CLASSIFICATION AND STRUCTURE OP FATTY AClDS 11 



Rothman^'^ that the free fatty acids obtained from human hair vary in 

 length from C7 to C22; traces of C7, C9, and Cu fatty acids are present, 

 while appreciable proportions of the C13, Cib, and C17 acids are also found. 

 Both saturated and unsaturated odd-carbon fatty acids are present. 



The fatty acids found most commonly as components of the fat mole- 

 cule are listed in Table 1, together with data on their discovery and also 

 their chief natural sources. 



The lower members of the fatty acids are liquid at ordinary tempera- 

 tures, while those with 10 carbons and more are solids having progres- 

 sively higher melting points with an increase in the length of the fatty acid 

 chain. It is interesting that the even progression of melting points and 

 the length of the carbon chain give a smooth curve only if one considers the 

 fatty acids with an even number of carbon atoms. When all the fatty 

 acids are included, the curve becomes a step-like one, each odd-carbon acid 

 having a melting point somewhat lower than that of the even-chain acid 

 immediately preceding it. 



Like other members of the lipids, the fatty acids are characterized, in 

 the main, by their solubility in fat solvents and their insolubility in water. 

 However, butyric acid is completely miscible with water in all proportions. 

 Caproic acid is soluble to the extent of 0.9%, while the higher acids (above 

 lauric acid) are practically insoluble in water. On the other hand, the 

 alkali salts or soaps of the fatty acids are in general quite soluble in water. 

 However, other metallic salts do not possess a similar property. Magne- 

 sium and calcium salts are largely insoluble in water, while lead soaps of the 

 saturated acids are insoluble in alcohol. This fact affords a method of 

 separation of the saturated from the unsaturated acids, since the lead 

 soaps of the latter are alcohol-soluble. 



Most of the fatty acids dissolve in absolute or 95% ethyl alcohol, al- 

 though this is the case only for palmitic acid and higher members of the 

 series when the alcohol is hot. All fatty acids are soluble in diethyl ether, 

 chloroform, benzene, and petroleum ether, although the last solvent is not a 

 satisfactory one for hydroxy-acids. 



(3) Monoethenoid Acids 



This series is comprised of the simplest group of unsaturated acids which 

 contain only one double bond. The empirical formula for the series is 

 C„H2„_202. It includes only acids having an even number of carbon atoms. 

 The acids having a chain shorter than 10 carbon atoms are not represented 

 among the acids found naturall.y. A number of isomers of several of the 

 acids occur naturally where the unsaturated bond is located at different 

 positions in the molecule. Table 2 lists the common acids in this group. 



'" A. W. Weitkiimp, A. M. Siniljanif, and S. Rothinan J. Am. Chem. Sac, 69, 1936- 

 1939 (1947). 



