New insights on the mechanism of oxidation of d-galacturonic acid by hypervalent chromium

Abstract

The pollutant Cr(VI) is known to be very carcinogenic. In conditions of excess of Cr(VI), oxidation of D-galacturonic acid (Galur), the major metabolite of pectin, yields d-galactaric acid (Galar) and Cr(III). The redox reaction takes place through a multistep mechanism involving formation of intermediate Cr(II/IV) and Cr(V) species. The mechanism combines one- and two-electron pathways for the reduction of Cr(IV) by the organic substrate: Cr(VI)→ Cr(IV)→ Cr(II) and Cr(VI)→ Cr(IV)→ Cr(III). This is supported by the observation of the optical absorption spectra of Cr(VI) esters, free radicals, CrO(2)(2+) (superoxoCr(III) ion) and oxo-Cr(V) complexes. Cr(IV) cannot be directly detected; however, formation of CrO(2)(2+) provides indirect evidence for the intermediacy of Cr(II/IV). Cr(IV) reacts with Galur much faster than Cr(V) and Cr(VI) do. The analysis of the reaction kinetics via optical absorption spectroscopy shows that the Cr(IV)-Galur reaction rate inversely depends on [H(+)]. Nevertheless, high [H(+)] still does not facilitate accumulation of Cr(IV) in the Cr(VI)-Galur mixture. Cr(VI) and the intermediate Cr(V) react with Galur at comparable rates; therefore the build-up and decay of Cr(V) accompany the decay of Cr(VI). The complete rate laws for the Cr(VI), Cr(V) and Cr(IV)-Galur redox reaction are here derived in detail. Furthermore, the nature of the five-co-ordinated oxo-Cr(V) bischelate complexes formed in Cr(VI)-Galur mixtures at pH 1-5 is investigated using continuous-wave and pulsed electron paramagnetic resonance (EPR) and density functional theory (DFT).

Quinic acid and hypervalent chromium: a spectroscopic and kinetic study

“…The UV-vis absorption spectrum of the QA/Cr VI reaction mixture in an acidic medium (HClO 4 ) showed the characteristic behavior of Cr VI in an acidic medium [25][26][27] with a shoulder at 420/450 nm and a band at 350 nm (Fig. 2).…”

“…As was previously described in the main text, 18,26,27,37 aqueous CrO 2+ species can be generated in situ by rapid oxidation of Cr 2+ using oxygen. To generate the required Cr 2+ , it is necessary to employ a highly reducing medium.…”

“…11,[13][14][15][16][17] The formation of Cr V and Cr II/IV intermediates during the oxidation of a variety of organic compounds by Cr VI has been observed, [18][19][20][21][22] and their involvement in Cr-induced cancers 1,23 has aroused much curiosity in the chemistry and biochemistry of chromium. 24 In fact, the detection by continuous-wave electron paramagnetic resonance (CW-EPR) or electron spin resonance (ESR) of a long-lived Cr V species [25][26][27][28] is focused on the probable role(s) performed by Cr V species in carcinogenesis brought about by Cr VI . A similar situation has arisen with Cr IV .…”

Oxidation and Reduction

“…The nature of the five-coordinated oxo-Cr(V)-bischelate complexes formed in Cr(VI)/gl mixtures at pH 1-5 was investigated using continuous-wave and pulsed ESR (EPR) and DFT. 39 The Cr(VI) oxidation of m-toluidine in aqueous nitric acid is inhibited by substrate at low [H + ] and believed to be proceeded by two routes to give the coloured product; consecutive reactions have been discussed. 40 For chromic anhydride oxidation of pyridines and their deuterium isotopomers in H 2 SO 4 , oxidation of both the Me group and pyridine nucleus is believed to involve fast nucleophilic attack of the oxidant, followed by a slow separation of hydride ion in the transition state resulting in two-electron reduction of Cr(VI) to Cr(IV).…”

“…The mechanism (Scheme 5) of the oxidative C−H transformation of allyl arenes to alkenyl aldehydes by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (37) and catalysed by FeCl 2 in C 2 H 4 Cl 2 had the initial reaction in which (37) oxidized allyl benzene (38), or [(Z)-prop-1-enyl]benzene (39), assisted by FeCl 2 , through a singleelectron transfer to the corresponding allyl radical (40) and Fe(III) complex (41). Next, (41) oxidized (40) to the allyl cation (42) and was reduced to Fe(I) complex (43).…”