View source for Chemistry:Hydrogenation

{{short description|Chemical reaction between molecular hydrogen and another compound or element}}
{{Reactionbox|Name=Hydrogenation|Type=|Section3={{Reactionbox Identifiers
   | OrganicChemistryNamed = 
   | RSC_ontology_id = 
 }}|Section1={{Reactionbox Conditions
   | Reference =
   | Catalyst= Ni, Pd, Pt
   | Solvent = 
   | Temperature = 
 }}}}{{this|addition of neutral H<sub>2</sub>|addition of H<sup>+</sup>|protonation}}

[[File:Hydrogenation on catalyst.svg|thumb|
Steps in the hydrogenation of a C=C double bond at a catalyst surface, for example Ni or Pt :<br />
(1) The reactants are [[Chemistry:Adsorption|adsorbed]] on the catalyst surface and H<sub>2</sub> dissociates.<br />
(2) An H atom bonds to one C atom. The other C atom is still attached to the surface.<br />
(3) A second C atom bonds to an H atom. The molecule leaves the surface.]]{{Infobox industrial process
|name         = Catalysed hydrogenation
|image        =
|caption      =
|type         = Chemical
|sector       = Food industry, petrochemical industry, [[Chemistry:Pharmaceutical|pharmaceutical]] industry, agricultural industry
|technologies = Various transition metal catalysts, high-pressure technology
|feedstock    = Unsaturated [[Chemistry:Substrate|substrates]] and [[Chemistry:Hydrogen|hydrogen]] or hydrogen donors
|product      = Saturated hydrocarbons and derivatives
|companies    =
|facility     =
|inventor     = [[Biography:Paul Sabatier (chemist)|Paul Sabatier]]
|year         = 1897
|developer    =
}}

'''Hydrogenation''' is a [[Chemistry:Chemical reaction|chemical reaction]] between molecular [[Chemistry:Hydrogen|hydrogen]] (H<sub>2</sub>) and another compound or element, usually in the presence of a [[Chemistry:Catalyst|catalyst]] such as [[Chemistry:Nickel|nickel]], [[Chemistry:Palladium|palladium]] or [[Chemistry:Platinum|platinum]]. The process is commonly employed to [[Chemistry:Redox|reduce]] or [[Chemistry:Saturated and unsaturated compounds|saturate]] [[Chemistry:Organic compound|organic compound]]s. Hydrogenation typically constitutes the addition of pairs of [[Chemistry:Hydrogen|hydrogen]] [[Physics:Atom|atom]]s to a molecule, often an [[Chemistry:Alkene|alkene]]. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces [[Chemistry:Double bond|double]] and [[Chemistry:Triple bond|triple]] bonds in [[Chemistry:Hydrocarbon|hydrocarbon]]s.<ref>{{cite book|last=Hudlický|first=Miloš|title=Reductions in Organic Chemistry|publisher=[[Organization:American Chemical Society|American Chemical Society]]|year=1996|location=Washington, D.C.|pages=429|isbn=978-0-8412-3344-7}}</ref>

==Process==
Hydrogenation has three components, the [[Chemistry:Saturated and unsaturated compounds|unsaturated]] substrate, the hydrogen (or hydrogen source) and, invariably, a [[Chemistry:Catalyst|catalyst]]. The [[Chemistry:Redox|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 [[Chemistry:Isomerization|isomerization]] of the alkenes from [[Cis–trans isomerism|cis to trans]]. This process is of great interest because hydrogenation technology generates most of the [[Chemistry:Trans fat|trans fat]] in foods. A reaction where bonds are broken while hydrogen is added is called ''[[Chemistry:Hydrogenolysis|hydrogenolysis]]'', a reaction that may occur to carbon-carbon and carbon-heteroatom ([[Chemistry:Oxygen|oxygen]], [[Chemistry:Nitrogen|nitrogen]] or [[Chemistry:Halogen|halogen]]) bonds. Some hydrogenations of polar bonds are accompanied by hydrogenolysis.

===Hydrogen sources===
For hydrogenation, the obvious source of hydrogen is {{chem2|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 {{chem2|H2}}, usually conveyed from the cylinders and sometimes augmented by "booster pumps". Gaseous hydrogen is produced industrially from hydrocarbons by the process known as [[Chemistry:Steam reforming|steam reforming]].<ref name=Rylander>Paul N. Rylander, "Hydrogenation and Dehydrogenation" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim, 2005. {{doi|10.1002/14356007.a13_487}}</ref>  For many applications, hydrogen is transferred from donor molecules such as [[Chemistry:Formic acid|formic acid]], isopropanol, and dihydroanthracene.<ref>Beck, Shay. Organometallic Chemistry. United Kingdom, EDTECH, 2019.</ref> These hydrogen donors undergo [[Chemistry:Dehydrogenation|dehydrogenation]] to, respectively, [[Chemistry:Carbon dioxide|carbon dioxide]], [[Chemistry:Acetone|acetone]], and [[Chemistry:Anthracene|anthracene]]. These processes are called [[Chemistry:Transfer hydrogenation|transfer hydrogenation]]s.

