Pulse-radiolytic investigations of catechols and catecholamines. 4. 3,4-Dihydroxybenzaldehyde and 3,4-dihydroxyacetophenone.

The model catechol compounds for adrenalone, 3,4-dihydroxybenzaldehyde and 3,4-dihydroxyacetophenone, were investigated by pulse radiolysis combined with kinetic spectroscopy. The rate constants with the oxidizing primary radicals in neutral aqueous solutions, hydroxyl radicals (4.8 × 109-5.9 × 109 M-1 s-1) and superoxide anions (1.4 × 107-2.9 × 107 M-1 s-1), differ by no more than a factor of about 2 from those of adrenalone itself. The mechanism for the oxidation by ·OH radicals or O2 - was identical for all three substances. The reduction rates with eaq - were determined both by kinetic analysis of the decay of eaq - and by competition experiments. The fact that the model compounds exclusively form the ketyl radical upon reduction, whereas adrenalone also reveals side-chain cleavage, was not reflected in the respective transient spectra. The second-order decay rates of the three ketyl radicals in alkaline solution all exceed 109 M-1 s-1 with a fivefold acceleration in near neutral solutions