184 ASCORBIC ACID 



halogens chlorine, bromine, or iodine in neutral or in acid solution. ^^ This 

 reaction with the halogens, which does not proceed in non-aqueous solvents 

 unless a carbonate such as lead carbonate is present to remove the acid, is 

 one of oxidation and not of addition of halogen, for two equivalents of halo- 

 gen acid are liberated. Thus two atoms of hydrogen are removed from each 

 mole of ascorbic acid (CeHsOe) to give dehydroascorbic acid (CeHeOe). 

 The oxidation reaction is a reversible one, for if the oxidized solutions are 

 treated with hydrogen sulfide the original vitamin is regenerated.^^ If iodine 

 has been used as the oxidizing agent, reversal of the reaction can be effected 

 by simply evaporating the oxidized solution in vacuo. The hydriodic acid 

 acts as the reducing agent, and iodine and ascorbic acid are formed. The 

 same well-defined oxidation of ascorbic acid can be carried out with a large 

 number of oxidizing agents such as acid permanganate,-^* quinone, phenol- 

 indophenoP and its dichloro derivative, ^^ methylene blue,^*^ and ferric salts." 

 It is because of this oxidation arrest point that these and other reagents 

 (see p. 248) have been utilized in devising methods for the assay and deter- 

 mination of vitamin C.^^ Dehydroascorbic acid, which is the first oxidation 

 product of ascorbic acid, retains the same antiscorbutic activity as the 

 original vitamin, and it is believed that some of the \'itamin C occurring 

 in nature exists in this oxidized form. Aqueous solutions and extracts of 

 ascorbic acid readily undergo oxidation when left exposed to the air, es- 

 pecially if traces of copper are present. ^^ In alkaline solutions the oxidation 

 is greatly accelerated and extensive degradation eventually takes place, 

 leading to complete breakdown of the molecule. 



The structure of ascorbic acid can be deduced from the following evi- 

 dence. The compound contains four hydroxyl groups as evidenced ]>y the 

 fact that vigorous aeetylation gives rise to a tetraacetate.*" Two of the hy- 

 droxyl groups are different from the others, for under milder acetjdation 

 conditions ascorbic acid gives a well-defined diacetate.*^ The two hydroxyl 



32 R. W. Herbert, E. L. Hirst, E. G. V. Percival, R. J. W. Reynolds, and F. Smith, 

 /. Chem. Soc. 1933, 1270. 



33 E. L. Hirst and S. S. Zilva, Biochem. J. 27, 1271 (1933). 



3^ K. S. Murty and C. R. Viswanadham, Proc. Natl. Inst. Set. India 10, 217 (1944). 

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39 R. Strohecker and H. Schmidt, Z. Lebensm. Untersuch. n. Forsch. 86, 370 (1943). 

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