===Substrates===
An important characteristic of alkene and [[Chemistry:Alkyne|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.<ref>''Advanced Organic Chemistry'' Jerry March 2nd Edition{{full citation needed|date=February 2022}}</ref> This reaction can be performed on a variety of different [[Chemistry:Functional group|functional group]]s.
{|class="wikitable toccolours"
|+Substrates for and products of hydrogenation
|-
! Substrate || Product || Comments||Heat of hydrogenation<br>(kJ/mol)<ref>Scott D. Barnicki "Synthetic Organic Chemicals" in Handbook of Industrial Chemistry and Biotechnology edited by James A. Kent, New York : Springer, 2012. 12th ed. {{ISBN|978-1-4614-4259-2}}.</ref>
|-
|align=center| {{chem2|R2C\dCR'2}}<br>([[Chemistry:Alkene|alkene]])
|align=center| {{chem2|R2CHCHR'2}}<br>(alkane)
|align=center| large application is production of margarine
| align=center| −90 to −130
|-
|align=center| {{chem2|RC\tCR'}}<br>([[Chemistry:Alkyne|alkyne]])
|align=center| {{chem2|RCH2CH2R'}}<br>(alkane)
| align=center| semihydrogenation gives {{chem2|''cis''\-RHC\dCHR'}}<br>
| align=center| −300<br>(for full hydrogenation)
|-
|align=center| {{chem2|RCH\dO}}<br>([[Chemistry:Aldehyde|aldehyde]]) 
|align=center| {{chem2|RCH2OH}}<br>(primary alcohol)
| align=center| often employs [[Chemistry:Transfer hydrogenation|transfer hydrogenation]]
| align=center| −60 to −65
|-
|align=center| {{chem2|R2CO}}<br>([[Chemistry:Ketone|ketone]])
|align=center| {{chem2|R2CHOH}}<br>(secondary alcohol)
| align=center| often employs [[Chemistry:Transfer hydrogenation|transfer hydrogenation]]
| align=center| −60 to −65
|-
|align=center| {{chem2|RCO2R'}}<br>([[Chemistry:Ester|ester]])
|align=center| {{chem2|RCH2OH + R'OH}}<br>(two alcohols)
| align=center| often applies to production of [[Chemistry:Fatty alcohol|fatty alcohol]]s
| align=center| −25 to −105
|-
|align=center| {{chem2|RCO2H}}<br>([[Chemistry:Carboxylic acid|carboxylic acid]])
|align=center| {{chem2|RCH2OH}}<br>(primary alcohol)
|align=center|  applicable to fatty alcohols
|align=center| −25 to −75
|-
|align=center| {{chem2|RNO2}}<br>([[Chemistry:Nitro compound|nitro]])
|align=center| {{chem2|RNH2}}<br>(amine)
|align=center| major application is [[Chemistry:Aniline|aniline]]<ref>{{cite journal | year = 2000 | title = Hydrogenation of nitrobenzene using polymer bound Ru(III) complexes as catalyst | journal = Ind. Jr. Of Chem. Tech. | volume = 7 | page = 280 }}</ref><ref>{{cite journal | last1 = Patel | first1 = D. R. | year = 1998 | title = Hydrogenation of nitrobenzene using polymer anchored Pd(II) complexes as catalyst | journal = Journal of Molecular Catalysis | volume = 130 | issue = 1–2| page = 57 | doi=10.1016/s1381-1169(97)00197-0}}</ref>
| align=center| −550
|-
|}

===Catalysts===
With rare exceptions, {{chem2|H2}} is unreactive toward organic compounds in the absence of metal catalysts. The unsaturated substrate is [[Physics:Chemisorption|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. [[Chemistry:Platinum|Platinum]], [[Chemistry:Palladium|palladium]], rhodium, and [[Chemistry:Ruthenium|ruthenium]] form highly active catalysts, which operate at lower temperatures and lower pressures of {{chem2|H2}}. Non-precious metal catalysts, especially those based on [[Chemistry:Nickel|nickel]] (such as [[Chemistry:Raney nickel|Raney nickel]] and [[Chemistry:Urushibara nickel|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:<ref>{{OrgSynth|author=C. F. H. Allen and James VanAllan|title=m-Toylybenzylamine|collvol=3|collvolpages=827|year=1955 |prep=CV3P0827}}</ref><ref>{{OrgSynth|author=A. B. Mekler, S. Ramachandran, S. Swaminathan, and Melvin S. Newman|title=2-Methyl-1,3-Cyclohexanedione|collvol=5|collvolpages=743|year=1973|prep=CV5P0743}}</ref>

Catalysts are usually classified into two broad classes: [[Chemistry:Homogeneous catalysis|homogeneous]] and [[Chemistry:Heterogeneous catalysis|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.

