290 III. CHEMISTRY OF NEUTRAL FATS 



dant activity proved to be quite variable. However, the isolation^'* of the 

 several forms of vitamin E, which are now referred to as tocopherols, has 

 largely explained the discrepancies, since it was found that a-tocopherol is 

 most effective as an antisterility agent while j8- and 7-tocopherols excel 

 as antioxidants. ^^^ The recent demonstration of the presence of another 

 tocopherol, 5-tocopherol, in soybean oil,*^^ which has a higher antioxidant 

 value than 7-tocopherol but an extremely low biological activity, gives 

 further credence to the belief that the tocopherols are actually important 

 natural antioxidants, in spite of the irregularities of biological and anti- 

 oxidant assays in different oils. 



H Hs 



HOC6 P 3CHs 



I il I /CHj 



HC7. ,C, ?C\ 



Tocol 



The formula for the tocopherols can readily be compared by considering 

 them as derivatives of tocol : 



a-Tocopherol 5,7,8-Trimethyltocol 



^-Tocopherol S.S-DimethyltocoH" 



7-Tocopherol 7,8-DimethyltocoH" 



5-Tocopherol S-MethyltocoH^^ 



The tocopherols resemble the phenolic inhibitors in furnishing an aro- 

 matic ring and one free hydroxyl group (position 6) . However, it has been 

 shown from a study of the parent chromans^^^ that the oxygen in the hetero- 

 cyclic ring (position 1) is required for antioxygenic activity, just as the 

 second hydroxyl is essential in the diphenols, catechol, and hydroquinone. 

 Moreover, all ethers and esters (except the allophanates) are inactive, as is 

 also the case with the phenolic inhibitors. 5-Tocopherol with one methyl 

 group is the most effective of the series, followed by the /3- and 7-com- 

 pounds, while a-tocopherol is the least active. The relative effectiveness 

 of the tocopherols as antioxidants has been sho\vn by Hove and Hove^^^ to 

 be related to temperature. Whereas the relative activities of a-, /3-, and 

 7-tocopherols were found to be approximately equal in protecting carotene 

 at 35° and 60°C., 7-tocopherol was shown to be the best at 98°C., followed 

 by /3-tocopherol, while the a-compound had the lowest activity. A similar 



«* H. M. Evans, O. H. Emerson, and G. A. Emerson, J. Biol. Chem., 113, 319-332 

 (1936). 



«5 H. S. Olcott and O. H. Emerson, J. Am. Chem. Soc, 59, 1008-1009 (1937). 



«« M. Stern, C. D. Robeson, L. Weisler, and J. G. Baxter, /. Am. Chem. Soc, 69, 

 869-874 (1947). 



«' O. H. Emerson and L. I. Smith, J. Am. Chem. Soc, 62, 1869-1872 (1940). 



«8 C. Golumhic, /. Am. Chem. Soc, 63, 1163-1164 (1941). 



«s E. L. Hove and Z. Hove, J. Biol. Chem., 156, 623-632 (1944). 



