88 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



correlation of the tetrabromide value and the iodine number with olive oil, 

 although an excellent agreement obtained with corn, sesame, cottonseed, 

 grapeseed, and poppyseed oils. These investigators believed that olive 

 oil contains isomeric linoleic acid. On the other hand, Kass ct al.^^^ con- 

 cluded that only one octadecadienoic acid is present in olive oil, as well as in 

 almond, coconut, corn, cottonseed, peanut, poppyseed and sunflowerseed 

 oils, and cacao butter, in spite of irregularities in the yield of tetrabromo- 

 stearic acid. These workers believe that the tetrabromide method gives 

 unreliable results. However, in spite of this debatable circumstantial 

 evidence for the presence of linoleic acid isomers, no isomer has been 

 isolated. In any event, the amounts and the distribution of the disputed 

 products are quite insignificant, and one must conclude that practically all 

 of the octadecadienoic acid found naturally is linoleic acid. 



(6) Trieihenoid Acids. Linolenic acid and elaeostearic acids are the 

 most important triethenoid acids. While linolenic acid is 9,12,15-octadec- 

 atrienoic acid, elaeostearic acid has the structure of 9,11,13-octadeca- 

 trienoic acid. These are the only two positional isomers known, although 

 the possibilities of such compounds are far greater with three double bonds 

 as compared with two unsaturated linkages. 



The differences in arrangement of the double bonds in linolenic and 

 elaeostearic acid are responsible for considerable variation in their j)roper- 

 ties. While the unsaturated bonds are separated in linolenic acid by a 

 methylene group, they occupy positions between alternate carbon atoms 

 in the case of elaeostearic acid. This latter arrangement is spoken of as 

 conjugation. Acids in which such conjugate double bonds occur do not add 

 halogens to complete saturation, in contradistinction to the quantitative 

 response of acids where the unsaturated linkages are not conjugated. 

 However, acids with conjugate double bonds add oxygen more readily and 

 pol3anerize more easily than do their non-conjugate isomers. This 

 property renders them especially satisfactory for use in quick-drying paints. 

 Other properties of acids with double bonds in alternating positions also 

 differentiate them from their non-conjugated compounds. This is particu- 

 larly noticeable as regards the melting point. Whereas linolenic acid 

 melts at — 11°C., a-elaeostearic acid melts at 48-49°C., and the jS-form 

 at 71-72°C. 



The opportunities for geometric isomers increase exponentially with the 

 rise in the number of double bonds. Eight such isomers are theoretically 

 possible in the case of the triethenoid acids, including cis-cis-cis and trans- 

 trans-trans, as well as all intermediate configurations. The geometrical 

 structure of a-linolenic acid is not known with certainty, although it is as- 

 sumed to be the cis-cis-cis form. Kass, Nichols, and Burr'^^^ have pre- 

 cis J. P. Kass, W. O. Lundberg, and G. O. Burr, Oil & Soap, 17, 50-53 (1940). 

 3»9 J. P. Kass, J. Nichols, and G. O. Burr, /. Am. Chem. Soc, 63, 1060-1063 (1941). 