====Homogeneous catalysts====
Some well known homogeneous catalysts are indicated below.  These are [[Chemistry:Coordination complex|coordination complex]]es that activate both the unsaturated substrate and the {{chem2|H2}}.  Most typically, these complexes contain platinum group metals, especially Rh and Ir. 
<gallery caption="Homogeneous hydrogenation catalysts and their precursors" widths="180px" heights="120px">
File:Dichlorotris(triphenylphosphine)ruthenium(II).png|[[Chemistry:Dichlorotris(triphenylphosphine)ruthenium(II)|Dichlorotris(triphenylphosphine)ruthenium(II)]] is a precatalyst based on ruthenium.
File:Crabtree.svg|[[Chemistry:Crabtree's catalyst|Crabtree's catalyst]] is a highly active catalyst featuring iridium.
File:Cyclooctadiene-rhodium-chloride-dimer-2D-skeletal.png|[[Chemistry:Cyclooctadiene rhodium chloride dimer|Cyclooctadiene rhodium chloride dimer]], {{chem2|Rh2Cl2(cod)2}} is a precursor to many homogeneous catalysts.
File:(S)-iPr-PHOX.svg|[[Chemistry:(S)-iPr-PHOX|(S)-iPr-PHOX]] is a typical chelating phosphine ligand used in asymmetric hydrogenation.
</gallery>
{{clear left}}
[[File:WilkinsonCycleJMBrown.png|thumbnail|right|480px|Mechanism for the hydrogenation of a [[Chemistry:Terminal alkene|terminal alkene]] using 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 {{Nowrap|[[Chemistry:L-DOPA|L-DOPA]]}}.<ref>{{cite journal|last1=Knowles|first1=W. S.|title=Application of organometallic catalysis to the commercial production of L-DOPA|journal=Journal of Chemical Education|date=March 1986|volume=63|issue=3|pages=222|doi=10.1021/ed063p222|bibcode=1986JChEd..63..222K}}</ref> To achieve asymmetric reduction, these catalyst are made chiral by use of chiral diphosphine ligands.<ref name="Textbook" /> Rhodium catalyzed hydrogenation has also been used in the herbicide production of S-metolachlor, which uses a Josiphos type ligand (called Xyliphos).<ref>{{cite journal|last1=Blaser|first1=Hans-Ulrich|last2=Pugin|first2=Benoît|last3=Spindler|first3=Felix|last4=Thommen|first4=Marc|title=From a Chiral Switch to a Ligand Portfolio for Asymmetric Catalysis|journal=Accounts of Chemical Research|date=December 2007|volume=40|issue=12|pages=1240–1250|doi=10.1021/ar7001057|pmid=17715990}}</ref>  In principle asymmetric hydrogenation can be catalyzed by chiral heterogeneous catalysts,<ref name=Mallet>{{cite journal | doi = 10.1021/cr0683663 | title = Asymmetric Catalysis at Chiral Metal Surfaces | year = 2007 | last1 = Mallat | first1 = T. | last2 = Orglmeister | first2 = E. | last3 = Baiker | first3 = A. | journal = Chemical Reviews | volume = 107 | issue = 11 | pages = 4863–90 | pmid = 17927256}}</ref> but this approach remains more of a curiosity than a useful technology.

====Heterogeneous catalysts====
Heterogeneous catalysts for hydrogenation are more common industrially. In industry, precious metal hydrogenation catalysts are deposited from solution as a fine powder on the support, which is a cheap, bulky, porous, usually granular material, such as [[Physics:Activated carbon|activated carbon]], alumina, [[Chemistry:Calcium carbonate|calcium carbonate]] or [[Chemistry:Barium sulfate|barium sulfate]].<ref>{{cite web|url=https://www.alfa.com/en/platinum-heterogeneous-catalysts/|title=Platinum Heterogeneous Catalysts - Alfa Aesar|website=www.alfa.com|access-date=28 April 2018|url-status=live|archive-url=https://web.archive.org/web/20180118181608/https://www.alfa.com/en/platinum-heterogeneous-catalysts/|archive-date=18 January 2018}}</ref> For example, platinum on carbon is produced by reduction of [[Chemistry:Chloroplatinic acid|chloroplatinic acid]] ''in situ'' in carbon. Examples of these catalysts are 5% [[Chemistry:Ruthenium|ruthenium]] on activated carbon, or 1% [[Chemistry:Platinum|platinum]] on alumina. Base metal catalysts, such as [[Chemistry:Raney nickel|Raney nickel]], are typically much cheaper and do not need a support. Also, in the laboratory, unsupported (massive) precious metal catalysts such as [[Physics:Platinum black|platinum black]] are still used, despite the cost.

As in homogeneous catalysts, the activity is adjusted through changes in the environment around the metal, i.e. the [[Chemistry:Coordination sphere|coordination sphere]]. Different [[Physics:Miller index|faces]] of a crystalline heterogeneous catalyst display distinct activities, for example. This can be modified by mixing metals or using different preparation techniques. Similarly, heterogeneous catalysts are affected by their supports.

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 [[Chemistry:Lindlar catalyst|Lindlar's catalyst]]. For example, when the catalyst [[Chemistry:Palladium|palladium]] is placed on [[Chemistry:Barium sulfate|barium sulfate]] and then treated with [[Chemistry:Quinoline|quinoline]], the resulting catalyst reduces alkynes only as far as alkenes. The Lindlar catalyst has been applied to the conversion of [[Chemistry:Phenylacetylene|phenylacetylene]] to [[Chemistry:Styrene|styrene]].<ref>{{OrgSynth|author=H. Lindlar and R. Dubuis|title=Palladium Catalyst for Partial Reduction of Acetylenes|collvol=5|collvolpages=880|year=1973|prep=CV5P0880}}</ref>

