Kinetics and mechanism of the reduction of chromium(VI) by d-fructose

The kinetics of the reduction of chromium(VI) to chromium(III) by d-fructose in HClO4 was studied at various concentrations of reactants and acidities. The redox reaction takes place through a complex multistep mechanism, which involves the formation of intermediate CrIV and CrV species. CrIV reacts with d-fructose much faster than CrV and CrVI do, and cannot be directly detected. Superoxochromium(III), CrO22+, has been detected by UV spectroscopy, thus evidencing that CrII is an intermediary in these reactions and provides indirect evidence for the intermediary of CrIV reaction path. The complete rate law for the CrVI oxidation reaction is expressed by −d[CrVI]/dt = (a + b [d-Fru])[H+]2 [CrVI]T, where a = (1.66 ± 0.04) × 10−4 M−2 s−1 and b = (1.79 ± 0.06) × 10−2 M−3 s−1. The EPR spectra show that five-coordinate oxo-CrV bischelates are formed in rapid pre-equilibria before the redox step: Cr(V) to Cr(III).

The reduction of chromium(VI) to chromium(III) by d-fructose in HClO4 takes places through a complex multistep mechanism.Figure optionsDownload as PowerPoint slide