CHEMICAL TROPERTIES OF FATTY ACIDS AND RELATED COMPOUNDS 119 



e', Prelog and Piantanida Method r^"^ This procedure involves the de- 

 composition by heat of the tetramethylammonium salt. It is most useful 

 with the polyterpenoid acids and sapogenins, such as oleanolic and related 

 acids, which are stable at the temperature required for the decomposition 

 of the tetramethylammonium salt. It is not a satisfactory procedure for 

 the esters of n-aliphatic acids. 



Glycol esters were prepared as early as 1859 by Wurtz/" while Berthe- 

 lot^"^ synthesized some of the esters of erythritol, dulcitol, mannitol, and 

 glucose 4 years earlier. The synthesis of triglycerides by esterification of 

 fatty acids with glycerol antedated both of these discoveries by a decade. ^°' 

 A description of esterification methods insofar as they are related to glyc- 

 erol is given in Chapter III. A comprehensive review of the higher fatty 

 acid esters of the polyhydric alcohols, which includes a discussion of prep- 

 aration, properties, and industrial applications, was prepared by Gold- 

 smith.^io 



(c) Properties of Esters, a'. Monoesters: The esters are, in general, 

 neutral substances. On standing, they may slowly hydrolyze to their acid 

 and alcohol components. The rate of hydrolysis is greatly accelerated by 

 an increase in temperature in the presence of moisture, by the presence of 

 an inorganic catalyst, or an enzyme, or by the action of alkalies. The 

 splitting of the esters of low molecular weight takes place much more readily 

 than in the case of those composed of longer carbon chains. 



The monoesters are relatively stable toward heat, and most of them 

 can be distilled without decomposition. For this reason, fatty acid mix- 

 tures are usually converted to their methyl esters for separation and 

 identification. The melting and boiling points of the methyl and ethyl 

 esters are lower than those of the corresponding acids. However, when 

 the length of the aliphatic chain in the alcohol is increased, the melting and 

 boiling points of the esters are gradually increased until they exceed those 

 of the free acids. For example, the several esters of palmitic acid have the 

 following melting points in °C.: methyl, 30.6; ethyl, 25; butyl, 27.5; 

 octyl, 22.5; decyl, 30; dodecyl, 41; tetradecyl, 48; pentadecyl, 55.5; 

 hexadec3d, 56.5; and triacontyl, 72. Palmitic acid melts at 63.1°C. 



The solubility of the esters corresponds, to some extent, to those of the 

 fatty acids. However, the one exception is that the esters composed of the 

 short-chain acids and alcohols are insoluble in water, in contradistinction 

 to their component acids and alcohols, which are water-soluble. Esters 

 readily dissolve in most organic solvents and, in fact, they are themselves 



406 V. Prelog and M. Piantanida, Z. physiol Chem., 2U, 56-58 (1936). 



«^ A. Wurtz, Ann. chim. phys. [3], 55, 400-478 (1859). 



«8 M. Berthelot, Compt. rend., 41, 452-456 (1855). 



«' J. Pelouze and A. G^lis, Ann. chim. phys. [3], 10, 434-456 (1844). 



"» H. A. Goldsmith, Chem. Revs., 33, 257-349 (1943). 