<gallery caption="Illustrative hydrogenations" widths="162px" heights="110px">
File:CarvoneH2.png|Selective hydrogenation of the less hindered alkene group in [[Chemistry:Carvone|carvone]] using a homogeneous catalyst ([[Chemistry:Wilkinson's catalyst|Wilkinson's catalyst]]).<ref>{{OrgSynth|author=S. Robert E. Ireland and P. Bey|title=Homogeneous Catalytic Hydrogenation: Dihydrocarvone|collvol=6|collvolpages=459|year=1988|prep=CV6P0459}}</ref>
File:PhC2HH2.png|Partial hydrogenation of [[Chemistry:Phenylacetylene|phenylacetylene]] using the [[Chemistry:Lindlar catalyst|Lindlar catalyst]].
File:ImineH2.png|Hydrogenation of an imine using a [[Chemistry:Raney nickel|Raney nickel]] catalyst, a popular heterogeneous catalyst.
File:ResorcinolH2.png|Partial hydrogenation of a [[Chemistry:Resorcinol|resorcinol]] derivative using a Raney-Nickel catalyst.
File:SuccPdH2.png|Hydrogenation of [[Chemistry:Maleic acid|maleic acid]] to [[Chemistry:Succinic acid|succinic acid]].<ref>{{cite journal | last1 = Amoa | first1 = Kwesi | year = 2007 | title = Catalytic Hydrogenation of Maleic Acid at Moderate Pressures A Laboratory Demonstration | journal = Journal of Chemical Education | volume = 84 | issue = 12| page = 1948 | doi = 10.1021/ed084p1948 | bibcode = 2007JChEd..84.1948A }}</ref>
</gallery>

===Transfer hydrogenation===
{{Main article|Chemistry:Transfer hydrogenation}}
[[File:THintermed.png|thumb|300px|right|The transition state of two transfer-hydrogenation reactions from ruthenium-hydride complexes onto carbonyls]]
''Transfer hydrogenation'' uses hydrogen-donor molecules other than molecular {{chem2|H2}}. These "sacrificial" hydrogen donors, which can also serve as [[Physics:Solvent|solvent]]s for the reaction, include [[Chemistry:Hydrazine|hydrazine]], [[Chemistry:Formic acid|formic acid]], and alcohols such as isopropanol.<ref name="Wang+Astruc 2015">{{cite journal | last1=Wang | first1=Dong | last2=Astruc | first2=Didier | title=The Golden Age of Transfer Hydrogenation | journal=Chem. Rev. | volume=115 | issue=13 | date=2015 | issn=0009-2665 | doi=10.1021/acs.chemrev.5b00203 | pages=6621–6686| pmid=26061159 }}</ref>

In [[Chemistry:Organic synthesis|organic synthesis]], transfer hydrogenation is useful for the [[Physics:Asymmetric hydrogenation|asymmetric hydrogenation]] of polar unsaturated substrates, such as [[Chemistry:Ketone|ketone]]s, [[Chemistry:Aldehyde|aldehyde]]s and [[Chemistry:Imine|imine]]s, by employing [[Physics:Enantioselective synthesis#Enantioselective catalysis|chiral catalysts]].

===Electrolytic hydrogenation===
[[Chemistry:Chemical polarity|Polar]] substrates such as nitriles can be hydrogenated [[Chemistry:Electrosynthesis#Cathodic reductions|electrochemically]], using [[Chemistry:Protic solvent|protic solvent]]s and reducing equivalents as the source of hydrogen.<ref>{{cite journal | doi = 10.1021/ed081p1350 | title = Catalytic Hydrogenation of Organic Compounds without H2 Supply: An Electrochemical System | year = 2004 | last1 = Navarro | first1 = Daniela Maria do Amaral Ferraz | last2 = Navarro | first2 = Marcelo | journal = Journal of Chemical Education | volume = 81 | issue = 9 | pages = 1350| bibcode = 2004JChEd..81.1350N | s2cid = 93416392 }}</ref>

==Thermodynamics and mechanism==
The addition of hydrogen to double or triple bonds in hydrocarbons is a type of [[Chemistry:Redox|redox]] reaction that can be thermodynamically favorable. For example, the addition of hydrogen to ethene has a [[Physics:Gibbs free energy|Gibbs free energy]] change of -101&nbsp;kJ·mol<sup>−1</sup>, which is highly exothermic.<ref name=Textbook>{{cite book|last1=Atkins|first1=Peter W.|title=Shriver & Atkins' inorganic chemistry.|date=2010|publisher=W. H. Freeman and Co.|location=New York|isbn=978-1-4292-1820-7|page=696|edition=5th}}</ref> In the hydrogenation of vegetable oils and fatty acids, for example, the heat released, about 25 kcal per mole (105 kJ/mol), is sufficient to raise the temperature of the oil by 1.6–1.7&nbsp;°C per [[Chemistry:Iodine number|iodine number]] drop.

However, the reaction rate for most hydrogenation reactions is negligible in the absence of catalysts.  The [[Physics:Reaction mechanism|mechanism]] of metal-catalyzed hydrogenation of alkenes and alkynes has been extensively studied.<ref>Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001</ref> First of all isotope labeling using [[Physics:Deuterium|deuterium]] confirms the regiochemistry of the addition:
:<chem>RCH=CH2 + D2 -> RCHDCH2D</chem>

===Heterogeneous catalysis===
On solids, the accepted mechanism is the Horiuti-[[Biography:Michael Polanyi|Polanyi]] mechanism:<ref>Gallezot, Pierre. "Hydrogenation - Heterogeneous" in Encyclopedia of Catalysis, Volume 4, ed. Horvath, I.T., John Wiley & Sons, 2003.</ref><ref>{{cite journal | doi = 10.1039/TF9343001164 | title = Exchange reactions of hydrogen on metallic catalysts | year = 1934 | last1 = Horiuti | first1 = Iurô | last2 = Polanyi | first2 = M. | journal = Transactions of the Faraday Society | volume = 30 | pages = 1164}}</ref>
# Binding of the unsaturated bond
# Dissociation of {{chem2|H2}} on the catalyst
# Addition of one atom of hydrogen; this step is reversible
# Addition of the second atom; effectively irreversible.

