Issue 3, 2003

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

Abstract

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.

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

Article information

Article type
Paper
Submitted
14 Jun 2002
Accepted
16 Oct 2002
First published
23 Dec 2002

Dalton Trans., 2003, 348-353

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

O. Impert, A. Katafias, P. Kita, A. Mills, A. Pietkiewicz-Graczyk and G. Wrzeszcz, Dalton Trans., 2003, 348 DOI: 10.1039/B205786G

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