Glycerides

GLYCERIDES, glycerol esters of fatty acids. Glycerides are the main constituent of animal and vegetable fats and fatty oils. Depending on the number of acid radicals introduced into the glycerol, mono-, di-, and triglycerides are distinguished. Synthetic methods for obtaining glycerides consist in the following: 1) heating glycerol with the corresponding acids (depending on concentration conditions and temperature, one can obtain a prevailing amount of mono-, di-, or triglyceride); 2) heating alpha-monochlorohydrin, CH₂Cl.CH(OH).CH₂OH (forming a monoglyceride), or alpha-dichlorohydrin, CH₂Cl.CH(OH).CH₂Cl (forming a dichlorohydrin), or tribromohydrin, CH₂Br.CHBr.CH₂Br, with salts of the corresponding acids. Glycerides of lower fatty acids are colorless liquids, while higher ones are crystalline bodies, insoluble in water, poorly soluble in alcohol, and readily soluble in ether. Some glycerides exhibit a peculiar phenomenon—a double melting point; thus, melted and then rapidly cooled tristearin melts first at 55°, upon further heating the melt becomes turbid, and at 71° it becomes transparent again. This phenomenon is apparently connected with the existence of two different modifications, which in individual cases could be isolated. Among chemical properties, for glycerides as complex esters, the ability to undergo hydrolytic cleavage (saponification) into acid and glycerol is characteristic: C₃H₅(O.CO.R)₃ + 3H₂O = C₃H₅(OH)₃ + 3R.COOH. With water at ordinary or low temperatures, this cleavage proceeds very slowly. Practically, it is carried out by heating at elevated temperatures in autoclaves or in the presence of mineral acids or alkalis, which act as catalysts. Certain enzymes (esterases or lipases) also possess the property of splitting glycerides into acid and glycerol. This property was first observed by Bernard and Berthelot in pancreatic juice. Later, Muntz, Siegmunt, and Green proved the presence of lipolytic enzymes in plant seeds. Currently, biochemical methods of fat cleavage by means of plant lipases have become widespread in fat technology. Upon heating glycerides with alcohols (methyl or ethyl) containing a small (1–2.5%) amount of hydrogen chloride, the glycerol radical (C₃H₅O₃) is displaced in the form of glycerol, and its place is taken by the group -OCH₃ or -OC₂H₅ depending on the nature of the taken alcohol: C₃H₅(O.CO.R)₃ + 3C₂H₅OH = C₃H₅(OH)₃ + 3R.CO.OC₂H₅. This conversion of a glycerol ester into an ethyl (or methyl) ester is called alcoholysis. Among lower glycerides, the best known are glycerol esters of formic acid (formins), formed by heating oxalic acid with glycerol, and glycerol esters of acetic acid (acetins); triacetin boils at 258–259°, is found in small quantities in natural fats and oils, and 100 cubic centimeters of a saturated solution at 15° contain 7.17 g of triacetin. Both formins and acetins are liquids at ordinary temperatures. Higher glycerides—see Fats.

S. Medvedev.