A kinetic study of the oxidation of 2-pyridinemethanol, 2-pyridinecarboxaldehyde and 2,6-pyridinedimethanol by chromium(VI) in acidic aqueous media

Abstract

Oxidation of 2-pyridinemethanol (2-pyol), 2,6-pyridinedimethanol (2,6-pydol) and 2-pyridinecarboxaldehyde (2-pyal) by CrVI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various H+ aq 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 parabolic dependence of the pseudo-first-order rate constant (k obs) versus [H+] was observed for all the reductants. A linear dependence of k obs on [2,6-pydol] and, unusually, higher than first-order dependence on [2-pyol] and [pyal] was established. The apparent activation parameters for reactions studied at constant [H+] at I = 1.2 and 2.1 M were determined. The presence of chromium species at the 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. Comparison of the available second-order rate constants for aromatic alcohols and aldehydes demonstrated that chelating abilities of the reductant facilitates the redox process, whereas the electron-withdrawing effect caused by protonating the pyridine nitrogen atom acts in the opposite direction. The unusual low reactivity of 2-pyol was ascribed to intramolecular hydrogen bond formation.