LIPIDES 75 



when combined with an alcohol. Occasionally free fatty acids exist 

 in immature seeds. By and large, fatty acids are straight-chain ali- 

 phatic monocarboxylic acids, although in recent years a few branched- 

 chain acids have been found. Most of the members of this series 

 contain an even number of carbons in the molecule. Thus, the first 

 member of the series, butyric acid, contains four carbons; the second 

 member, caproic acid, contains six carbons in the molecule. Although 

 the low-molecular-weight members of the series are found occasionally 

 in natmal prtxlucts, they are not nearly so abundant as higher-molec- 

 ular-weight acids such as palmitic acid (Ci5H;{iCOOH), stearic acid 

 (C17H35COOH), and oleic acid (Cj^Hj^COOH). 



Classification 



Fatty acids may be divided into foiu" classes according to their mo- 

 lecular structme. There are saturated, unsaturated, hydroxy, and 

 cyclic acids (see classification in Table 4-1). Of these, the saturated 

 and unsaturated acids are the most common in natural products. 



The first three members of the saturated fatty acid series, butyric, 

 caproic, and caprylic acids, are liquid at 20°C., while the fourth mem- 

 ber, capric acid, is a semisolid at this temperature. These four acids 

 are volatile with steam and are referred to as the volatile fatty acids. 

 This property of volatility is utilized in testing for the presence of 

 these acids in such natural products as butterfat. 



The volatile fatty acids are easily recognized by their odor. Butyric 

 acid has a strong, penetrating odor quite suggestive of rancid butter. 

 Caproic, caprylic, and capric acids are powerfully reminiscent of a 

 goat and are, in fact, referred to as the goat acids. 



Saturated fatty acids with more than ten carbons in the molecule 

 are not volatile with steam, are odorless, and have melting points well 

 above 20°C. The most abundant saturated acids occurring in the 

 lipides are the sixteen- and eighteen-carbon members, palmitic and 

 stearic acids. The hard consistency of many fats may be attributed 

 to the presence of large percentages of these relatively high-melting 

 components esterified with glycerol. 



When a fatty acid has one or more double bonds in the molecule, 

 it is said to be unsaturated. Generally speaking, the unsaturated fatty 

 acids are of high molecular weight, having twelve or more carbons 

 in the molecule. The predominant members of this class have eighteen 

 carbons, for example, oleic acid (one double bond), linolic acid (two 

 double bonds), and linolenic acid (three double bonds). 



The presence of one or more double bonds in the fatty acid confers 

 quite different properties on the molecule from those evident in a 



