All About Hydrogenations

Introduction:

The method is frequently used to reduce or saturated organic compounds. Hydrogenation is the process of adding pairs of hydrogen atoms to a molecule, most commonly an alkene. Catalysts are required for the reaction to be useful; non-catalytic hydrogenation occurs only at extremely high temperatures. Hydrogenation breaks down hydrocarbon double and triple bonds.

Saturated Versus Unsaturated Hydrocarbon:

Saturated hydrocarbons are compounds with only one bond in their structure. The alkane functional group is a prime example of a saturated hydrocarbon. Alkanes are organic compounds with single bonds between carbon and hydrogen atoms.

 Unsaturated hydrocarbons are organic compounds with double bonds in their structure. A good example of an unsaturated hydrocarbon is the alkene functional group. In their structure, alkenes have at least one double bond between the carbon atoms. Here are some saturated and unsaturated hydrocarbon examples:

Nickel as catalyst:

Catalytic hydrogenation in the presence of hydrogen is known to prepare partially hydrogenated unsaturated fatty materials, such as oils or fatty acids, and especially edible oils, such as soybean oil, rapeseed oil, palm oil, canola oil, or fish oil. Hydrogenation can occur using traditional hydrogenation catalysts such as nickel or precious metal catalysts.Catalysts for the hydrogenation of edible oil should have small particle size, large pores, high catalytic metal dispersion, and excellent filtration properties. Nickel powder catalysts that have been coated with fat (droplets) to prevent oxidation are frequently used in the hydrogenation of oils and fatty acids. These catalysts typically contain between 16 and 25 weight percent Ni, based on the weight of the droplets including the coated catalyst. A nickel powder catalyst is typically prepared by co-precipitation of nickel and a soluble support, with the amount of Ni loaded onto the support being approximately 60% by weight of the calcined catalyst. Nysosel 800 catalyst is a commercially available example.

Metal free hydrogenation:

For most practical purposes, hydrogenation necessitates the use of a metal catalyst. However, hydrogenation can occur without the use of a catalyst in the case of some hydrogen donors, such as diimide and aluminium isopropoxide, as demonstrated by the Meerwein–Ponndorf–Verley reduction. In academic research, some metal-free catalytic systems have been investigated. One such system for reducing ketones uses tert-butanol, potassium tert-butoxide, and extremely high temperatures. The hydrogenation of benzophenone is depicted in the following reaction:

According to a chemical kinetics study, this reaction is first-order in all three reactants, implying a cyclic 6-membered transition state.

Another metal-free hydrogenation system is based on the phosphine-borane compound 1, also known as a frustrated Lewis pair. At low temperatures, it accepts dihydrogen reversibly to form phosphonium borate 2, which can reduce simple hindered imines.

Fullerene, its mono-anion, atmospheric hydrogen, and UV light have all been reported to catalyse the reduction of nitrobenzene to aniline.

Uses of hydrogenation:

Catalytic hydrogenation has a wide range of industrial applications. Typically, heterogeneous catalysts are used in industrial hydrogenation:

 The food industry:

Vegetable oils are hydrogenated in the food industry to create solid or semi-solid fats that can be used in spreads, candies, baked goods, and other products such as margarine. Polyunsaturated fatty acids are used to make vegetable oils (having more than one carbon-carbon double bond). Some of these double bonds are removed during hydrogenation.

 The petroleum-chemical industry:

Hydrogenation is used in petrochemical processes to convert alkenes and aromatics into less toxic and reactive saturated alkanes (paraffins) and cycloalkanes (naphthenes). Saturated hydrocarbons have superior storage properties when it comes to liquid fuels that are sometimes stored for long periods of time in air. Alkenes, on the other hand, tend to form hydroperoxides, which can form gums that clog fuel handling equipment. Mineral turpentine, for example, is typically hydrogenated. Another application is the hydrocracking of heavy residues into diesel. Some hydrogen pressure is maintained during the isomerization and catalytic reforming processes to hydrogenolysis coke formed on the catalyst and prevent its accumulation.

 Chemistry of organic compounds:

Hydrogenation is an effective method for converting unsaturated compounds to saturated derivatives. Substrates include not only alkenes and alkynes, but also aldehydes, imines, and nitriles, all of which are converted into the corresponding saturated compounds, namely alcohols and amines. Thus, alkyl aldehydes, which can be synthesised from carbon monoxide and an alkene using the oxo process, can be converted to alcohols. For example, 1-propanol is made from propionaldehyde, which is made from ethene and carbon monoxide. Xylitol, a polyol, is made by hydrogenating the sugar xylose, which is an aldehyde. Primary amines can be made by hydrogenating nitriles, whereas nitriles are easily made from cyanide and a suitable electrophile.Isophorone diamine, a precursor to the polyurethane monomer isophorone diisocyanate, is produced from isophorone nitrile via a tandem nitrile hydrogenation/reductive amination by ammonia reaction, in which hydrogenation converts both the nitrile and the imine formed from the aldehyde and ammonia into another amine.

Conclusion:

Hydrogenation is a chemical reaction in which hydrogen is added to a molecule.

At room temperature, hydrogenation is not thermodynamically favourable, so a catalyst is required. This catalyst is usually made of metal.Margarine, mineral turpentine, and aniline are examples of hydrogenated products.Although hydrogenation has many applications, most people are familiar with it as the reaction used to convert liquid oils into semi-solid and solid fats. The hydrogenation of unsaturated dietary fats to produce saturated fats and trans fats may pose some health risks.