A novel kinetic and mechanistic study of the oxidation of pinacol by acid bromate—homogeneous catalysis of maganous ions

A detailed kinetic study of the Mn(II)-catalyzed and -uncatalyzed oxidation of pinacol by bromate has been carried out in aqueous acetic acid media containing Hg(II) ions. The uncatalyzed reaction exhibits 1.5 order that is, 0.5 order in pinacol] and 1.0 in bromate]. A decrease in k₁ by increasing bromate] has been accounted for due to the formation of Br₂O₅, which is inactive toward reduction. Mn(II)-catalyzed oxidation follows first order in oxidant], 0.5 order in manganous ion], and variable order with respect to pinacol]. At lower pinacol] (0.005–0.025M) the order is 0.5, but at higher concentration (0.03–0.15M) it becomes negative (?1.0). These observations can be accounted for qualitatively by the formation of 1:1 and 1:2 Mn(II)–pinacol complexes of which only 1:1 is active toward bromate oxidation. At higher pinacol] the ratio of 1:2 and 1:1 complexes reached 98.2. All reactions were accelerated with acidity, and the rate constant follows the h₀ function. Participation of H₂O in the rate-limiting step and a free-radical mechanism were proposed for the manganous-ion-catalyzed reaction, whereas for the uncatalyzed reaction this was not true. The effects of NaClO₄, Na₄P₂O₇, and the dielectric constant of the media are also in accordance with the proposed mechanism.