hydrogenation n : a chemical process that adds hydrogen atoms to an unsaturated oil; "food producers use hydrogenation to keep fat from becoming rancid"

English

Pronunciation

• Rhymes: -eɪʃǝn

Noun

hydrogenation

1. the chemical reaction of hydrogen with another substance, especially with an unsaturated organic compound, and usually under the influence of temperature, pressure and catalysts

Translations

• French: hydrogénation
• Italian: idrogenazione
• Spanish: hidrogenación

or with 4-(trimethylsilyl)-3-butyn-1-ol: The next reaction featuring carvone is an example of homogeneous catalysis i.e. the Wilkinson's catalyst:

Hydrogenation is sensitive to steric hindrance explaining the selectivity for reaction with the exocyclic double bond but not the internal double bond.

The compound 1-naphthol is completely reduced to a mixture of decalin-ol isomers. 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:

An effective catalyst is the Lindlar catalyst for example in the conversion of phenylacetylene to styrene. Hydrogenation is also used in organic reduction of nitro compounds, for instance aromatic nitro compounds in combination with palladium on carbon and formaldehyde:

or the reduction of imines, for example in a synthesis of m-tolylbenzylamine:

or the reduction of nitriles for instance in a synthesis of phenethylamine with Raney nickel and ammonia:

In the food industry

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 restricting the amount of hydrogen (or reducing agent) allowed to react with the fat.

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.

Health implications

A side effect of incomplete hydrogenation having implications for human health is the isomerization of the remaining unsaturated carbon bonds. The cis configuration of these double bonds predominates in the unprocessed fats in most edible fat sources, but incomplete hydrogenation partially converts these molecules to trans isomers, which have been implicated in circulatory diseases including heart disease (see trans fats). 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.

In 2006, New York City adopted the US's first major municipal ban on most artificial trans fats in restaurant cooking.

Hydrogenation of coal

Main article: Bergius process

History

The earliest hydrogenation is that of platinum catalyzed addition of hydrogen to oxygen in the Döbereiner's lamp, a device commercialized as early as 1823. The French 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.

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 ketones consists of tert-butanol and potassium tert-butoxide and very high temperatures. The reaction depicted below describes the hydrogenation of benzophenone:

A chemical kinetics study found this reaction is 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.

See also

• Dehydrogenation
• Transfer hydrogenation
• Hydrogenolysis
• Hydrodesulfurization, Hydrotreater and Oil desulfurization

References

Further reading

• Cholesterol Won't Kill You, But Trans Fat Could