In the third step, the alkyl group can revert to alkene, which can detach from the catalyst. Consequently, contact with a hydrogenation catalyst allows ''cis-trans''-isomerization. The ''trans''-alkene can reassociate to the surface and undergo hydrogenation. These details are revealed in part using D<sub>2</sub> (deuterium), because recovered alkenes often contain deuterium.

For aromatic substrates, the first hydrogenation is slowest. The product of this step is a cyclohexadiene, which hydrogenate rapidly and are rarely detected. Similarly, the [[Chemistry:Cyclohexene|cyclohexene]] is ordinarily reduced to cyclohexane.

===Homogeneous catalysis===
In many homogeneous hydrogenation processes,<ref>Johannes G. de Vries, Cornelis J. Elsevier, eds. The Handbook of Homogeneous Hydrogenation Wiley-VCH, Weinheim, 2007. {{ISBN|978-3-527-31161-3}}</ref> the metal binds to both components to give an intermediate alkene-metal(H)<sub>2</sub> 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 via [[Chemistry:Oxidative addition|oxidative addition]] (preceding the oxidative addition of {{chem2|H2}} is the formation of a [[Chemistry:Dihydrogen complex|dihydrogen complex]]):
:{{chem2|L_{''n''}M + H2 -> LnMH2}}
* binding of alkene:
:{{chem2|L_{''n''}MH2 + CH2\dCHR -> L_{''n''}MH2(CH2\dCHR)}}
* transfer of one hydrogen atom from the metal to carbon (migratory insertion):
:{{chem2|L_{''n''}MH2(CH2\dCHR) -> L_{''n''}M(H)(CH2\sCH2R)}}
* transfer of the second hydrogen atom from the metal to the alkyl group with simultaneous dissociation of the alkane ("reductive elimination")
:{{chem2|L_{''n''}M(H)(CH2\sCH2R) -> L_{''n''}M + CH3\sCH2R}}

Alkene isomerization often accompanies hydrogenation. This important side reaction proceeds by beta-hydride elimination of the alkyl hydride intermediate:<ref>{{cite book | title=Organotransition Metal Chemistry: From Bonding to Catalysis | publisher=University Science Books | author=Hartwig, John | year=2010 | location=New York | pages=1160 | isbn=978-1-938787-15-7}}</ref>
:{{chem2|L_{''n''}M(H)(CH2\sCH2R)  ->  L_{''n''}MH2(CH2\dCHR)}}
:{{chem2|L_{''n''}MH2(CH2\dCHR)  ->  L_{''n''}MH2  +  CH2\dCHR }}
Often the released olefin is trans.

==Inorganic substrates==
The hydrogenation of nitrogen to give ammonia is conducted on a vast scale by the Haber–Bosch process,<ref name="Appl">{{Ullmann |first=Max |last=Appl |title=Ammonia |doi=10.1002/14356007.a02_143.pub2 |year=2006}}</ref> consuming an estimated 1% of the world's energy supply.
:<chem title="Hydrogenation of nitrogen">
\underset{nitrogen}{N{\equiv}N} + \underset{hydrogen\atop (200 atm)}{3H2} ->[\ce{Fe\ catalyst}][350-550^\circ\ce C] \underset{ammonia}{2NH3}
</chem>
Oxygen can be partially hydrogenated to give [[Chemistry:Hydrogen peroxide|hydrogen peroxide]], although this process has not been commercialized. One difficulty is preventing the catalysts from triggering decomposition of the hydrogen peroxide to form water.<ref>Noritaka Mizuno Gabriele Centi, Siglinda Perathoner, Salvatore Abate  "Direct Synthesis of Hydrogen Peroxide: Recent Advances"  in ''Modern Heterogeneous Oxidation Catalysis: Design, Reactions and Characterization'' 2009, Wiley-VCH. {{doi|10.1002/9783527627547.ch8}}</ref><ref name="Hutchings">{{cite journal |last1=Edwards |first1=Jennifer K. |last2=Solsona |first2= Benjamin |last3=N |first3=Edwin Ntainjua |last4=Carley |first4=Albert F. |last5=Herzing |first5=Andrew A. |last6=Kiely |first6=Christopher J. |last7=Hutchings |first7=Graham J. |date=20 February 2009 |title=Switching Off Hydrogen Peroxide Hydrogenation in the Direct Synthesis Process |journal=Science |doi=10.1126/science.1168980 |pmid=19229032 |bibcode=2009Sci...323.1037E |volume=323 |number=5917 |pages=1037–1041|s2cid=1828874 }}</ref>

==Industrial applications==
Catalytic hydrogenation has diverse industrial uses. Most frequently, industrial hydrogenation relies on heterogeneous catalysts.<ref name=Rylander/>

=== Food industry <span class="anchor" id="In the food industry"></span>===
The food industry hydrogenates vegetable oils to convert them into solid or semi-solid fats that can be used in spreads, candies, baked goods, and other products like margarine.  Vegetable oils are made from polyunsaturated fatty acids (having more than one carbon-carbon double bond).  Hydrogenation eliminates some of these double bonds.<ref name=free2005>Ian P. Freeman "Margarines and Shortenings" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2005, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a16_145}}</ref>

:[[File:H2forMargerin.png|thumb|320px|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.]]

