SYNTHESIS OF THE GLYCERIDES 173 



by fractional crystallization, until recently there was little evidence of the 

 positional configuration of the molecules. However, since 1927, the struc- 

 ture of a number of natural mixed triglycerides has been elucidated by the 

 Hilditch school, while the synthesis of a large group of such mixed tri- 

 glycerides by King and his associates has helped to identify the natural 

 compounds of unknown structure. Practically all the mixed triglycerides 

 which have been separated from natural sources are composed of only two 

 fatty acids, one of which is present to the extent of two molecules. Like- 

 wise, the products which have been synthesized have consisted almost en- 

 tirely of this type of compound. Little is known of triglycerides which 

 contain three different fatty acid components, although Wynter BIyth and 

 Robertson ^^ prepared a solid glyceride from butter fat, which they identified 

 as an oleyl-butyryl-palmitin. 



Two types of mixed glycerides are known, namely, the symmetrical, in 

 which the single acid molecule is in the |S-position, and the unsymmetrical 

 form, where the single fatty acid occupies the a-position. 



It has been pointed out by Longenecker''° that a very large number of 

 different mixed triglycerides is possible in any natural fat. If one repre- 

 sents the number of different kinds of acids as n, then the theoretical 

 number of combinations resulting from an even distribution of acids in the 



Table 1 

 Numerical Relationship between Component Fatty Acids and Glycerides" 



« H. E. Longenecker, Chem. Revs., 29, 201-224 (1941). 



molecules would be n^. However, since the a- and a '-positions on glycerol 

 are equivalent, the total number of molecules chemically distinguishable 

 would be V2(^^ + ^^)- The numbers of such possible compounds in rela- 

 tion to the component acids are given in Table 1. 



^' A. Wynter Blyth and G. H. Robertson, Proc. Chem. Soc. London, 5, 5 (1889). 

 6» H. E. Longenecker, Chem. Revs., 29, 201-224 (1941). 



