166 III. CHEMISTRY OF NEUTRAL PATS 



the fatty acids is to be found in those reactions which are referable to the 

 free carboxyl group. The triglycerides are unable to react with the 

 alkalies or with the various metallic salts to form soaps without a pre- 

 liminary saponification. The formation of fatty acid aldehydes and alco- 

 hols, acid amides, cyanides, and related compounds is precluded unless a 

 preliminary saponification and separation of the resulting fatty acids has 

 been effected. 



Physical properties, such as solubility, show marked variations between 

 the triglycerides and the fatty acids. For example, the lower fatty acids 

 such as acetic, propionic, and butyric are soluble in water, while the cor- 

 responding triglycerides are largely insoluble. In the case of the long- 

 chain fatty acids such as palmitic and stearic, solubility can readily be ef- 

 fected in dilute aqueous alkali in the cold state, with the formation of water- 

 soluble soaps. Neutral fats, on the other hand, cannot be dissolved in a 

 similar solvent in the cold state except over long periods, and then only 

 after hydrolysis of the molecule has been produced. Even at boiling 

 temperature a considerable period may elapse before solution is effected, 

 and in this case, also, it will not occur until after saponification has taken 

 place. 



The preparation of pure triglycerides from natural mixtures and the 

 separation and identification of the isomeric triglycerides are much more 

 difficult than in the case of the fatty acids. Fractionation of the natural 

 triglycerides by distillation under reduced pressure can usually not be car- 

 ried out, and this procedure offers no such satisfactory means of separation 

 as it does with the methyl esters of the fatty acids. It is only by such 

 means as differences in solubility and melting point that partial separation 

 of specific triglycerides may be accomplished. 



2. Nomenclature 



The sjaithelic mono-, di-, and triglycerides and the natural fats consist 

 of combinations of glycerol and the component fatty acids in ester linkages. 

 Two isomers are possible with the monoglycerides, depending upon whether 

 the ester combination is on one of the terminal carbon atoms of glycerol (in 

 which case they are called unsymmetrical or a-monoglycerides) or on the 

 central carbon of the glycerol, when the compound is known as a symmet- 

 rical or a /3-monoglyceride. In the case of the diglycerides containing a 

 single kind of fatty acid, two isomers may occur also. In one case, the acid 

 constituents are combined with the two outside carbons of glycerol; these 

 isomers are called symmetrical or a, a '-diglycerides. In the second type, 

 the fatty acids are esterified with the middle carbon and one of the outside 

 carbons of glycerol. These esters are spoken of as unsymmetrical or a,/3- 

 diglycerides. When the two substituent acids are different, four isomers 



