Refractive Indices of l -Ascorbic Acid

E. G. Cox1 states that l-ascorbic acid, is optically negative with = 1.462, = 1.68, and >1.70. We have examined a material isolated from peppers after the method of A. Szent-Györgyi2 by Dr. A. G. Grollmann, of the Johns Hopkins Medical School. This substance gives the characteristic absorption spectrum of l-ascorbic acid with a maximum coefficient3 at 2650 A.; the melting point is 188°, and the analysis (Mrs. M. S. Sherman) C, 40.80 per cent, H, 4.78 per cent (calculated for C6H8O6; C, 40.89 per cent, H, 4.58 per cent). The compound as crystallised from methyl alcohol or acetone has = 1.465, = 1.600±0.006, and = 1.747 for 5780 A. The optical sign is either positive or negative within the limit of experimental error, as is verified by the lack of curvature of the isogyre in a centred optic axis interference figure. No evidence was found for structural polymorphism, but the possibility was not rigorously eliminated. These constants, which are for crystals of the type shown in Szent-Györgyi's Fig. 2a2, are published since they are of value in identification of ascorbic acid and since they substantiate Cox's deduction of a plane configuration for the molecule, which is in accord with the accepted furanose ring structure.