CLASSIFICATION^ AND STRUCTURE OF FATTY ACIDS 



tion which can be made is that the natural fats and oils of animal as well 

 as of vegetable oi-igin, almost without exception, are composed only of 

 fatty acids having an even number of carbon atoms. Furthermore, they 

 consist almost exclusively of straight-chain acids rather than of forked- 

 chain components; only in a few isolated cases are any ring compounds 

 found to occur as part of the fatty acid molecule. Although fats with an 

 odd number of carbon atoms can be synthesized in the laboratory and 

 would appear to be metabolized in the animal body, they are almost never 

 deposited in the tissues as such. Some exceptions have, howe^'er, been 

 noted (see page 237). 



The fatty acids most frequently occurring as components of natural fats 

 and oils can be classified into several series. The first of these is referred 

 to as the saturated fatty acid series or simply as the fatt}'' acid series. 

 These have no unsaturated linkages and cannot be altered by hydrogena- 

 tion or halogenation. The second group is characterized by the presence of 

 one double bond, and is called the oleic acid series (or monoethenoid acids). 

 A third group is the linoleic acid or diethenoid acid series, which is char- 

 acterized by two unsaturated linkages. The triethenoid and tetraethenoid 

 acids may be classified under the general group of polyethenoid acids. 

 The acids having more than one double bond make up some of the most im- 

 portant of the fatty acids, since they cannot be synthesized by the higher 

 animals, and some of them are required by the animal; hence they are of 

 considerable importance from a nutritional standpoint. 



(i) Nomenclature 



Most of the fatty acids were known before their structural relations were 

 established and before the adoption by the Geneva Congress^'^ in 1892, of 

 the system of nomenclature which is still more or less in general use. Ac- 

 cording to the Geneva classification, the aliphatic acid is regarded as a 

 derivative of the hydrocarbon where a terminal " — CH3" group is replaced 

 by a carboxyl group " — COOH." The name of the acid is derived from 

 that of the hydrocarbon by replacing the terminal e by the suffix oic. 

 Thus, the fatty acid related to the hydrocarbon, butane (CH3CH2CH2C/f3), 

 is named butanoic acid and has the formula CH3CH2CH2COO//. The chief 

 advantage of the Geneva system, other than that of furnishing a uniform 

 system which also relates the acid to the corresponding hydrocarbon, is 

 that, in all cases above the 4-carbon members, the name of the hydrocar- 

 bon (and consequently of the acid) is derived from the Latin word for the 

 number which corresponds to the number of carbon atoms in the specific 

 compound. This affords an easy method for the recognition of the formula 



7 A. Pictet, Arch. set. phys. nat., 27, 485-520 (1892). 

 » F. Tiemann, Ber., 26, 1595-1631 (1893). 



