Hydrogenation

Heterogeneous Catalytic Hydrogenation

Due to the high stability of the hydrogen molecule ( the dissociation enthalpy for the reaction ΔH0 H2 → 2H * is 434 kJ · mol -1) is required for the hydrogenation is usually a catalyst. The hydrogen molecule binds intermediates in the catalyst metal atom, while the bond between the two hydrogen atoms is weakened that she experiences a shortage of electrons and can now interact with an electron-rich multiple bond. The actual hydrogenation takes place when a formal two hydrogen atoms are transferred to the corresponding multiple bond of the catalyst - hydrogen complex. The oxidation numbers of the affected elements are reduced by one.

Homogeneous Catalytic Hydrogenation

The homogeneous catalytic hydrogenation, for example using the Wilkinson catalyst is mainly used for hydrogenation of alkenes with hydrogen.

Elimination of a labile ligand hydrogen are added by oxidative addition in a subsequent step. The olefin (or olefin ) then co-ordinated to the metal first, and then reacts to the insertion into the metal -hydrogen bond. By reaction with the second bonded hydrogen, the alkane is released by reductive elimination and the original catalytically active complex recovered.

With the Wilkinson's catalyst selectively terminal double bonds can be hydrogenated.

Applications of hydrogenation

Application in Organic Chemistry

Hydrogenation in organic chemistry is used to store hydrogen at multiple bonds. In most cases, there is the heterogeneous catalytic hydrogenation and in exceptional cases, the homogeneous catalytic hydrogenation to use. The catalysts used are mainly transition metals, usually applied to carrier materials such as activated carbon, optionally also in combination with catalyst poisons, which improve the selectivity. For the hydrogenation most elemental hydrogen is used, the required pressures and temperature vary within wide ranges.

Especially on a laboratory scale catalytic hydrogenations also with hydrazine, cyclohexadiene or formic acid as the hydrogen source have a practical significance. In these methods, arise with nitrogen for hydrazine, for benzene and carbon dioxide to formic acid, cyclohexadienes thermodynamically very stable compounds, which provide the driving force.

Alkenes and alkynes can be used in the case of alkanes, and alkynes also selectively in (E) - and ( Z)- alkenes are converted. But can be catalytically hydrogenated and functional groups containing hetero atoms, such as nitro compounds to amines, or imines.

Hydrogenation catalysts

Suitable hydrogenation catalysts are, in particular elements of the 8th subgroup of the periodic system are used. Here palladium and nickel have become particularly important. And also platinum, rhodium, ruthenium, cobalt, iron, copper and zinc chromite further be used as hydrogenation catalysts.

Catalyst poisons

Hydrogenation of carbon-carbon multiple bonds

An important application of the catalytic hydrogenation, the hydrogenation of carbon-carbon multiple bonds; it is technically applied on a large scale, for example in the fat hardening. The hydrogenation is generally in the range of hydrogen and a non-soluble catalyst such as palladium, platinum, iridium or nickel ( Raney nickel).

In the hydrogenation of alkynes is a double bond is formed from the first triple bond, which may be in the E or the Z form. With appropriate catalysts, one or the other stereoisomer may preferably be obtained. Usually the alkenes are rapidly further hydrogenated to alkanes; with special catalysts ( Lindlar catalyst ), the hydrogenation can also be stopped at the stage of the alkene.

One, especially on a large scale, much more important than the hydrogenation of alkynes has the catalytic hydrogenation of alkenes to alkanes, such as the operated on a large scale fat hardening. Here, oily unsaturated fatty acid esters are converted to the saturated fatty acid esters which have a significantly higher melting point. This is an important step in the margarine production ( see lower portion).

The hydrogenation ( Hydrierentalpie ) can be used as a measure of the stability of various unsaturated compounds. It can be calculated, and will be concluded with the comparison to the hydrogenation of benzene to the resonance stabilization by the aromatic system in the benzene by comparing of hydrogenation of cyclohexene and cyclohexa- 1 ,3-diene hydrogenation of the theoretical non-existent Cylohexatriens.

Depending on the steric hindrance, the catalytic hydrogenation is easy and possible at normal pressure and room temperature or requires drastic conditions. Easily, monosubstituted olefins to hydrogenate and much heavier then with increasing number of carbon substituents to hydrogenate olefins.

In the laboratory, for the catalytic hydrogenation is usually palladium on activated carbon and hydrogen used in a compressed gas cylinder. In the technical chemistry here one uses more optimized method. The catalyst is palladium supported on activated carbon is commercially available and contains, depending on the quality and thus the price 1-10 % palladium in an oxidation state of Pd (II) and has in the case of complex ligands such as nitro groups, previously hydrogenated palladium prior to the actual hydrogenation itself ( = reduced to palladium (0)) be.

Hydrogenation of other multiple bonds

By catalytic hydrogenation of carbon and hetero atom - hetero atom -hetero atom multiple bonds can be reduced. Of particular importance here are the hydrogenation of nitro groups, imines and oximes to the corresponding amines and heterocyclic aromatic compounds to the corresponding saturated ring systems.

Hydrogenation as a method to remove protective groups

Due to the often mild conditions, the catalytic hydrogenation and removal of protecting groups suitable. Attention must also be mentioned that protect in the case of benzyl ethers, benzyl alcohols or carboxylic acids, or in peptide chemistry, the benzyloxycarbonyl group or Z group (Cbz ) called the benzyl group.

Margarine production by Hydrogenation

The hydrogenation is also used in the Hydrogenation of vegetable oils for the production of margarine ( Normann Wilhelm, 1901). In this case, the double bonds are saturated in the fatty acid chain of the fat molecules with hydrogen. Since the compounds without double bonds having a higher melting point is obtained from the liquid oil a solid fat. The hydrogenation is carried out over nickel as a catalyst at 120 to 180 ° C and a hydrogen pressure of 6 to 7 bar. Incomplete hydrogenation contributes to that as unwanted by- products E- fatty acids formed ( trans fatty acids ). These are suspected to increase the risk for cardiovascular disease. Through optimized hydrogenation today the formation of undesired by-products can be reduced to a few percent.