A kinetic study of the oxidation of 3- and 4-pyridinemethanol, and 3- and 4-pyridinecarboxaldehyde by chromium(VI) in acidic aqueous media

Abstract

Oxidation of 3-pyridinemethanol (3-pyol), 4-pyridinemethanol (4-pyol), 3-pyridinecarboxaldehyde (3-pyal) and 4-pyridinecarboxaldehyde (4-pyal) by CrVI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various Haq+ concentrations; [CrVI] = 8 × 10−4 M, [reductant] = 0.025–0.20 M, [HClO4] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A linear dependence of the pseudo-first-order rate constant (kobs) on [reductant] and a parabolic function of kobsversus [H+] lead to the rate law: −d[CrVI]/dt = (a + b[H+]2)[reductant][CrVI], where a and b describe the reaction paths via HCrO4− and H3CrO4+ species respectively, and are composite values including rate constants and equilibrium constants. The apparent activation parameters were determined from second-order rate constants at 1.0 and 2.0 M HClO4, at three temperatures within the 293–323 K range. The presence of chromium species with intermediate oxidation states – CrV, CrIV and CrII, was deduced based on e.s.r. measurements and the kinetic effects of MnII or O2 (Ar), respectively. The alcohols were oxidized to the aldehydes, and carboxylic acids and the aldehydes to the carboxylic acids. Chromium(III) was in the form of the [Cr(H2O)6]3+ complex.