Hydrogenation

HYDROGENATION, hydrogenization, the addition of hydrogen to simple and complex chemical substances. Only a few elements possess the property of directly combining with hydrogen. The larger part of both simple and complex chemical substances (of the latter, organic compounds are of particular interest) combines with hydrogen only in the presence of special substances—catalysts. Thus, under H. is usually understood the catalytic binding of hydrogen by organic compounds. The wide application of H. methods, both in laboratories and in chemical technology, began in the early 20th century, after Sabatier and Sanderan developed the technique for hydrogenating organic compounds in the vapor state in the presence of finely divided non-noble metals, mainly nickel. The existing methods of H. at present can be divided into two groups: H. in the vapor (gaseous) state and H. in the liquid state, in solution. Both methods are often used in organic synthesis and serve as one of the methods for determining the structure of complex products of plant and animal origin. H. in the vapor state (Sabatier) consists in passing the vapors of the substance to be hydrogenated together with hydrogen through a catalyst (metals: cobalt, iron, copper, and especially nickel). The scope of application of the Sabatier method is large but limited, since not all organic substances can be converted to the vapor state without decomposition. In these cases, the method of H. in solution is used, the development of which is associated with the names of Fokin, Ipatiev, Willstätter, Paal, and Skeat. Catalysts are platinum or palladium in a finely divided (Fokin-Willstätter) or colloidal (Paal-Skeat) state. In technology, nickel is used almost exclusively. This method is particularly convenient because H. is carried out at ordinary or low temperatures, thereby eliminating the possibility of rearrangements and other side phenomena that complicate research. H. processes are widely used in technology. H. is used to convert liquid vegetable oils into solid products, since in this process glycerides of unsaturated acids, which are the predominant component of vegetable oils, are converted into glycerides of saturated fatty acids with a higher melting point. The hydrogenation of oils in technology is usually carried out by passing hydrogen in the form of small bubbles through the oil in which the catalyst (nickel) is suspended. The operation is carried out in special autoclaves at low pressures. Hydrogenated fats are suitable for food. The amounts of nickel remaining in the fats are completely negligible (0.02-0.675 mg of nickel per 1 kg of fat). Many organic products obtained as a result of H. have found wide application in technology as solvents—decalin, tetralin, etc. In recent years, great attention has been paid to the possibility of obtaining synthetic products analogous to natural petroleum products, both by catalytic reduction of carbon monoxide (F. Fischer) and by direct H. of hard and brown coals (Bergius). The latter process is carried out on a technical scale.