CLASSIFICATION AXD STRUCTURE OF FATTY ACIDS 21 



geometrical isomers: cis-cis; cis-trans; trans-cis; and trans-trans. Natu- 

 rally occurring linoleic acid (a-linoleic acid) is the cis-cis isomer. The re- 

 sults of Brown and FrankeP" seem to indicate that only one type of linoleic 

 acid is present in the natural seed oils; this view is supported by the 

 findings of McCutcheon.''^ The /3-acid, which is also known as isolinoleic 

 acid, has a trans-trans structure, according to Suzuki et o//--^^ and Maru- 

 yama/* On elaidinization with Poutet's reagent or selenium, according to 

 Kass and Burr,*^ two geometrical isomers resulted, namely trans-9-trans- 

 12-linolelaidic acid and /?'ans-9-C2's-12-linolelaidic acid. This indicates that 

 the double bond on carbon 12 cannot elaidinize without a previous change 

 on the 9 carbon. Since no c?'s-9-c?'.s-12-octadecadienoic acid was found, 

 Kass and Burr concluded that the change does not involve an equilibrium 

 reaction. 



The presence of the unsaturated bonds enables linoleic acid to take up 

 oxygen from the air. The fat, especially when spread in a thin layer, 

 forms a hard and glossy surface, which is the chemical reaction taking place 

 when paint hardens. Linoleic acid is generally credited with being the 

 most important component in drying oils. In order to serve satisfactorily 

 in this capacity, however, commercial oils should contain 40 to 65% of 

 linoleic acid. Fats having less than 25% of linoleic acid are considered 

 unsuitable for use in paints. Another requirement for a satisfactory 

 drying oil is that it also contain appreciable amounts of the triethenoid 

 acid, linolenic, in addition to the necessary proportion of linoleic acid. 

 Only linseed, perilla, and hempseed oils meet these qualifications. When 

 less than 15% of linolenic acid is present, the oils are unsatisfactory for 

 paints, even though the diethenoid acid constitutes a considerable propor- 

 tion of the fatty acids. Even walnut oil, in which linoleic acid values as 

 high as 75% have been reported, but in w4iich the linolenic acid content is 

 only 2 to 10%,'"^^''^ has a limited application in the drying oil industry. 



^'egetabIe oils having the requisite quantity of linoleic acid but lacking a 

 sufficient quota of linolenic acid have only limited application as drying 

 oils, and hence are referred to as semidrying oils. 



« J. B. Brown and J. Frankel, /. Am. Chem. Soc, 60, 54-56 (1938). 



^' J. W. McCuicheon, Can. J. Research, B16, 158-175 (1938). 



« Y. Inoue and B. Suzuki, Proc. Imp. Acad. Tokyo, 7, 15-18 (1931). 



" T. Maruvama and B. Suzuki, Proc. Imp. Acad. Tokyo, 7, 379-382 (1931); 8, 186- 

 189 (1932). 



'"■ T. Maruvama, /. Chem. Soc. Japan, 54, 1082-1087 (1933); Chem. AbsL, 28, 1015 

 (1934). 



« J. P. Kass and G. O. Burr, /. Am. Chem. Soc, 61, 1062-1066 (1939). 



*^ S. L. Ivanov and E. Berdichevski, Schriften zentral. hiochem. Forsch. Inst. Nahr. 

 71. GenussmiUelind, U. S. S. R., 3, 246-250 (1933); Chem. AbsL, 28, 2557 (1934). 



^' G. S. Jamieson and R. S. McKinney, Oil & Soap, 13, 202 (1936). 



*8 G. S. Jamieson and R. S. McKinney, Oil & Fat Industries, 6, No. 2, 21-23 (1929). 



« H. N. Griffiths and T. P. Hilditch, J. Soc. Chem. Ind., 63, 75-81T (1934). 



