ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Influence of medium on the kinetics of oxidation of iron(II) ion with t-butyl hydroperoxide

B. Mihaljević and D. Ražem

Radiation Chemistry and Dosimetry Laboratory, Department of Chemistry, Ruder Bošković Institute, 10000 Zagreb, Croatia

E-mail: mihozeg@irb.hr

Received: 5 August 2005  Revised: 28 March 2006  Accepted: 5 April 2006

Abstract: The oxidation of iron(II) with tert-butyl hydroperoxide was investigated in the absence of oxygen in water, methanol, and the dichloromethane—methanol solvent mixture (φ_r = 2:1). The oxidation rate depends on solvent polarity; measured in the presence of SCN⁻ at constant 0.8 mmol dm⁻³ HCl, the rate constant increases with the polarity decrease passing from water and methanol to the dichloromethane—methanol solvent mixture. Further, in non-aqueous solutions at this acid concentration the rate constant was higher than the rate constant in the presence of Cl⁻ only. The oxidation rate measured in the [FeCl]²⁺ complex in dichloromethane—methanol was slow in acidic medium and increased by decreasing the acid concentration. Approaching the physiological pH conditions the rate constant attained the value of an order of magnitude of 10³ dm³ mol⁻¹ s⁻¹, while very little alteration of stoichiometry of the oxidation reaction was observed. The rate constant measured in the presence of Cl⁻ strongly depends on electrolyte concentration at concentrations less than 0.5 mmol dm⁻³ HCl, both in MeOH and the solvent mixture. Based on these results, a possible mechanism of the influence of solvent, acidity, and ligand type on the rate constant is discussed. We assume that the oxidation proceeds by an inner-sphere mechanism considering that the breakdown of the successor inner-sphere complex forming reactive alkoxyl radicals is probably the rate-limiting step. Presented at the 20th International Conference on the Coordination and Bioinorganic Chemistry organized by the Slovak Chemical Society, Slovak University of Technology, Comenius University, and the Slovak Academy of Sciences, Smolenice Castle, 5–10 June 2005.

Full paper is available at www.springerlink.com.

DOI: 10.2478/s11696-006-0045-5

Chemical Papers 60 (4) 253–267 (2006)