Hydrogenation 235968 224552095 2008-07-09T11:07:20Z 217.150.100.154 [[Image:MaleicAcidHydrogenation.svg|right|Hydrogenation of maleic acid]]'''Hydrogenation''' is a [[redox|reductive]] [[chemical reaction]] which results in an addition of [[hydrogen]] (H<sub>2</sub>), usually in order to [[Saturation (chemistry)|saturate]] [[organic compound]]s. The process constitutes the addition of [[hydrogen]] [[atom]]s to the [[double bond]]s of a molecule through the use of a [[catalyst]]. [[Hydrogen]] also adds to [[triple bond]]s if they are present. Typical substrates include [[alkenes]], [[alkyne]]s, [[ketone]]s, [[nitrile]]s, and [[imine]]s.<ref>{{cite book|last=Hudlický|first=Miloš|title=Reductions in Organic Chemistry|publisher=[[American Chemical Society]]|date=1996|location=Washington, D.C.|pages=429|id=ISBN 0-8412-3344-6}}</ref> Most hydrogenations involve the direct addition of diatomic hydrogen (H<sub>2</sub>) but some involve the alternative sources of hydrogen, not H<sub>2</sub>: these processes are called [[transfer hydrogenation]]s. The reverse reaction, removal of hydrogen, is called [[dehydrogenation]]. A reaction involving hydrogen and cleavage of a [[carbon-oxygen bond]] or [[carbon-nitrogen bond]] is called [[Hydrogenolysis]]. Hydrogenation differs from [[protonation]] or [[hydride]] addition (e.g. use 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 [[alkene|unsaturated]] [[chemical bond|bonds]] between [[carbon]] [[atom]]s, converting [[alkenes]] to [[alkanes]]. A simple example is the hydrogenation of [[maleic acid]] to [[succinic acid]] depicted on the right.<ref>''Catalytic Hydrogenation of Maleic Acid at Moderate Pressures A Laboratory Demonstration'' Kwesi Amoa 1948 [[Journal of Chemical Education]] • Vol. 84 No. 12 December '''2007'''</ref> Numerous important applications are found in the [[petrochemical]], pharmaceutical and food industries. Health concerns associated with the hydrogenation of [[unsaturated fat]]s to produce [[saturated fat]]s and [[trans fat]]s 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 H<sub>2</sub> are the hydrogenation and [[hydrogenolysis]] reactions associated with both heavy and fine chemicals industries. Hydrogenation is the addition of H<sub>2</sub> to [[unsaturated]] [[organic compound]]s such as [[alkene]]s to give [[alkane]]s and [[aldehyde]]s to give [[alcohol]]s. Hydrogenation reactions require metal catalysts, often those composed of [[platinum]] or similar [[precious metal]]s. The addition of H<sub>2</sub> to an [[alkene|alk''e''ne]] affords an [[alkane|alk''a''ne]] in the protypical reaction: :RCH=CH<sub>2</sub> + H<sub>2</sub> → RCH<sub>2</sub>CH<sub>3</sub> (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.<ref>''Advanced Organic Chemistry'' Jerry March 2nd Edition</ref> ===Catalysts=== With rare exception, no reaction below 480 °C occurs between H<sub>2</sub> and organic compounds in the absence of metal catalysts. The catalyst simultaneously binds ''both'' the H<sub>2</sub> and the unsaturated substrate and facilitates their union. Platinum group metals, particularly [[platinum]], [[palladium]], [[rhodium]] and [[ruthenium]], are highly active catalysts. Highly active catalysts operate at lower temperatures and lower pressures of H<sub>2</sub>. 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. 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 "[[Homogeneous catalysis|"homogeneous"]]" catalyst are sometimes employed, such as the [[rhodium]]-based compound known as [[Wilkinson's catalyst]], or the [[iridium]]-based [[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 [[miller index|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. Homogeneous catalysts are affected by their [[ligand]]s. 