726 VTI. VITAMINS A 



6. Properties of Vitamins A and Related Compounds 



(1 ) General Physical Properties 



The several forms of vitamin A are very soluble in practically all fat 

 solvents and are insoluble in Avater. It is impossible to separate vitamin A 

 from vitamin A2, or from /3-carotene, on the basis of differential solubility 

 alone. The same is true for the stereoisomers of vitamin A. However, a 

 partial separation of carotene from vitamin A can be effected by precipitat- 

 ing the carotenoid from a mixture in a methanol solution at the tempera- 

 ture of solid carbon dioxide. 



Vitamin Ai alcohol crystallizes from methanol in the form of pale yellow 

 platelets which melt at 7.5 to 8°C.^^ and which contain methyl alcohol of 

 crystallization. When the crystallization occurs from ethyl formate or 

 propylene glycol solutions of vitamin A, pure vitamin A crystals result 

 which melt at 63-64°C.^^ 



In addition to the reaction with antimony trichloride, which is discussed 

 on page 732, vitamin A develops a blue color with acid earths when dis- 

 solved in non-polar solvents. This Avas originally discovered by Meunier.^^'' 

 The explanation for the reaction which Meunier proposed was that some 

 acid earths possess incomplete electronic octets which enable them to give 

 rise to an intensely blue color when they come in contact with vitamin A 

 in a non-polar solvent. When unshared electrons are donated to such 

 adsorbents, the vitamin molecule undergoes polarization and forms strongly 

 charged resonating structures. Lowman,^'* independently, recommended 

 this method for the detection of vitamin A and for its rough quantitative 

 estimation. The latter worker used a non-polar solvent and the commer- 

 cial adsorbent Super-Filtrol. Kreider^^^ confirmed the results of Lowman. 

 Zechmeister and Sandoval^^" have pointed out that this reaction is not 

 specific for vitamin A but that it is also given by carotene (as noted by the 

 earlier workers), and also by a new fluorescent polyene which has been 

 identified as phytofluene (see Chapter VI). Moreover, a positive reaction 

 can be obtained on purified Super-Filtrol with a benzene solution of di- 

 phenyloctatetraene, C6H5(CH : CH)4 • CeHs. 



Because of its alcohol group, vitamin A readily forms esters, not only with 

 fatty acids but also with other organic acids. Such esters occur naturally 

 in fish liver oils, as Avell as in the blood and tissues of higher animals. Some 

 pure crystalline vitamin A esters have recently been prepared by Baxter 

 and Robeson^'' (see Table 1). Vitamin A cannot be precipitated by digi- 



2" P. Meunier, Compt. rend., 215, 470-473 (1942). 



218 A. Lowman, Science, 101, 183-184 (1945). 



219 H. R. Kreider, Science, 101, 377 (1945). 



"0 L. Zechmeister and A. Sandoval, Science, 101, 585 (1945). 



