POLYUXSATURATED FATTY ACIDS 787 



procedures. One of these methods, which involves low-temperature crys- 

 tallization from organic solvents, was developed in the laboratories of J. 

 B. Brown twenty years ago. The features of the procedure were reviewed 

 by Brown. 2^ The low-temperature fractionation not only facilitates 

 separation of saturated from unsaturated acids but also renders possible 

 the preparation of samples of oleic and linoleic acid of high purity. Ho^^'- 

 ever, highly purified linolenic acid cannot be prepared by this method. 



c. The Urea-Adduct Method. The use of the urea inclusion compounds 

 for the preparation of piu'e saturated and unsaturated compounds has 

 IM'oved to be a simple and highly effective method for the preparation of a 

 number of saturated and unsaturated acids. According to Schlenk,^^ 

 who recently published a comprehensi^'e re\'iew on this subject, the com- 

 pounds which form urea adducts must necessarily have more than four to 

 six carbon atoms, and the carbons must be linked to form a straight chain. 

 The reaction of urea to form compounds with fats was discovered by Ben- 

 gen (as described b}^ Schlenk"'-^) in the case of milk fats. It was later 

 noted by Bengen and Schlenk^^ that urea inclusion compounds were formed 

 with straight-chain but not with branched-chain molecules. Bengen-^" 

 patented these findings under the title Method for Separation of Aliphatic 

 Oxygen-Containing Compounds and of Straight-Chain Hydrocarhmis of at 

 Least 6 Carbon Atoms from Mixtures Containing Them. 



Schlenk and Holman^^ described the separation of urea inclusion com- 

 pounds, of increasing degrees of unsaturation, by the fractional precipita- 

 tion of a mixture of fatty acids with methanol solutions of urea. Reiser,^^ 

 in studies of fat absorption, employed a similar procedure for the prepara- 

 tion of conjugated linoleic acid. Swern and Parker^* prepared linoleic 

 acid of 85% to 959o purity from corn oil, in a 72% to 50% yield, and 87% 

 to 89% pure Unolenic acid, in a yield of 71% to 55%, from perilla oil. 

 Achaya et al.^^ obtained linoleic acid with an iodine number of 172 to 175 

 (theory 181.4) in a single operation from safflowerseed fatty acids. 



25 J. B. Brown, Chem. Revs., 29, 333-354 (1941). 



26 H. Schlenk, Progr. Chem. Fats and Other Lipids, 2, 243- 2G7 (1954). 



27 W. Schlenk, Jr., Ann., 565, 204-240 (1949). 



2« W. Schlenk, Jr., Angew, Chem., 62A, 299-301 (1950). 

 29 F. Bengen and W. Schlenk, Jr., Experientia, 5, 200 (1949). 



•» F. Bengen, German Patent appl. (to I. G. Farbenindustrie, A. G.), 3, WO, 197 IV 

 c?//^ (March 18, 1940). 



31 H. Schlenk and R. T. Holman, /. Am. Chem. Soc, 72, 5001-5004 (1950). 



32 R. Reiser, Pro . Soc. Exptl. Biol. Med., 74, 666-669 (1950). 



" D. Swern and W. E. Parker, J. Am. Oil Chemists' Soc, 30, 5-7 (1953). 

 '** K. T. Achaj-a, B. P. Baliga, S. A. Saletore, and S. H. Zaheer, Personal communica- 

 tion to the author (H. J. D.), 1954. 