===Petrochemical industry===
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. Relevant to liquid fuels that are stored sometimes for long periods in air, saturated hydrocarbons exhibit superior storage properties.  On the other hand, alkenes tend to form [[Chemistry:Hydroperoxide|hydroperoxide]]s, which can form gums that interfere with fuel handling equipment.  For example, mineral turpentine is usually hydrogenated. Hydrocracking of heavy residues into diesel is another application. In [[Chemistry:Isomerization|isomerization]] and [[Chemistry:Catalytic reforming|catalytic reforming]] processes, some hydrogen pressure is maintained to [[Chemistry:Hydrogenolysis|hydrogenolyze]] [[Chemistry:Petroleum coke|coke]] formed on the catalyst and prevent its accumulation.

===Organic chemistry===
Hydrogenation is a useful means for converting unsaturated compounds into saturated derivatives.  Substrates include not only alkenes and alkynes, but also aldehydes, imines, and nitriles,<ref>{{cite journal|last1=Werkmeister|first1=Svenja|last2=Junge|first2=Kathrin|last3=Beller|first3=Matthias|title=Catalytic Hydrogenation of Carboxylic Acid Esters, Amides, and Nitriles with Homogeneous Catalysts|journal=Organic Process Research & Development|date=2 February 2014|volume=18|issue=2|pages=289–302|doi=10.1021/op4003278|s2cid=102012512 |doi-access=}}{{open access}}</ref> which are converted into the corresponding saturated compounds, i.e. alcohols and amines.  Thus, alkyl aldehydes, which can be synthesized with the oxo process from [[Chemistry:Carbon monoxide|carbon monoxide]] and an alkene, can be converted to alcohols. E.g. 1-propanol is produced from propionaldehyde, produced from ethene and carbon monoxide. [[Chemistry:Xylitol|Xylitol]], a [[Chemistry:Polyol|polyol]], is produced by hydrogenation of the sugar [[Chemistry:Xylose|xylose]], an aldehyde. Primary amines can be synthesized by [[Chemistry:Nitrile reduction|hydrogenation of nitriles]], while nitriles are readily synthesized from cyanide and a suitable electrophile. For example, isophorone diamine, a precursor to the [[Chemistry:Polyurethane|polyurethane]] monomer [[Chemistry:Isophorone diisocyanate|isophorone diisocyanate]], 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.

===Hydrogenation of coal===
{{main|Chemistry:Bergius process}}

==History==

===Heterogeneous catalytic hydrogenation===
The earliest hydrogenation was that of the platinum-catalyzed addition of hydrogen to oxygen in the [[Physics:Döbereiner's lamp|Döbereiner's lamp]], a device commercialized as early as 1823. The French chemist [[Biography:Paul Sabatier (chemist)|Paul Sabatier]] is considered the father of the hydrogenation process. In 1897, building on the earlier work of James Boyce, an American chemist working in the manufacture of soap products, he discovered that traces of nickel catalyzed the addition of hydrogen to molecules of gaseous hydrocarbons in what is now known as the Sabatier process. For this work, Sabatier shared the 1912 Nobel Prize in Chemistry. [[Biography:Wilhelm Normann|Wilhelm Normann]] 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 worldwide industry. The commercially important Haber–Bosch process, first described in 1905, involves hydrogenation of nitrogen. In the [[Chemistry:Fischer–Tropsch process|Fischer–Tropsch process]], reported in 1922 carbon monoxide, which is easily derived from coal, is hydrogenated to liquid fuels.

In 1922, Voorhees and Adams described an apparatus for performing hydrogenation under pressures above one atmosphere.<ref>{{cite web |url=http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1922/44/i06/f-pdf/f_ja01427a021.pdf |title=Archived copy |access-date=2008-08-05 |url-status=live |archive-url=https://web.archive.org/web/20080910124034/http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/1922/44/i06/f-pdf/f_ja01427a021.pdf |archive-date=2008-09-10 }}</ref> 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 [[Biography:Murray Raney|Murray Raney]] developed a finely powdered form of nickel, which is widely used to catalyze hydrogenation reactions such as conversion of nitriles to amines or the production of margarine.

===Homogeneous catalytic hydrogenation===
In the 1930s, Calvin discovered that copper(II) complexes oxidized H<sub>2</sub>. The 1960s witnessed the development of well defined homogeneous catalysts using transition metal complexes, e.g., [[Chemistry:Wilkinson's catalyst|Wilkinson's catalyst]] (RhCl(PPh<sub>3</sub>)<sub>3</sub>). Soon thereafter cationic Rh and Ir were found to catalyze the hydrogenation of alkenes and carbonyls.<ref name=ACS>{{cite journal|last1=Schrock|first1=Richard R.|last2=Osborn|first2=John A.|title=Catalytic hydrogenation using cationic rhodium complexes. I. Evolution of the catalytic system and the hydrogenation of olefins|journal=Journal of the American Chemical Society|date=April 1976|volume=98|issue=8|pages=2134–2143|doi=10.1021/ja00424a020}}</ref> In the 1970s, asymmetric hydrogenation was demonstrated in the synthesis of {{Nowrap|L-DOPA}}, and the 1990s saw the invention of [[Physics:Noyori asymmetric hydrogenation|Noyori asymmetric hydrogenation]].<ref>C. Pettinari, F. Marchetti, D. Martini "Metal Complexes as Hydrogenation Catalysts" Comprehensive Coordination Chemistry II, 2004, volume 9. pp. 75–139. {{doi|10.1016/B0-08-043748-6/09125-8}}</ref> The development of homogeneous hydrogenation was influenced by work started in the 1930s and 1940s on the oxo process and Ziegler–Natta polymerization.

