Hydrogenation is a class of chemical reactions which result in an addition of hydrogen (H2) usually to unsaturated organic compounds. Typical substrates include alkenes, alkynes, ketones, nitriles, and imines. Most hydrogenations involve the direct addition of diatomic hydrogen (H2) but some involve the alternative sources of hydrogen, not H2: these processes are called transfer hydrogenations. The reverse reaction, removal of hydrogen, is called dehydrogenation.
The classical example of a hydrogenation is the addition of hydrogen on unsaturated bonds between carbon atoms, converting alkenes to alkanes. Numerous important applications are found in the petrochemical, pharmaceutical and food industries. Health concerns associated with the hydrogenation of unsaturated fats to produce saturated fats and trans fats is an important aspect of current consumer awareness. 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 hydrogenation process
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 platinum, palladium, rhodium and ruthenium, are highly active catalysts. Highly active catalysts 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.
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"" 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 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 ligands. 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.
Catalysts
The catalytic hydrogenation of organic sulfur compounds to form gaseous hydrogen sulfide (H2S) is very widely used in petroleum refineries, petrochemical plants and other industries to desulfurize various final products, intermediate products and process feedstocks by converting sulfur compounds to gaseous hydrogen sulfide which is then easily removed by distillation. The gaseous hydrogen sulfide is subsequently recovered in an amine treater and finally converted to elemental sulfur in a Claus process unit. In those industries, desulfurization process units are often referred to as hydrodesulfurizers (HDS) or hydrotreaters (HDT). In the petroleum refining and petrochemical industries, cobalt-molybdenum or nickel-molybdenum catalysts are commonly used for hydrogenation and hydrogenolysis catalysts.
Mechanism of reaction
The obvious source of H2 is the gas itself, often under pressure. Hydrogen can also be transferred from hydrogen-donor molecules, such as hydrazine, Transfer hydrogenation can be metal catalysed. Hydrogenation does proceed from some hydrogen donors without catalysts, examples being diimide and aluminium isopropoxide.
Hydrogen sources
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.
Temperatures
Hydrogenation is widely applied to the processing of vegetable oils and fats. Complete hydrogenation converts unsaturated fatty acids to 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 adding hydrogen atoms which bond to the carbon, thus occupying a place in the outer orbital of the carbon which would have otherwise been used to bond with the next carbon in the fatty acid chain.
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.
Unsaturated fat
- Monounsaturated fat
Polyunsaturated fat
Trans fat
Omega: 3, 6, 9
Saturated fat
- Interesterified fat
Fatty acid
Essential fatty acid
Hydrogenation in the food industry
- Interesterified fat
Lefthit