A kinetic study of the oxidation of 2-(2-hydroxyethyl)pyridine by chromium(VI) in strongly acidic aqueous solutions of HClO4

Abstract

Oxidation of 2-(2-hydroxyethyl)pyridine (pyeol) to 2-pyridylacetaldehyde (pyeal) by CrVI has been studied in the 0.5–2.0 M HClO4 range at I = 2.2 M and in super acidic media within the 3–7 M HClO4 range. In all cases the reaction has been examined under pseudo-first order conditions keeping the alcohol and H+aq in excess. CrIII-complexes formed during reduction of CrVI by pyeol at different molar ratios of the reactants, were isolated chromatographically and identified as [Cr(H2O)6]3+ and [Cr(pyeac)(H2O)4]2+ ions (pyeac = 2-pyridylacetic acid). Free 2-pyridylacetaldehyde (pyeal) was separated and determined as its 2,4-dinitro-phenylhydrazone derivative. A dependence of the rate constants on [pyeol] and [H+] has been established at I = 1.2 M and I = 2.2 M. The apparent activation parameters at [H+] = 1 and 2 M have been determined. A rate law of the form d[CrVI]/dt= (k1[H+]+k2[H+]2)[pyeol][CrVI] is proposed. A linear dependence of log kobs on H0 in the super acidic media is obeyed. A rate decrease is observed if oxygen instead of argon is in the reaction cell. The reaction mechanism has been discussed.