rules, which will be phased in, require that restaurants eliminate margarines and shortenings that contain more than a trace of trans fats by July 1, 2007, and remove all items from their menus that exceed a limit of a half-gram of trans fat per serving by July 1, 2008.

[edit] See also

[edit] References

  1. ^ Hudlický, Miloš (1996). Reductions in Organic Chemistry. Washington, D.C.: American Chemical Society, 429. ISBN 0-8412-3344-6.
  2. ^ Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001
  3. ^ Agostic complex
  4. ^ Leggether, B. E.; Brown, R. K. Can. J. Chem. 1960, 38, 2363.
  5. ^ Kuhn, L. P. J. Am. Chem. Soc. 1951, 73, 1510.
  6. ^ Davies, R. R.; Hodgson, H. H. J. Chem. Soc. 1943, 281.
  7. ^ van Es, T.; Staskun, B. Org. Syn., Coll. Vol. 6, p.631 (1988); Vol. 51, p.20 (1971). (Article)

[edit] External links

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