A kinetic study on the reactivity of chromium(IV)

Abstract

The inhibition effect of Mn2+ on the rates of the Cr(VI) oxidations of 13 different organic reductants (RH) in aqueous perchloric acid has been studied. The reductants were formaldehyde, formic acid, maleic acid, and 10 alcohols (8 primary and 2 secondary). The second-order rate constants decrease with increasing [Mn2+] according to the law[Formula: see text]Parameter b′ is interpreted as the ratio between the rate constants for the Cr(IV)–Mn2+ and Cr(IV)–RH reactions. Cr(IV) is reduced by Mn2+ (and also by Ce3+) much faster than by organic reductants RH. The Cr(IV) oxidations of primary alcohols follow Taft's equation with a negative reaction constant (ρ* = −0.80 ± 0.06). The Cr(IV) present in aqueous perchloric acid is postulated as an oxo species (CrO2+) capable of reacting by one-electron transfer with both Mn2+ and Ce3+, and by hydride-ion transfer with organic reductants. Relevance of these results to the general mechanism for the Cr(VI) oxidations of organic substrates is discussed.