Kinetics and Mechanism of the Reduction of Chromium(VI) and Chromium(V) byD-Glucitol andD-Mannitol

Abstract

The oxidation of d-glucitol and d-mannitol by Cr VI yields the aldonic acid (and/or the aldonolactone) and Cr III as final products when an excess of alditol over Cr VI is used. The redox reaction occurs through a Cr VI 3 Cr V 3 Cr III path, the Cr VI 3 Cr V reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by:where k G2 H (8.5 AE 0.2)´10 À2 m À3 s À1 and k GH (1.8 AE 0.1)´10 À2 m À2 s À1 , at 338. The slow redox steps are preceded by the formation of a Cr VI oxy ester with l max 371 nm, at pH 4.5. In acid medium, intermediate Cr V reacts with the substrate faster than Cr VI does. The EPR spectra show that five-and six-coordinate oxo-Cr V intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxoCr V species are present at any [H ], whereas hexacoordinate ones are observed only at pH`2 and become the dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At higher pH, where hexacoordinate oxo-Cr V species are not observed, Cr V complexes are stable enough to remain in solution for several days to months.1. Introduction. ± Chromium(VI) (Cr VI ) is a potential hazard both in a biological and an ecological context [1]. The observation of Cr V and Cr IV intermediates in the selective oxidation of organic substrates by Cr VI and their implication in the mechanism of Cr-induced cancers [2 ± 3] has generated a considerable amount of interest in their chemistry and biochemistry [4 ± 7]. The biological reduction of Cr VI to lower states has been observed with a wide variety of naturally occurring cellular reductants [8 ± 12]. Ligands that possess two O-atoms able to form five-membered rings about the metal ion, such as 1,2-diols and a-hydroxy acids, are effective as nonenzymatic reductants and complexation agents towards hypervalent chromium and can stabilize the labile oxidation states of chromium [13 ± 19]. For this reason, it is interesting to look at the ability of polyhydroxy compounds to reduce Cr VI to Cr III , in order to know the role they may play in the chemistry of Cr VI .We are studying the possible fate of Cr VI and Cr V in biological systems by examining reactions of Cr VI with low-molecular molecules [20 ± 34]. Our studies on the reduction of Cr VI and intermediate Cr V by aldoses [20] [30] [31] [33], deoxyaldoses [20] [23] [25] [30] [31], sugar acids [22] [27] [29], and methyl glycosides [34] showed that the relative redox reactivities of these saccharides toward chromate is based on the relative rates of the oxidation vs. complexation processes [31] [32]. Under conditions of

Evidence for the involvement of CrII and free radicals as intermediates in the reduction of by saccharides, alcohols and hydroxyacids

“…However, this mechanism does not explain the polymerization induced by free radicals observed in the oxidation of a number of substrates by Cr VI [9][10][11][12][13][14][15][16][17][18][19][20], which implies that a pathway involving a one-electron step must also occur. Recently, based on the observation of CrO 2þ 2 and the successful trap of organic radicals using acrylonitrile, a combination of the Cr VI !…”

Influence of Homocysteine on the Function of Native Arteriovenous Fistula in Hemodialysis Patients

“…Generally, these reagents are capable of effecting chemospecific, regiospecific, and stereospecific oxidations in highly sensitive systems. Kinetic and mechanistic studies on chromium(VI) oxidations continue to be of interest 5–7, and similar investigations with these reagents are numerous. So are those dealing with the structure–reactivity relationships operating in these oxidations 8, 9.…”

Kinetic and mechanistic features of the thiolactic acid oxidation by chromium(vi) in acidic environment

“…The condensation complex formed (  3 X C r O ) further decomposes by transfer of electrons, this being often the rate determining step. Depending on the nature of the reducing agent and the molecularity of this electron transfer step, the number of electrons changed may vary from one [14,15] (to form Cr(V)) to two [16][17][18] (yielding Cr(IV)) or even three in some co-oxidations [19,20]. The involvement of the intermediate valence states Cr(V) and Cr(IV), both also oxidizing agents, as well as that of other very reactive intermediates, such as organic radicals, brings about complicated reaction schemes [6].…”