New insights on the mechanism of oxidation of D-galacturonic acid by hypervalent chromium
Faculty of Sciences. Physics
Journal of the Chemical Society: Dalton transactions. - London
, p. 7033-7045
University of Antwerp
The pollutant CrVI is known to be very carcinogenic. In conditions of excess of CrVI, oxidation of D-galacturonic acid (Galur), the major metabolite of pectin, yields D-galactaric acid (Galar) and CrIII. The redox reaction takes place through a multistep mechanism involving formation of intermediate CrII/IV and CrV species. The mechanism combines one- and two-electron pathways for the reduction of CrIV by the organic substrate: CrVI → CrIV → CrII and CrVI → CrIV → CrIII. This is supported by the observation of the optical absorption spectra of CrVI esters, free radicals, CrO22+ (superoxoCrIII ion) and oxo-CrV complexes. CrIV cannot be directly detected; however, formation of CrO22+ provides indirect evidence for the intermediacy of CrII/IV. CrIV reacts with Galur much faster than CrV and CrVI do. The analysis of the reaction kinetics via optical absorption spectroscopy shows that the CrIVGalur reaction rate inversely depends on [H+]. Nevertheless, high [H+] still does not facilitate accumulation of CrIV in the CrVIGalur mixture. CrVI and the intermediate CrV react with Galur at comparable rates; therefore the build-up and decay of CrV accompany the decay of CrVI. The complete rate laws for the CrVI, CrV and CrIVGalur redox reaction are here derived in detail. Furthermore, the nature of the five-co-ordinated oxo-CrV bischelate complexes formed in CrVIGalur mixtures at pH 15 is investigated using continuous-wave and pulsed electron paramagnetic resonance (EPR) and density functional theory (DFT).