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Micellar effect on the kinetics and mechanism of chromium(VI) oxidation of organic substrates

“…Depending on the conditions, Mn(II) can catalyze or inhibit or remain without any effect in the reduction of Cr(VI) [2,15(a), 18,43,44,48,49,[77][78][79][80][81][82][83][84][85][86]. For the one equivalent reduction of Cr(VI) at the rate determining step, many workers have suggested that the rate of Cr(VI) reduction is not affected by Mn(II) [82].…”

“…For the oxidation of a single substrate, this 3e-transfer pathway at the rate determining step is preferred at lower Cr(VI) concentrations in less acidic media while the 2e-transfer pathway (Scheme 7) is preferred at higher Cr(VI) concentrations in strongly acid media. Though this 3e-transfer pathway has been disputed by some workers [1], some authors are still in favor of this mechanism in many cases [79,80,91,[121][122][123][124][125][126]. Recently, we have shown [121] that the micellar effect is an indirect evidence in favor of the 3e-transfer pathway for the cooxidation of oxalic acid and formic acid in aqueous acidic media.…”

“…The rate retarding effect of the inorganic salts like NaCl, NaBr, NaNO 3 , Na 2 SO 4 on the observed micellar catalysis has been explained by considering the removal of glycolic acid from the reaction site. The micellar effect on the kinetics of Mn(II)-catalyzed oxidation of citric acid by Cr(VI) has been studied by the same group in different conditions [79]. The rate determining step involves the one-step three electron transfer in the complex HCrO 4 − -citric acid-Mn(II) that is formed in pre-equilibrium steps.…”

Electron transfer reaction in the chromium(VI)-manganese(II) system in the presence of ethylenediaminetetraacetic acid (EDTA)

“…In the redox reactions of chromium with organic reductants, manganese(II) also plays an important role [11][12][13][14][15]. The formation of chromium(IV) as an intermediate has been decided by the addition of manganese(II) [16][17][18][19][20]. It is widely accepted that reduction proceeds via three consecutive one-electron steps in the presence of transition metal ions [1].…”

“…The kinetics of oxidation of some a-hydroxy acids (mandelic [12], lactic [22], glycolic [23], malic [24], tartaric [25] and citric [26]) by chromic acid have been reported in the presence of manganese(II). However, the details of manganese(II) oxidation by chromium (VI) are not yet known.…”