CLASSIFICATION' AND STRUCTURE OF FATTY ACIDS 13 



All of the members of this series have melting points considerably 

 lower than those of the corresponding saturated acids. The main reason 

 for the fact that vegetable oils are liquid while most animal fats are solid 

 is to be traced to the much greater proportion of unsaturated acids in the 

 former. 



In addition to the acids listed in Table 2, one probably should include 

 crotonic acid (CHoCH : CHC'OOH or 2-butenoic acid). This is the simplest 

 monoethenoid acid found naturally, but because it is present only in the 

 inedible croton oil (from which it is named) it is not of sufficient importance 

 to be listed. However, it is an acid which is readily metabolized." It can 

 be synthesized from the important physiological compound, /3-hydroxy- 

 butyric acid (CHaCHOHCH.COOH), while Blunden'i found that the re- 

 verse transformation could be demonstrated in the fasting rat. 



a. Octadecenoic Acids. Oleic acid, or 9-octadecenoic acid, is the com- 

 monest one of the monoethenoid series. This acid is so widely distributed 

 in natural oils as to be considered the predominant acid. It frequently 

 comprises as much as 50% of the total acid content of oils, and seldom con- 

 stitutes less than 10% of the total fatty acid content. It is present to the 

 extent of nearly 85% in olive oil,^- and of 74% in cashew kernel oil (Aiui- 

 cardium occidentale) J^ Oleic acid is widely distributed in animal fats, 

 as well as in tlie vegetable oils. It seems to be the chief component of the 

 fats of warm-blooded animals, in contrast to 9-hexadecenoic acid (palmit- 

 oleic acid) which is largely present in the fat of cold-blooded animals. 

 The favored position for the unsaturated linkage would seem to be be- 

 tween carbons 9 and 10. Not only does the double bond occur in this 

 position in oleic and palmitoleic acids, which from a quantitative stand- 

 point account for most of the monoethenoid acids, but it is found at the 

 same position in four other acids (including gadoleic acid) where the chain 

 length varies from 10 to 20 carbon atoms. 



Because of the presence of a double bond in the molecule, the mono- 

 ethenoid acids can exist in either a cis or a trans form. Oleic acid is the 

 CIS isomer of 9-octadecenoic acid, while elaidic acid is its trans isomer. 

 Although the cis — ^ trails rearrangement may be readily accomplished by 

 chemical means, it apparently cannot be brought about in the animal 

 organism. Elaidic acid is never found in natural products; however, if 

 fed to animals, it may be laid down in the depot fat. Most of the mono- 

 ethenoid acids occur onl^' in the cis form. This is the case with palmitoleic, 



" H. D. Blunden, The Intermediary Metabolism of Butyric Acid as Determined from 

 the Physiological Behavior of the Various Four-Carbon Acids, Dissertation, Dept. Bio- 

 chemistry, Univ. Southern California, April, 1938. 



>2 G. S. Jamieson and W. F. Baughman, J. Oil & Fat Ind., 2, 40-44 (1925); Chem. 

 Abst., 20, 2083-2084 (1926). 



13 C. K. Patel, J. J. Sudborough, and H. E. Watson, J. Indian Insi, Sci., Part 6, 6, 

 111-129 (1923); Chem. Abst., i 7, 3616 (1923). 



