Kinetics of the autoxidation of adrenaline and [copper(II)(adrenaline)]2+ in alkaline aqueous and micellar media

Abstract

The kinetics of autoxidation of adrenaline and [Cu(adrenaline)]2+ complex by dissolved oxygen in alkaline aqueous and micellar media has been studied. The reaction is initiated by the removal of amino-H+ protons of adrenaline by hydroxide ion, followed by cyclization. The values of (1/k obs) for the autoxidation of both species were found to be linearly dependent upon 1/[OH−]. The reaction follows a consecutive pathway in which the intermediate adrenochrome remains stable for few minutes, and then undergoes further reactions to yield adrenolutin and other products. The [Cu(adrenochrome)]+ complex is stable for a few hours. Studies on the effects of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) on the reactivity of both species revealed different behaviors. The micelles of CTAB catalyzed the rates of autoxidation for both species, whereas SDS micelles inhibited the autoxidation of adrenaline but catalyzed the rate of autoxidation of [Cu(adrenaline)]2+. Addition of the reactive counterion surfactant, cetyltrimethylammonium hydroxide (CTAOH) initially increased the rate constant with the increasing [CTAOH], until it reached a plateau for k ψ −[CTAOH]. Salts such as NaCl, NaBr, tetramethyl ammonium bromide, and tetraethyl ammonium bromide increased the rate when added at lower concentrations, but had negligible effect at higher concentrations. The results obtained in micellar media were treated according to Berezin’s Pseudophase Model. The various kinetic parameters for the reactions occurring in aqueous and in micellar media are reported.