Kinetics and mechanism of a fast leuco-Methylene Blue oxidation by copper(II)–halide species in acidic aqueous media

The kinetics of a fast leuco-Methylene Blue (LMB) re-oxidation to Methylene Blue (MB) by copper(II)–halide (Cl–, Br–) complexes in acidic aqueous media has been studied spectrophotometrically using a stopped-flow technique. The reaction follows a simple first order rate expression under an excess of the copper(II) species (and H+(aq)), and the pseudo-first order rate constant (kobs) is largely independent of the atmosphere used (air, oxygen, argon). The rate law, at constant Cl–(Br–) anion concentration, is given by the expression: (d[MB+])/dt= ((kaK[H+]+kb)/(1 +K[H+]))·[CuII][LMB]=kobs[LMB], where K is the protonation constant, and ka and kb are the pseudo-second order rate constants for protonated and deprotonated forms of LMB, respectively. The rate law was determined based on the observed kobsvs.[CuII] and [H+] dependences. The rate dramatically increases with [Cl–] over the range: 0.1–1.5 M, reflecting the following reactivity order: Cu2+(aq) CuCl+(aq) < < CuCl42–. The slow re-oxidation of LMB by oxygen has also been briefly examined at different [H+]. ESR results provide clear evidence for the formation of an intermediate radical. The mechanistic consequences of all these results are discussed.