II. CHEMISTRY 271 



debromination method of Rollet.^ Advantage is taken of the differences in 

 petroleum ether sohibihty of tetrabromostearic acid formed from hnoleic 

 acid and the solubiUties of the saturated acids and the dibromostearic acid 

 formed from oleic acid. This method has been carefully worked out and 

 standardized by McCutcheon.^ Best practice requires constant protection 

 of the unsaturated material from oxygen. Thus during saponification, de- 

 bromination, \\ashing, and transferring, the material should be kept under 

 a blanket of nitrogen. The resulting methyl ester after final distillation 

 through an efficient fractionation column should have an iodine number of 

 172 to 173, less than 0.1 % conjugated acids, and consist of only Cis acid. 

 Pi'eparations of linoleate or more highly unsaturated acids should always 

 be kept in evacuated or nitrogen-filled ampules at low temperature to 

 prevent oxidation of these very easily oxidizable esters or acids. 



RoUet^ and McCutcheon^ have developed similar bromination-debromi- 

 nation procedures for preparation of linolenic acid from the acids of linseed 

 oil. Arachidonic acid is prepared by bromination-debromination of the 

 acids from beef adrenal phospholipids^ or from fresh hog liver fatty acids. 

 With these preparations the danger of oxidation is even greater and 

 extreme care must be taken to prevent entry of oxygen. The quality of the 

 preparation is roughly indicated by its color — the lighter the color, the 

 better the preparation. Spectral absorption should be low in the ultra- 

 violet, and iodine number should be close to theoretical. Hydrogen number 

 is a better characteristic constant for the highly unsaturated acids than is 

 iodine number. 



Low-temperature crystallization methods developed by Brown and his 

 coworkers'" "12 provide a means of preparing these acids without danger 

 of isomerization by chemical treatment. The low-temperature crystalliza- 

 tion methods yield natural isomers, but numerous recrystallizations are 

 required to approach purity. 



Chromatographic procedures yield very pure natural products, but as yet 

 the method has not been used on a large scale. Swift et al. and Riemen- 

 schneider ct al. have prepared linoleate'^ and linolenate '* in a high degree of 



6 A. Rollet, Hoppe-Seyler's Z. physiol. Chem. 62, 410 (1909). 



6 J. W. McCutcheon Org. Syntheses 22, 75 (1942). 



7 A. Rollet, Hoppe-Seyler's Z. physiol. Chcm. 62, 422 (1909). 



8 J. W. McCutcheon, Org. Syntheses 22, 82 (1942). 



^ W. C. Ault and J. B. Brown, J. Biol. Chem. 107, 615 (1934). 

 '» J. B. Brown and J. Frankel, ./. Am. Chem. Sac. 60, 54 (1938). 

 " G. Y. Shinowara and J. B. Brown, J. Am. Chem. Sac. 60, 2734 (1938). 

 »2 D. T. Mowry, W. R. Brode, and J. B. Brown, J. Biol. Chem. 142, 671 (1942). 

 " C. E. Swift, W. G. Rose, and G. S. Jamieson, Oil & Soap 20, 249 (1943). 

 ^* R. W. Riemenschaeider, S. F. Herb, and P. L. Nichols, /. Am. Oil Chemists' Soc. 

 26, 371 (1949). 