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 [[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 H<sub>2</sub>, has been the subject of considerable study, focusing on the [[reaction mechanism]]s of by which metals mediate these reactions.<ref>Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001</ref> First of all [[isotope labeling]] using [[deuterium]] can be used to determine the [[regiochemistry]] of the addition: :RCH=CH<sub>2</sub> + D<sub>2</sub> → RCHDCH<sub>2</sub>D Essentially, the metal binds to both components to give an intermediate alkene-metal(H)<sub>2</sub> complex. The general sequence of reactions is: *binding of the hydrogen to give a dihydride complex ("oxidative addition"): :L<sub>n</sub>M + H<sub>2</sub> → L<sub>n</sub>MH<sub>2</sub> *binding of alkene: :L<sub>n</sub>M(η<sup>2</sup>H<sub>2</sub>) + CH<sub>2</sub>=CHR → L<sub>n-1</sub>MH<sub>2</sub>(CH<sub>2</sub>=CHR) + L *transfer of one hydrogen atom from the metal to carbon (migratory insertion) :L<sub>n-1</sub>MH<sub>2</sub>(CH<sub>2</sub>=CHR) → L<sub>n-1</sub>M(H)(CH<sub>2</sub>-CH<sub>2</sub>R) *transfer of the second hydrogen atom from the metal to the alkyl group with simultaneous dissociation of the alkane ("reductive elimination") :L<sub>n-1</sub>M(H)(CH<sub>2</sub>-CH<sub>2</sub>R) → L<sub>n-1</sub>M + CH<sub>3</sub>-CH<sub>2</sub>R Preceding the oxidative addition of H<sub>2</sub> is the formation of a [[dihydrogen complex]]. ===Hydrogen sources=== The obvious source of H<sub>2</sub> is the gas itself, often under pressure. Hydrogen can also be transferred from hydrogen-donor molecules, such as [[hydrazine]],<ref>Leggether, B. E.; Brown, R. K. ''Can. J. Chem.'' '''1960''', ''38'', 2363.</ref><ref>Kuhn, L. P. ''[[J. Am. Chem. Soc.]]'' '''1951''', ''73'', 1510.</ref> [[dihydronaphthalene]], [[dihydroanthracene]], [[isopropanol]], and [[formic acid]].<ref>Davies, R. R.; Hodgson, H. H. ''[[J. Chem. Soc.]]'' '''1943''', 281.</ref><ref>van Es, T.; Staskun, B. ''[[Org. Syn.]]'', Coll. Vol. 6, p.631 (1988); Vol. 51, p.20 (1971). ([http://www.orgsyn.org/orgsyn/prep.asp?prep=cv6p0631 Article])</ref> [[Transfer hydrogenation]] can be metal catalysed. Hydrogenation does proceed from some hydrogen donors without catalysts, examples being [[diimide]] and [[aluminium isopropoxide]]. Some reactions (e.g. reduction of [[thioketal|thioketals]]) do not require the addition of hydrogen if a freshly prepared [[Raney Nickel]] catalyst is used; sufficient hydrogen is deposited on the catalyst as it is prepared. ===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. ==Scope== [[Alkyne]]s can be selectively converted into [[alkene]]s in a so-called '''semihydrogenation''', for instance with the compound ''Ethyl 2-Butynoate'' and catalyst [[palladium]] on [[barium sulfate]] and [[quinoline]] (which deactivates the catalyst enhancing [[chemoselectivity]]):<ref>[[Organic Syntheses]], Coll. Vol. 7, p.226 (1990); Vol. 64, p.108 (1986).http://orgsynth.org/orgsyn/pdfs/CV7P0226.pdf</ref> :[[Image:EthylisocrotonateSynthesis.svg|500px|Ethyl isocrotonate synthesis by ethyl 2-butynoate hydrogenation]] or with ''4-(trimethylsilyl)-3-butyn-1-ol'':<ref>[[Organic Syntheses]], Coll. Vol. 8, p.609 (1993); Vol. 68, p.182 (1990). http://orgsynth.org/orgsyn/pdfs/CV8P0609.pdf</ref> :[[Image:Alkynealkenehydrogenation.svg|550px|4-(trimethylsilyl)-3-butyn-1-ol hydrogenation]] The next reaction featuring [[carvone]] is an example of [[homogeneous catalysis]] i.e. the [[Wilkinson's catalyst]]:<ref>Organic Syntheses, Coll. Vol. 6, p.459 (1988); Vol. 53, p.63 (1973). http://orgsynth.org/orgsyn/pdfs/CV6P0459.pdf</ref> :[[Image:CarvoneHydrogenation.svg|400px|Carvone hydrogenation]] Hydrogenation is sensitive to [[steric hindrance]] explaining the selectivity for reaction with the [[exocyclic]] double bond but not the internal double bond. The compound [[Naphthol|1-naphthol]] is completely reduced to a mixture of [[decalin]]-ol [[isomer]]s.