Ascorbiс Acid Degradation in N, N-Dimethylformamide Solutions

Abstract

Objective: Investigate the mechanisms of L-ascorbic acid transforтmation and formation of coloured enamines in N, N-dimethyl-formamide solutions.
 Methods: An automatic polarimeter Atago POL-1/2 was used for polarimetric investigation. Electronic spectra were recorded by UV-spectrometer Cary 60 (Agilent). The statistical analysis was carried out using the OriginPro 9.1 packages.
 Results: The Biot’s law violation was found in below 0.1% solutions of L-ascorbic acid (AA) in N, N-dimethylformamide (DMF). During the day, the specific rotation of 1% AA solution varied from+37 to-1.0. Gradually, the solution acquired the red colour, and its intensity depended on the AA concentration. Spectrophotometrically, it was shown that after 15 min AA was absent in the n·10-3% solutions. The decomposition followed the first-order kinetics (k1=1.83·10-2с-1). At the same time, new absorption bands appeared at 273, 390, 533 nm. Model solutions containing dimethylamine (DMA) had a similar spectrum, and the intensity of the absorption bands increased in proportion to the concentration of DMA.
 Conclusion: The results show that the first step in the decomposition of ascorbic acid AA in DMF follows first-order kinetics. Numerous decomposition products are optically active compounds and reverse the sign of the optical rotation of the solution. The water resulting from the decomposition of AA is involved in the hydrolysis of the solvent. The hydrolysis product, the secondary amine DMA, interacts with the carbonyl groups of the AA decomposition products to form coloured enamines. Magnesium (II) accelerates the formation of coloured products.