COPPER(II)-CATALYZED OXIDATION OF CATECHOL BY MOLECULAR-OXYGEN IN AQUEOUS-SOLUTION

The oxidation of catechol by molecular oxygen in the presence of catalytic amounts of copper(II) ions has been followed by use of a Clark-type oxygen electrode and spectrophotometric methods at 25.0-degrees-C and with the ionic strength maintained at 1.0 M (KNO3). The overall stoichiometry of the reaction was established to be [GRAPHICS] The rate of reaction was strictly of half-order in O2 concentration but of no simple integral order with respect to either total copper(II) or total catechol concentration. However, the observed kinetic data were explicable in terms of a chain reaction involving the copper(I)-dioxygen species CuO2+, which reacts with the free ligand and its copper(II) complexes. The copper(I) is formed in the initiation step by the reaction of free copper(II) with the complex CuA, and the chain is terminated by bimolecular elimination of the activated dioxygen complex, with the formation of copper(II) and peroxide ion. This reaction scheme is shown to lead to the rate equation -d[O2]/dt = {k1'[HA-] + k2'[CuA] + k3'[CuA2(2-)]}{[Cu2+][CuA][O2]-1/2, with k1' = (3.15 +/- 0.15) x 10(3) M-3/2 s-1, k2' = (15.5 +/- 0.7) M-3/2 s-1, and k3' = (2.95 +/- 0.28) x 10(2) M-3/2 s-1.