PHYSICAL PROPERTIES OF FATTY ACIDS 85 



(a) Optical Isomerism. This type of isomerism is possible when at least 

 one asymmetric carbon atom is present in the molecule. An asymmetric 

 carbon atom is one in which each of four valences is satisfied by different 

 atoms or groups. When more than one asymmetric atom occurs in the 

 same molecule, the number of possible isomers is increased to a total of 2", 

 where n is the number of such asymmetric carbon atoms. Optical isomers 

 are characterized by differences in the rotation of the plane of polarized 

 light as well as by variations in most other physical properties. Examples 

 of the importance of optical isomerism are the methyl- or hydroxy-sub- 

 stituted acids. In such compounds, this property may be useful for their 

 identification as well as for their quantitative estimation. 



(6) Geometric or Cis-Trans Isomerism. Geometric isomerism is possible 

 when a compound has two carbon atoms attached by double bonds. A 

 restricted rotation of these carbon atoms occurs because of the double 

 bonds, and so they are not able to develop a free rotation. The shape of 

 the molecule will differ according to whether the combination is of such a 

 nature that the molecule is partially folded back on itself {cis) or extended 

 to the maximum length (trans). The best known examples of such isomers 

 are oleic and elaidic acids, which represent, respectively, the cis and trans 

 forms of 9-octadecenoic acid. 



a'. Elaidinization: The process of isomerization of a cis to a trans 

 isomer is usually referred to as elaidinization. The general reaction was 

 discovered over a hundred years ago by Poutet,^*^ who reported that the 

 treatment of olive oil with mercurous nitrate resulted in its change to a 

 semisolid fat resembling pork fat. Poutet's reagent has had wide applica- 

 tion over a number of years ; apparently, it is superior to other catalysts 

 since it does not result in the formation of addition products. Nitrous acid, 

 286-289 g^g ^ygji g^g nltrous oxide, ^^""^^^ have been used successfully to bring 

 about the isomerization of oleic acid, as well as in the transformation of 

 them acid, erucic (13-docosenoic acid) to the trans acid, brassidic acid.^^^"® 

 Lidoff^^" had discovered its application to linoleic acid, and this work 



285 J. J. E. Poutet, Ann. chim. phys. [2] 12, 58-68 (1819). 



286 F. Boudet, Ann., 4, 1-33 (1832). 



287 H. Meyer, Ann., 35, 174-188 (1840). 



288 J. Gottlieb, Ann., 57, 33-67 (1846). 



289 F. Varrentrapp, Ann., 35, 196-215 (1840). 



29« A. Lidoff, /. Russ. Pfiys. Chem. Soc, 24, 515-524 (1892); cited by K. S. Markley, 

 Fatty Acids, Interscience, New York, 1947, p. 517. 



291 A. Lidoff, /. Russ. Phys. Ctiem. Soc, 24, 524-526 (1892); cited by K. S. Markley, 

 Faiiy Acids, Interscience, New York, 1947, p. 517. 



292 A. Lidoff, J. Russ. Ptiys. Chem. Soc, 27, 177-182 (1895); cited Ijy K. S. Markley, 

 Fatty Acids, Interscience, New York, 1947, p. 517. 



293 J. Jegerow, J. pratct. Chem. [2], 86, 521-539 (1912). 

 2"^ A. Fitz, Ber., 4, 442-446 (1871). 



295 J. J. Sudborough and J. M. Gittins, J. Chem. Soc, 05, 315-321 (1909). 



296 G. Rankoff, J. prakt. Chem. [2], ISl, 293-300 (1931). 



