818 IX. THE VITAMIN E GROUP 



recovered from an olive oil solution kept in an oven at 55°C. in contact with 

 oxygen for periods of 88 and of 108 hours. These data are summarized in 

 Table 8. The results of this test parallel the behavior of these toco- 

 pherols as antioxidants. The tocopherols which can best protect them- 

 selves are also the most effective as antioxidants. 



TABLE 8 

 Relative Resistance of the Tocopherols to Oxidation in Oil at 55 "C." 



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



869-874(1947), p. 871. 



On oxidation of the tocopherols with gold chloride, p-quinones are 

 formed which apparently result from the rupture of the 1 : 2 linkage. These 

 products are readily distinguishable by a maximum absorption band at 

 about 260 m/i. Stem et al.^^ have reported a similar behavior for 5- 

 tocopherol, in which case the maximum absorption band is at 257 mju. 

 Wlien the quinone is reduced with sodium hydrosulfite, a hydroquinone is 

 formed : 



E (1%, 1 cm., 294 m/x) = 81.6 for S-tocopherol.^s This can be con- 

 densed with sulfonic acid in ethyl alcohol to produce the original toco- 

 pherol. The gold chloride method has been used by Karrer et aZ.^"^-"'' for 

 the quantitative determination of tocopherol. 



Vitamin E also develops a characteristic red color when treated with 

 silver nitrate and nitric acid. The product obtained by such treatment of 

 5-tocopherol has the spectral absorption properties of an o-quinone; the 

 corresponding compounds from a-, ^-, and 7-tocopherol have likewise been 

 identified as o-quinones."''"^ The yellow color which is produced has an 

 intensity proportional to the amount of tocopherol present; this can be 

 used for the quantitative determination of vitamin E. The maximum ab- 

 sorption is from 460 to 480 mju^" for a-, /S-, and 7-tocopherols, and the ex- 

 tinction coefficient values are from 11 to 15; in the case of 5-tocopherol, the 

 maximum absorption is at 435 m^ and the E (1%, 1 cm.) is about 15. 



10^ P. Karrer, R. Escher, H. Fritzsche, H. Keller, B. H. Ringier, and H. Salomon, 

 Helv. Chim. Acta, 21, 939-953 (1938). 



ii» P. Karrer and H. Keller, Helv. Chim. Acta, 21, 1161-1169 (1938). 



111 L. I. Smith, W. B. Irwin, and H. E. Ungnade, J. Avi. Chem. Soc, 61, 2424-2429 

 (1939). 



112 L. I. Smith, W. B. Irwin, and H. E. Ungnade, Science, 90, 334-335 (1939). 



