152 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



ethenoid acids may be due to the shifting of the double bonds. Thus, the 

 hydrogen ation of linoleic acids results not only in the production of 9- or 

 12-octadecenoic acids but also in the formation of the 8- and 10-octadec- 

 enoic acids. ^^^ The latter two isooleic acids have likewise been demon- 

 strated by Hilditch and Vidyarthi*^" when methyl oleate was hydrogenated 

 in the presence of metallic nickel at 217-220 °C. These results were later 

 substantiated by Hilditch,''-^ although he used a lower temperature (180- 

 200 °C.). Van der Veen"^*' suggested that the formation of the 8- and 10- 

 octadecenoic acids is best explained by a shift in the double bonds of oleic 

 acid as it is formed. According to Moore®^^ such isomerization can occur 

 only during the actual hydrogenation. On the other hand, normal oleic 

 acid results during the hydrogenation of isooleic acids. However, although 

 this has been considered to be an equilibrium reaction, the ratio of iso- 

 oleic to oleic acid differs widely, depending upon whether oleic or isooleic 

 acid is the initial material hydrogenated. This fact would indicate that at 

 least one component of the isooleic acid mixture can revert to the original 

 oleic acid. The extent of the reaction in either direction varies with the 

 experimental conditions. A high temperature and, to a lesser extent, a 

 large amount of catalyst, are conducive to the formation of a higher propor- 

 tion of the iso-acids than of ordinary oleic acid. Ethyl esters are reported 

 to form more iso-acids than is the case with the triglycerides.^'" 



In addition to the shifting of the double bond during hydrogenation, 

 geometrical isomerism is also a feature. Thus, wlien methyl oleate is 

 partially hydrogenated to the extent of 30%, elaidic acid (^rans-9-octadec- 

 enoic acid) is produced, as well as the aforementioned positional isomers.^" 

 Conversely, when elaidic acid esters are hydrogenated, the corresponding 

 oleic acid esters result. ■*'^^'' This transformation proceeds more smoothly 

 with the ethyl esters than with the triglycerides."'" 



The so-called "selective" hydrogenation was first described by Moore and 

 associates"'^ in 1917, and has been amply confirmed"'^""'^ since the pioneer 

 investigations. According to this concept, a&-as-9,12-octadecadienoic or 

 its esters are practically completely transformed to the monoethenoid acid 

 before a saturation of the second double bond occurs. According to 



626 H. Van dor Veen, Chem. Umschau, 38, 89-96 (1931); Chem. AbsL, 25, 2972 (1931). 



627 T. P. Hilditch and N. L. Vidyarthi, Proc. Roy. Soc. London, A122, 552-563 (1929). 



628 T. P. Hilditch, Chem. Umschau, 37, 354-356 (1930); Chem. Abst., 25, 1800 (1931). 



629 C. W. Moore, J. Chem. Soc. Ind., 38, 320-325T (1919). 



630 A. Steger and H. W. Scheffers, Chem. Umschau, 38, 45-53 (1931); Chem. AbsL, 25, 

 2315(1931). 



631 H. K. Moore, G. A. Richter, and W. B. Van Arsdel, /. Ind. Eng. Chem., 9, 451-462, 

 (1917). 



632 A. S. Richardson, C. A. Knuth, and C. H. Milligan, Ind. Eng. Chem., 16, 519-522, 

 (1924); 17, 80-83(1925). 



633 T. P. Hilditch and C. W. Moore, J. Soc. Chem. Ind., 42, 15-17 T (1923). 



634 D. R. Dhingra, T. P. Hilditch, and A. J. Rhead, /. Soc. Chem. Ind., 51, 195-198T, 

 (1932). 