==Metal-free hydrogenation==
For most practical purposes, hydrogenation requires a metal catalyst. Hydrogenation can, however, proceed from some hydrogen donors without catalysts, illustrative hydrogen donors being [[Chemistry:Diimide|diimide]] and [[Chemistry:Aluminium isopropoxide|aluminium isopropoxide]], the latter illustrated by the [[Chemistry:Meerwein–Ponndorf–Verley reduction|Meerwein–Ponndorf–Verley reduction]].  Some metal-free catalytic systems have been investigated in academic research. One such system for reduction of ketones consists of ''tert''-butanol and [[Chemistry:Potassium tert-butoxide|potassium tert-butoxide]] and very high temperatures.<ref>{{cite journal | doi = 10.1021/ja01072a028 | title = Homogeneous Hydrogenation in the Absence of Transition-Metal Catalysts | year = 1964 | last1 = Walling | first1 = Cheves. | last2 = Bollyky | first2 = Laszlo. | journal = Journal of the American Chemical Society | volume = 86 | issue = 18 | pages = 3750}}</ref> The reaction depicted below describes the hydrogenation of [[Chemistry:Benzophenone|benzophenone]]:

:[[File:BaseCatalyzedKetoneHydrogenation.svg|500px|Base-catalyzed hydrogenation of ketones.]]
A [[Chemistry:Chemical kinetics|chemical kinetics]] study<ref>{{cite journal | doi = 10.1021/ja016152r | title = Hydrogenation without a Transition-Metal Catalyst:&nbsp; On the Mechanism of the Base-Catalyzed Hydrogenation of Ketones | year = 2002 | last1 = Berkessel | first1 = Albrecht | last2 = Schubert | first2 = Thomas J. S. | last3 = Müller | first3 = Thomas N. | journal = Journal of the American Chemical Society | volume = 124 | issue = 29 | pages = 8693–8 | pmid = 12121113}}</ref> found this reaction is first-order in all three reactants suggesting a cyclic 6-membered [[Chemistry:Transition state|transition state]].

Another system for metal-free hydrogenation is based on the [[Chemistry:Phosphine|phosphine]]-[[Chemistry:Borane|borane]], compound '''1''', which has been called a ''[[Chemistry:Frustrated Lewis pair|frustrated Lewis pair]]''. It reversibly accepts dihydrogen at relatively low temperatures to form the [[Physics:Phosphonium|phosphonium]] [[Physics:Borate|borate]] '''2''' which can reduce simple hindered [[Chemistry:Imine|imine]]s.<ref>{{cite journal | doi = 10.1002/anie.200702908 | pmid = 17696181 | title = Metal-Free Catalytic Hydrogenation | year = 2007 | last1 = Chase | first1 = Preston A. | last2 = Welch | first2 = Gregory C. | last3 = Jurca | first3 = Titel | last4 = Stephan | first4 = Douglas W. | journal = Angewandte Chemie International Edition | volume = 46 | issue = 42 | pages = 8050–3}}</ref>

:[[File:MetalfreehydrogenationPhosphineBorane.svg|400px|Metal free hydrogenation Phosphine Borane]]

The reduction of [[Chemistry:Nitrobenzene|nitrobenzene]] to [[Chemistry:Aniline|aniline]] has been reported to be catalysed by [[Chemistry:Fullerene|fullerene]], its mono-anion, atmospheric hydrogen and UV light.<ref>{{cite journal | doi = 10.1021/ja9061097 | title = A Nonmetal Catalyst for Molecular Hydrogen Activation with Comparable Catalytic Hydrogenation Capability to Noble Metal Catalyst | year = 2009 | last1 = Li | first1 = Baojun | last2 = Xu | first2 = Zheng | journal = Journal of the American Chemical Society | volume = 131 | issue = 45 | pages = 16380–2 | pmid = 19845383| url = https://figshare.com/articles/A_Nonmetal_Catalyst_for_Molecular_Hydrogen_Activation_with_Comparable_Catalytic_Hydrogenation_Capability_to_Noble_Metal_Catalyst/2812501 }}</ref>

==Equipment used for hydrogenation==
Today's bench chemist has three main choices of hydrogenation equipment:
* Batch hydrogenation under atmospheric conditions
* Batch hydrogenation at elevated temperature and/or pressure<ref name=Apparatus>{{cite journal|last1=Adams|first1=Roger|last2=Voorhees|first2=V.|title=Apparatus for catalytic reduction|journal=Organic Syntheses|date=1928|volume=8|page=10|doi=10.15227/orgsyn.008.0010}}</ref>
* Flow hydrogenation

===Batch hydrogenation under atmospheric conditions===
The original and still a commonly practised form of hydrogenation in teaching laboratories, this process is usually effected by adding solid catalyst to a round bottom flask of dissolved reactant which has been evacuated using [[Chemistry:Nitrogen|nitrogen]] or [[Chemistry:Argon|argon]] gas and sealing the mixture with a penetrable rubber seal. Hydrogen gas is then supplied from a H<sub>2</sub>-filled balloon. 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 [[Chemistry:Copper sulfate|copper sulfate]] or with [[Engineering:Sight glass|gauges]] for each reaction vessel.

