Hydrogenation



Hydrogenation is a class of chemical reactions which result in an addition of sodium borohydride): in hydrogenation, the products have the same charge as the reactants.

The classical example of a hydrogenation is the addition of hydrogen on petrochemical, pharmaceutical and food industries.

Health concerns associated with the hydrogenation of trans fats is an important aspect of current consumer awareness.

Process

Hydrogenation has three components:

  • the unsaturated substrate,
  • the hydrogen (or hydrogen source) and, invariably,
  • a catalyst.

The largest scale technological uses of H2 are the hydrogenation and precious metals.

The addition of H2 to an alkane in the protypical reaction:

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

An important characteristic of alkene and alkyne hydrogenations both homogeneous and heterogeneous is that hydrogen addition takes place with syn addition with hydrogen entering from the least hindered side.[3]

Catalysts

With rare exception, no reaction below 480 °C occurs between H2 and organic compounds in the absence of metal catalysts. The catalyst simultaneously binds both the H2 and the unsaturated substrate and facilitates their union. Platinum group metals, particularly 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.

Two broad families of catalysts are known - 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. In the pharmaceutical industry and for special chemical applications, soluble "Crabtree's catalyst.

The activity and selectivity of catalysts can be adjusted by changing the environment around the metal, i.e. the coordination sphere. Different prochiral substrates, the selectivity of the catalyst can be adjusted such that one enantiomeric product is produced.

Mechanism of reaction

Because of its technological relevance, metal-catalyzed “activation” of H2, has been the subject of considerable study, focusing on the regiochemistry of the addition:

RCH=CH2 + D2 → RCHDCH2D

Essentially, the metal binds to both components to give an intermediate alkene-metal(H)2 complex. The general sequence of reactions is:

  • 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 dihydrogen complex.

Hydrogen sources

The obvious source of H2 is the gas itself, often under pressure. Hydrogen can also be transferred from hydrogen-donor molecules, such as aluminium isopropoxide.

Temperatures

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

Scope

chemoselectivity):[9]

or with 4-(trimethylsilyl)-3-butyn-1-ol:[10]

The next reaction featuring Wilkinson's catalyst:[11]

Hydrogenation is sensitive to exocyclic double bond but not the internal double bond.

The compound 1-naphthol is completely reduced to a mixture of decalin-ol isomers.[12]

The compound methyl iodide to 2-methyl-1,3-cyclohexandione:[13]

An effective catalyst is the styrene.[14]

Hydrogenation is also used in formaldehyde:[15]

or the reduction of imines, for example in a synthesis of m-tolylbenzylamine:[16]

or the reduction of ammonia:[17]

In the food industry

Types of fats in food
See also

Hydrogenation is widely applied to the processing of vegetable oils and fats. Complete hydrogenation converts unsaturated 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. This is done by restricting the amount of hydrogen (or reducing agent) allowed to react with the fat.

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, 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 in most commercial baked goods. Fat blends formulated for this purpose are called shortenings.

Health implications

Main article: trans fat

A side effect of incomplete hydrogenation having implications for human health is the 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 US's first major municipal ban on most artificial trans fats in restaurant cooking.[18]

History

The earliest hydrogenation is that of catalyzed addition of hydrogen to oxygen in the Döbereiner's lamp, a device commercialized as early as 1823. 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 in what is now known as the oxo process.[19]

Metal-free hydrogenation

Although for all practical purposes hydrogenation requires a metal catalyst there exist some metal-free catalytic systems that are investigated in academic research. One such system for reduction of benzophenone:

A transition state.


Another system is based on the imine.[22]

See also

References

  1. ^ Hudlický, Miloš (1996). Reductions in Organic Chemistry. Washington, D.C.: American Chemical Society, 429. ISBN 0-8412-3344-6. 
  2. ^ Catalytic Hydrogenation of Maleic Acid at Moderate Pressures A Laboratory Demonstration Kwesi Amoa 1948 Journal of Chemical Education • Vol. 84 No. 12 December 2007
  3. ^ Advanced Organic Chemistry Jerry March 2nd Edition
  4. ^ Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001
  5. ^ Leggether, B. E.; Brown, R. K. Can. J. Chem. 1960, 38, 2363.
  6. ^ Kuhn, L. P. J. Am. Chem. Soc. 1951, 73, 1510.
  7. ^ Davies, R. R.; Hodgson, H. H. J. Chem. Soc. 1943, 281.
  8. ^ van Es, T.; Staskun, B. Org. Syn., Coll. Vol. 6, p.631 (1988); Vol. 51, p.20 (1971). (Article)
  9. ^ Organic Syntheses, Coll. Vol. 7, p.226 (1990); Vol. 64, p.108 (1986).http://orgsynth.org/orgsyn/pdfs/CV7P0226.pdf
  10. ^ Organic Syntheses, Coll. Vol. 8, p.609 (1993); Vol. 68, p.182 (1990). http://orgsynth.org/orgsyn/pdfs/CV8P0609.pdf
  11. ^ Organic Syntheses, Coll. Vol. 6, p.459 (1988); Vol. 53, p.63 (1973). http://orgsynth.org/orgsyn/pdfs/CV6P0459.pdf
  12. ^ Organic Syntheses, Coll. Vol. 6, p.371 (1988); Vol. 51, p.103 (1971). http://orgsynth.org/orgsyn/pdfs/CV6P0371.pdf
  13. ^ Organic Syntheses, Coll. Vol. 5, p.743 (1973); Vol. 41, p.56 (1961). http://orgsynth.org/orgsyn/pdfs/CV5P0567.pdf
  14. ^ Organic Syntheses, Coll. Vol. 5, p.880 (1973); Vol. 46, p.89 (1966). http://orgsynth.org/orgsyn/pdfs/CV5P0880.pdf
  15. ^ Organic Syntheses, Coll. Vol. 5, p.552 (1973); Vol. 47, p.69 (1967). http://orgsynth.org/orgsyn/pdfs/CV5P0552.pdf
  16. ^ Organic Syntheses, Coll. Vol. 3, p.827 (1955); Vol. 21, p.108 (1941). http://orgsynth.org/orgsyn/pdfs/CV3P0827.pdf
  17. ^ Organic Syntheses, Coll. Vol. 3, p.720 (1955); Vol. 23, p.71 (1943). http://orgsynth.org/orgsyn/pdfs/CV4P0603.pdf
  18. ^ "New York City passes trans fat ban", msnbc.com, 2006-12-05. Retrieved on 2007-12-03. 
  19. ^ Hydrogen-Mediated C-C Bond Formation: A Broad New Concept in Catalytic C-C Coupling Ming-Yu Ngai, Jong-Rock Kong, and Michael J. Krische J. Org. Chem.; 2007; 72(4) pp. 1063–1072; (Perspective) doi:10.1021/jo061895m
  20. ^ Homogeneous Hydrogenation in the Absence of Transition-Metal Catalysts Cheves Walling, Laszlo Bollyky J. Am. Chem. Soc.; 1964; 86(18); 3750–3752. doi:10.1021/ja01072a028
  21. ^ Hydrogenation without a Transition-Metal Catalyst: On the Mechanism of the Base-Catalyzed Hydrogenation of Ketones Albrecht Berkessel, Thomas J. S. Schubert, and Thomas N. Muller J. Am. Chem. Soc. 2002, 124, 8693–8698 doi:10.1021/ja016152r
  22. ^ Metal-Free Catalytic Hydrogenation Preston A. Chase, Gregory C. Welch, Titel Jurca, and Douglas W. Stephan Angew. Chem. Int. Ed. 2007, 46, 8050–8053 doi:10.1002/anie.200702908
 
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