Evidence for the involvement of chromium(II) as an intermediate in the reduction of chromium(VI) to chromium(III) by formaldehyde

Abstract

The reaction between chromium(vi) and formaldehyde in aqueous perchloric acid is inhibited by manganese(i1) ion, according to the relation k, = a/(l + b[Mn2+]) + c; manganese(i1) ion and formaldehyde compete to reduce the intermediate Crlv, and Cr" is an intermediate.According to Westheimer's mechanisms,lJ there are in principle two possibilities for the reduction of CrVI to CrI" by two-equivalent reducing agents (R), one involving only CrV and CrlV as intermqdiates [eqn. (l)], and the other involving also Cr" as intermediate [eqn. (2)].

A kinetic study on the reactivity of chromium(IV)

“…By application of the steady-state approximation to the intermediate Cr(IV) in the case of Mechanism I, to both Cr(1V) and Cr(I1) for Mechanism 11, and to both Cr(IV) and R. for Mechanism 111, it is easy to demonstrate that the expression obtained for parameter b in the case of Mechanism I is whereas for both Mechanisms I1 and I11 we obtain where kin,, k,,, and k,, are the rate constants for the reactions of the intermediate chromium(1V) with manganese(I1) ion (inhibitor), chromium(V1) (oxidant), and the organic substrate (reductant), respectively (12).…”

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

“…A similar mechanism has been proposed by Perez-Benito and co-workers [43,44] for the Cr(VI) oxidation of different organic substrates where Cr(IV) is reduced to Cr(II) through a hydride transfer pathway. In the presence of a Co(III) complex such as [(NH 3 ) 5 CoF] 2+ , Cr(II) can be trapped in the following reaction [42,45].…”

“…At higher [Cr(VI)]/[S] ratio, the Westheimer mechanism is favored, while at the low values of the ratio, the Rocek mechanism is favored. In the Rocek mechanism (Scheme 7), the free-radicals are suggested to be responsible for the acrylonitrile polymerization while in the Perez-Benito and co-workers' mechanism [43,44], the carbo-cationic center generated through the hydride transfer towards Cr(IV) is supposed to be responsible for the polymerization of monomer.…”

Kinetics and mechanism of oxidation of N,α-diphenylnitrones by 4-(dimethylamino)pyridinium chlorochromate (DMAPCC) in aqueous DMF medium

“…The values of k obs are listed in Table 2. Manganese(II) plays an important role (inhibition [18] or catalysis [19] or no effect [20,21] attributed to the removal of chromium(IV) by reaction with manganese(II) [22].…”