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 (k′obs) 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] = k′obs[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 k′obsvs. [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.