Hydrogenation

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.
Catalysed hydrogenation
Process type Chemical
Industrial sector(s) Food industry, petrochemical industry, pharmaceutical industry, agricultural industry
Main technologies or sub-processes Various transition metal catalysts, high-pressure technology
Feedstock Unsaturated substrates and hydrogen or hydrogen donors
Product(s) Saturated hydrocarbons and derivatives
Inventor Paul Sabatier
Year of invention 1897

Hydrogenation – to treat with hydrogen – is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene. Catalysts are required for the reaction to be usable; non-catalytic hydrogenation takes place only at very high temperatures. Hydrogenation reduces double and triple bonds in hydrocarbons. [1]

Process

It has three components, the unsaturated substrate, the hydrogen (or hydrogen source) and, invariably, a catalyst. The 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 isomerization of the alkenes from cis to trans. This process is of great interest because hydrogenation technology generates most of the trans fat in foods (see below). A reaction where bonds are broken while hydrogen is added is called hydrogenolysis, a reaction that may occur to carbon-carbon and carbon-heteroatom ( oxygen, nitrogen or halogen) bonds. Some hydrogenations of polar bonds are accompanied by hydrogenolysis.

Hydrogen sources

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 steam reforming. [2] For many applications, hydrogen is transferred from donor molecules such as formic acid, isopropanol, and dihydroanthracene. These hydrogen donors undergo dehydrogenation to, respectively, carbon dioxide, acetone, and anthracene. These processes are called transfer hydrogenations.

Substrates

An important characteristic of alkene and 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. [3] Typical substrates are listed in the table

Substrates for and products of hydrogenation
Substrate Product Comments heat of hydrogenation [4]
R2C=CR'2
( alkene)
R2CHCHR'2
(alkane)
large application is production of margerine -90 to -130 kcal/mol
RC≡CR
( alkyne)
alkane semihydrogenation gives cis-RHC=CHR'
-300 kcal/mol for full hydrogenation
RCHO
( aldehyde)
RCH2OH
(primary alcohol)
often employs transfer hydrogenation -60 to -65 kcal/mol
R2CO
( ketone)
R2CHOH
(secondary alcohol)
often employs transfer hydrogenation -60 to -65 kcal/mol
RCO2R'
( ester)
RCH2OH + R'OH
(two alcohols)
often applies to production of fatty alcohols -25 to -105 kcal/mol
RCO2H
( carboxylic acid)
RCH2OH
(alcohol)
applicable to fatty alcohols 25 to -75 kcal/mol
RNO2
( nitro)
RNH2
(amine)
major application is aniline [5] [6] -550 kcal/mol

Catalysts

With rare exceptions, H2 is unreactive toward organic compounds in the absence of metal catalysts. The unsaturated substrate is 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. Platinum, palladium, rhodium, and ruthenium 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 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. Notice that the Raney-nickel catalysed hydrogenations require high pressures: [7] [8]

Catalysts are usually classified into two broad classes: homogeneous catalysts and heterogeneous catalysts. 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 coordination complexes that activate both the unsaturated substrate and the H2. Most typically, these complexes contain platinum group metals, especially Rh and Ir.

hydrogenation of propylene with 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 L-DOPA. [9] To achieve asymmetric reduction, these catalyst are made chiral by use of chiral diphosphine ligands. [10] Rhodium catalyzed hydrogenation has also been used in the herbicide production of S-metolachlor, which uses a Josiphos type ligand (called Xyliphos). [11] In principle asymmetric hydrogenation can be catalyzed by chiral heterogeneous catalysts, [12] but this approach remains more of a curiosity than a useful technology.

Heterogeneous catalysts

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 coordination sphere. Different 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.

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 example, when the catalyst palladium is placed on barium sulfate and then treated with quinoline, the resulting catalyst reduces alkynes only as far as alkenes. The Lindlar catalyst has been applied to the conversion of phenylacetylene to styrene. [13]

Transfer hydrogenation

Hydrogen also can be extracted ("transferred") from "hydrogen-donors" in place of H2 gas. Hydrogen donors, which often serve as solvents include hydrazine, dihydronaphthalene, dihydroanthracene, isopropanol, and formic acid. [16]

In organic synthesis, transfer hydrogenation is useful for the asymmetric reduction of polar unsaturated substrates, such as ketones, aldehydes, and imines. The hydrogenation of polar substrates such as ketones and aldehydes typically require transfer hydrogenation, at least in homogeneous catalysis. These catalysts are readily generated in chiral forms, which is the basis of asymmetric hydrogenation of ketones.

Transfer hydrogenation catalyzed by transition metal complexes proceeds by an "outer sphere mechanism."

Electrolytic hydrogenation

Polar substrates such as nitriles can be hydrogenated electrochemically, using protic solvents and reducing equivalents as the source of hydrogen. [17]

Other Languages
العربية: هدرجة
bosanski: Hidrogenacija
català: Hidrogenació
čeština: Hydrogenace
Deutsch: Hydrierung
Ελληνικά: Υδρογόνωση
español: Hidrogenación
français: Hydrogénation
Gaeilge: Hidriginiú
Bahasa Indonesia: Hidrogenasi
italiano: Idrogenazione
Nederlands: Hydrogenering
日本語: 水素化
norsk nynorsk: Hydrogenering
oʻzbekcha/ўзбекча: Gidrogenlash
polski: Uwodornianie
português: Hidrogenação
română: Hidrogenare
русский: Гидрирование
Simple English: Hydrogenation
slovenčina: Hydrogenácia
suomi: Hydraus
svenska: Hydrogenering
Türkçe: Hidrojenasyon
українська: Гідрогенізація
اردو: آبگریت
中文: 氢化