The air oxidation of 4,6-di (2-phenyl-2-propyl) pyrogallol. Spectroscopic and kinetic studies of the intermediates

Abstract

Two colored extinction bands, one with λmax near 520 nm and the other near 770 nm, form and decay during air oxidations of 4,6-di(2-phenyl-2-propyl) pyrogallol in alkaline methanol. The 770 nm band appears to be due to the semiquinone monoanion while the extinction near 500 nm is thought to be due to both the semiquinone and 3-hydroxy-o-quinone. Although the rates of formation and decay of the two bands are indistinguishable in the presence of excess dissolved oxygen, the 770 nm band disappears slightly before the 500 nm extinction when the pyrogallol reactant is initially in excess of the dissolved oxygen. Except at high methoxide concentrations, the kinetics indicate a system of two consecutive reactions with pseudo first-order rate constants k1 and k2. The pH′ dependence of k1 indicates that the neutral pyrogallol, its monoanion and dianion all react, the rate increasing with the negative charge of the species. The k2/k1 ratio is constant at 5 from pH′ 11 to 16. Extinction coefficients at both wavelengths increase with pH′. The 770 nm extinction disappears immediately after acidification while the 500 nm extinction decays at a rate consistent with oxidation. The rate constants k1 and k2 are insensitive to changes in pyrogallol concentrations, indicating an absence of irreversible semiquinone disproportionations. Increased oxygen concentration raises k1 and k2 to the same extent. © 1977 Carlsberg Laboratory.