PHYSICAL PROPERTIES OF FATTY ACIDS 87 



COOH. Other trans octadecenoic acids which are known have the double 

 bonds at positions 5-6, 7-8, 8-9, 10-11, or 12-13, respectively. 



The properties of the fatt}^ acids having a tra7is linkage are distinct from 

 those with the cis form. Elaidic acid, which is the best known and typical 

 trans representative, can be readil}^ distinguished from oleic acid by its 

 melting point (see Table 16), by the fact that it forms solid solutions with 

 saturated fatty acids,* in accordance with the findings of Mascarelli 

 et al. that trans isomers such as brassidic acid form solid solutions with 

 saturated compounds (behenic), and that cis forms, like erucic, do not,^^''"^^' 

 by its longer molecular length as determined by x-ray examination,'^^""* 

 by the difference in behavior of the monomolecular film,*^^ and by other 

 physical properties. 



d. Isomerism of the Polyethenoid Acids, (a) Linoleic Acid. There is 

 no clear-cut proof for position or geometric isomers of linoleic acid in 

 nature. Hilditch^ has cited considerable evidence to indicate that linoleic 

 acid from a Avide variety of plant oils conforms to the pattern of cis-9-cis- 

 12-octadecadienoic acid. Although a large variety of positional isomers 

 are theoretically possible, the 9,12-octadecadienoic acid is the only one 

 which occurs naturally. 



Four geometric isomers are theoretically possible in the case of the di- 

 ethenoid acids. Thus, linoleic acid may have the following configuration 

 of the double bonds at the 9 and 12 positions, respectively: cis-cis, cis- 

 trans, trans-cis, and trans-trans. In addition to the natural form (cis- 

 cis), the tran-trans isomer (linolelaidic acid) is well known also, since it 

 can be produced by the elaidinization reaction. The structures of the 

 above two acids have been established unequivocally. 



There is some question whether the cis-trans or the trans-cis isomers have 

 been demonstrated. The chief indication for their presence has been based 

 upon failures to obtain a correlation between the tetrabromide value, the 

 thiocyanogen number, and the iodine number. Inoue and Suzuki*^ in- 

 terpreted their results as evidence of a new linoleic acid isomer from silk- 

 worm pupae, since they were unable to obtain the tetrabromide in the usual 

 solid form. Similarly, Smith and ChibnalP** could not separate the 

 crystalline tetrabromide from the linoleic acid fraction of cocksfoot and 

 perennial rye grass. Moreover, Frankel et al.^^'' were unable to obtain a 



"0 G. Bruni and Y. Gorni, Atti accad. Lincei [5], 8, II, 181-190 (1899). 

 31' L. Mascarelli, Atti accad. Lincei [5], 23, II, 583-585 (1914). 

 312 L. Mascarelli and B. Toschi, Atti accad. Lincei [5], 23, II, 586-590 (1914). 

 3'3 L. Mascarelli and G. Sanna, Atti accad. Lincei [5], 24, II, 30-37 (1915). 

 "•• A. Mixller, J. Chem. Sac, 123, 2043-2047 (1923). 

 ''5 A. Muller and G. Shearer, /. Chem. Soc, 123, 3156-3164 (1923). 

 316 J. Marsden and E. K. Rideal, /. Chem. Soc, 1938, 1163-1171. 

 "' J. S. Frankel, W. Stoneburner, and J. B. Brown, J. Am. Chem. Soc, 65, 259-262 

 (1943). 



