Quinic acid and hypervalent chromium: a spectroscopic and kinetic study

The redox reaction between an excess of quinic acid (QA) and Cr(VI) involves the formation of intermediates, namely, Cr(IV) and Cr(V) species, which in turn react with the organic substrates. As observed with other substrates that have already been studied, Cr(IV) does not accumulate during this reaction because of the rate of the reaction. Its rate of disappearance is several times higher than that of the reaction of Cr(VI) or Cr(V) with QA. Kinetic studies indicate that the redox reaction proceeds via a combined mechanism that involves the pathways Cr(VI) → Cr(IV) → Cr(II) and Cr(VI) → Cr(IV) → Cr(III), which is supported by the observation of superoxo-Cr(III) (CrO(2)(2+)) ions, free radicals, and oxo-Cr(V) species as intermediates and the detection of Cr(VI) ester species. The present study reports the complete rate laws for the QA/chromium redox reaction.