<ref>Organic Syntheses, Coll. Vol. 6, p.371 (1988); Vol. 51, p.103 (1971). http://orgsynth.org/orgsyn/pdfs/CV6P0371.pdf</ref> :[[Image:NaphtolHydrogenation.svg|500px|1-naphthol hydrogenation]] The compound [[resorcinol]], hydrogenated with [[Raney nickel]] in presence of aqeous [[sodium hydroxide]] forms an [[enolate]] which is alkylated with [[methyl iodide]] to ''2-methyl-1,3-cyclohexandione'':<ref>Organic Syntheses, Coll. Vol. 5, p.743 (1973); Vol. 41, p.56 (1961). http://orgsynth.org/orgsyn/pdfs/CV5P0567.pdf</ref> :[[Image:ResorcinolHydrogenation.svg|600px|Resorcinol Hydrogenation]] An effective catalyst is the [[Lindlar catalyst]] for example in the conversion of [[phenylacetylene]] to [[styrene]].<ref>Organic Syntheses, Coll. Vol. 5, p.880 (1973); Vol. 46, p.89 (1966). http://orgsynth.org/orgsyn/pdfs/CV5P0880.pdf</ref> :[[Image:ApplicationLindlarCatalyst.svg|400px|Application Lindlar Catalyst]] Hydrogenation is also used in [[organic reduction]] of [[nitro]] compounds, for instance aromatic nitro compounds in combination with [[palladium on carbon]] and [[formaldehyde]]:<ref>Organic Syntheses, Coll. Vol. 5, p.552 (1973); Vol. 47, p.69 (1967). http://orgsynth.org/orgsyn/pdfs/CV5P0552.pdf</ref> :[[Image:Nitrohydrogenation.svg|500px|Nitro compound hydrogenation]] or the reduction of [[imine]]s, for example in a synthesis of ''m-tolylbenzylamine'':<ref>Organic Syntheses, Coll. Vol. 3, p.827 (1955); Vol. 21, p.108 (1941). http://orgsynth.org/orgsyn/pdfs/CV3P0827.pdf</ref> :[[Image:ImineReduction.svg|500px|Imine hydrogenation]] or the reduction of [[nitrile]]s for instance in a synthesis of [[phenethylamine]] with [[Raney nickel]] and [[ammonia]]:<ref>Organic Syntheses, Coll. Vol. 3, p.720 (1955); Vol. 23, p.71 (1943). http://orgsynth.org/orgsyn/pdfs/CV4P0603.pdf</ref> :[[Image:NitrileHydrogenation.svg|450px|Nitrile hydrogenation]] ==In the food industry== {{fats}} Hydrogenation is widely applied to the processing of vegetable oils and [[fat]]s. Complete hydrogenation converts unsaturated [[fatty acid]]s to [[saturated fat|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. 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 [[Vegetable oil|oil]]s 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 [[shortening]]s. ===Health implications=== {{Main|trans fat}} A side effect of incomplete hydrogenation having implications for human health is the [[isomerization]] of the remaining unsaturated carbon bonds. The [[Cis-trans isomerism|cis]] configuration of these [[double bond]]s predominates in the unprocessed fats in most edible fat sources, but incomplete hydrogenation partially converts these molecules to [[trans isomer]]s, which have been implicated in circulatory diseases including [[heart disease]] (see [[trans fat]]s). 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. Further, trans fats are banned in two European countries: [[Denmark]] and [[Switzerland]].<ref>{{cite news |url=http://www.independent.co.uk/life-style/health-and-wellbeing/healthy-living/deadly-fats-why-are-we-still-eating-them-843400.html |title=Deadly fats: why are we still eating them? |date=2008-06-10 |publisher=The Independent |accessdate=2008-06-16}}</ref> In 2006, [[New York City]] adopted the US's first major municipal ban on most artificial trans fats in restaurant cooking.