===Batch hydrogenation at elevated temperature and/or pressure===
Since many hydrogenation reactions such as [[Chemistry:Hydrogenolysis|hydrogenolysis]] of protecting groups and the reduction of aromatic 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 [[Physics:Pressure vessel|pressure vessel]]. 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.<ref name=Apparatus /> Recent advances in [[Chemistry:Electrolysis|electrolysis]] technology have led to the development of [https://www.globenewswire.com/news-release/2018/08/17/1553391/0/en/ThalesNano-Energy-Introduces-H-Genie-Revolutionary-Compact-High-Pressure-Hydrogen-Gas-Generation-Platform-for-Chemists.html high pressure hydrogen generators], which generate hydrogen up to 1,400 [[Pound per square inch|psi]] (100 bar) from water. Heat may also be used, as the pressure compensates for the associated reduction in gas solubility.

===Flow hydrogenation===
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 [[High-performance liquid chromatography|high-performance liquid chromatography]] technology, this technique allows the application of pressures from atmospheric to {{convert|1450|psi|bar|abbr=on}}. 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 pyrophoric catalysts.

===Industrial reactors===
Catalytic hydrogenation is done in a [[Chemistry:Plug flow reactor model|tubular plug-flow reactor]] 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.<ref>{{cite journal | doi = 10.1016/0009-2509(82)80171-1 | title = Mechanically agitated gas–liquid reactors | year = 1982 | last1 = Joshi | first1 = J.B. | last2 = Pandit | first2 = A.B. | last3 = Sharma | first3 = M.M. | journal = Chemical Engineering Science | volume = 37 | issue = 6 | pages = 813| bibcode = 1982ChEnS..37..813J }}</ref>

==See also==
{{Div col|colwidth=24em}}
* Carbon neutral fuel
* [[Chemistry:Dehydrogenation|Dehydrogenation]]
* H-Bio
* [[Chemistry:Hydrodesulfurization|Hydrodesulfurization]], hydrotreater and oil desulfurization
* [[Chemistry:Hydrogenation of carbon–nitrogen double bonds|Hydrogenation of carbon–nitrogen double bonds]]
* [[Chemistry:Josiphos ligands|Josiphos ligands]]
* [[Physics:Timeline of hydrogen technologies|Timeline of hydrogen technologies]]
* [[Chemistry:Transfer hydrogenation|Transfer hydrogenation]]
{{Div col end}}

==References==
{{Reflist|30em}}

==Further reading==
* {{cite journal|url=http://members.ift.org/NR/rdonlyres/27B49B9B-EA63-4D73-BAB4-42FEFCD72C68/0/crfsfsv4n1p00220030ms20040577.pdf|vauthors=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|url-status=dead|archive-url=https://web.archive.org/web/20081217185908/http://members.ift.org/NR/rdonlyres/27B49B9B-EA63-4D73-BAB4-42FEFCD72C68/0/crfsfsv4n1p00220030ms20040577.pdf|archive-date=2008-12-17}}
* examples of hydrogenation from Organic Syntheses:
** [http://orgsynth.org/orgsyn/pdfs/CV7P0226.pdf Organic Syntheses, Coll. Vol. 7, p.226 (1990).]
** [http://orgsynth.org/orgsyn/pdfs/CV8P0609.pdf Organic Syntheses, Coll. Vol. 8, p.609 (1993).]
** [http://orgsynth.org/orgsyn/pdfs/CV5P0552.pdf Organic Syntheses, Coll. Vol. 5, p.552 (1973).]
** [http://orgsynth.org/orgsyn/pdfs/CV4P0603.pdf Organic Syntheses, Coll. Vol. 3, p.720 (1955).]
** [http://orgsynth.org/orgsyn/pdfs/CV6P0371.pdf Organic Syntheses, Coll. Vol. 6, p.371 (1988).]
* early work on transfer hydrogenation: 
** {{cite journal | last1 = Davies | first1 = R. R. | last2 = Hodgson | first2 = H. H. | year = 1943 | title =  76. Catalytic reduction by formic acid under pressure. Part II. A comparison of copper and nickel as catalysts| doi = 10.1039/jr9430000281 | journal = J. Chem. Soc. | page = 281 }}
** {{cite journal | last1 = Leggether | first1 = B. E. | title = Reduction of Monohalogenated Nitrobenzenes with Hydrazine and Raney Nickel. A Convenient Preparation of Halogenated Anilines | last2 = Brown | first2 = R. K. | year = 1960 | doi = 10.1139/v60-320 | journal = Can. J. Chem. | volume = 38 | issue = 12| page = 2363 | doi-access =  }}
** {{cite journal | last1 = Kuhn | first1 = L. P. | year = 1951 | title =  Catalytic Reduction with Hydrazine| doi = 10.1021/ja01148a029 | journal = J. Am. Chem. Soc. | volume = 73 | issue = 4| page = 1510 }}

==External links==
* [https://books.google.com/books?id=4OIDAAAAMBAJ&pg=-PA107 "The Magic of Hydro"], ''Popular Mechanics'', June 1931, pp.&nbsp;107–109 – early article for the general public on hydrogenation of oil produced in the 1930s


[[Category:Addition reactions]]
[[Category:Homogeneous catalysis]]
[[Category:Industrial processes]]
[[Category:Hydrogen]]
[[Category:Organic redox reactions]]
[[Category:Synthetic fuel technologies]]

{{Sourceattribution|Hydrogenation}}