<ref>{{cite news |url=http://www.msnbc.msn.com/id/16051436/ |title=New York City passes trans fat ban |date=2006-12-05 |publisher=msnbc.com |accessdate=2007-12-03}}</ref> ==Hydrogenation of coal== :''Main article: [[Bergius process]]'' ==History== The earliest hydrogenation is that of [[platinum]] [[catalysis|catalyzed]] addition of hydrogen to oxygen in the [[Döbereiner's lamp]], a device commercialized as early as 1823. The French chemist [[Paul Sabatier (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 [[Sabatier process]]. For this work Sabatier won half of the 1912 [[Nobel Prize in Chemistry]]. [[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. The commercially very important [[Haber-Bosch process]] (ammonia hydrogenation) was first described in 1905 and less so [[Fischer-Tropsch process]] (carbon monoxide hydrogenation) in 1922. Another commercial application is the [[oxo process]] (1938), a hydrogen mediated coupling of aldehydes with alkenes. [[Wilkinson's catalyst]] was the first [[homogeneous catalyst]] developed in the 1960s and [[Noyori asymmetric hydrogenation]] (1987) one of the first applications in [[asymmetric synthesis]]. A 2007 review article advocated the use of more hydrogenations in C-C coupling reactions like the [[oxo process]].<ref>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}}</ref> ==Metal-free hydrogenation== For all practical purposes, hydrogenation requires a metal catalyst. Although, there are some metal-free catalytic systems that are investigated in academic research. One such system for reduction of [[ketone]]s consists of [[Tert-Butanol|''tert''-butanol]] and [[potassium tert-butoxide]] and very high temperatures.<ref>''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}}</ref> The reaction depicted below describes the hydrogenation of [[benzophenone]]: :[[Image:BaseCatalyzedKetoneHydrogenation.svg|500px|Base Catalyzed Ketone Hydrogenation]] A [[chemical kinetics]] study<ref>''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}}</ref> found this reaction is [[first order reaction|first order]] in all three reactants suggesting a cyclic 6-membered [[transition state]]. Another system is based on the [[phosphine]]-[[borane]] compound ('''1'''). It reversibly accepts dihydrogen at relatively low temperatures to form the [[phosphonium]] [[borate]] '''2''' which is able to reduce a simple hindered [[imine]].<ref>''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}}</ref> :[[Image:MetalfreehydrogenationPhosphineBorane.svg|500px|Metal free hydrogenation Phosphine Borane]] == See also == * [[Dehydrogenation]] * [[Transfer hydrogenation]] * [[Hydrogenolysis]] * [[Hydrodesulfurization]], [[Hydrotreater]] and [[Oil desulfurization]] == References == {{reflist}} == Further reading == * {{cite journal| url=http://members.ift.org/NR/rdonlyres/27B49B9B-EA63-4D73-BAB4-42FEFCD72C68/0/crfsfsv4n1p00220030ms20040577.pdf |format=PDF |author=Jang ES, Jung MY, Min DB |title=Hydrogenation for Low Trans and High Conjugated Fatty Acids |journal=Comprehensive Reviews in Food Science and Food Safety |volume=1 |year=2005}} *{{cite book |author=Fred A. Kummerow|title=Cholesterol Won't Kill You, But Trans Fat Could|publisher=Trafford |year=2008 |isbn=142513808}} [[Category:Addition reactions]] [[Category:Homogeneous catalysis]] [[Category:Chemical engineering]] [[Category:Industrial processes]] [[Category:Hydrogen]] [[Category:Oil refineries]] [[ar:إضافة حفزية للهيدروجين]] [[de:Hydrierung]] [[et:Hüdrogeenimine]] [[es:Hidrogenación]] [[fi:Hydraus]] [[it:Idrogenazione]] [[he:הידרוגנציה]] [[nl:Hydrogenering]] [[ja:水素化]] [[nn:Hydrogenering]] [[pt:Hidrogenação]] [[ru:Гидрогенизация]] [[th:ไฮโดรจีเนชัน]] [[tr:Hidrojenasyon]] [[zh:氢化]]