Original scientific paper

Kinetics and Mechanism of Oxidation of Hydroxyurea with Hexacyanoferrate(III) Ions in Aqueous Solution

Ana Budimir ; Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovačića 1, 10000 Zagreb, Croatia
Erim Bešić ; Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovačića 1, 10000 Zagreb, Croatia
Mladen Biruš ; Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovačića 1, 10000 Zagreb, Croatia

Abstract

Hydroxyurea (HU) effectively reduces Fe(CN)₆3− to Fe(CN)₆ 4− species in neutral and basic aqueous solution via an electron transfer process that includes the formation and subsequent fading out of a free radical, U•(U• ≡ H₂N−C(=O)N(H)O•). The EPR spectrum of U• in H₂O solutions suggests that the unpaired electron is located predominantly on the hydroxamate hydroxyl-oxygen atom. Visible spectrophotometric data reveal HU as a two-electron donor. Stoichiometry of the studied reaction can be formulated
as: 2Fe(CN)₆3− + NH₂CONHOH + ½H₂O → 2Fe(CN)₆4− + CO₂ + NH₃ + ½N₂O + 2H+. Lack of evidence for the formation of NO probably is a consequence of fast dimerization of HNO in comparison with the rate of its oxidation, which is slow due to the low reduction potential of the Fe(CN)₆3−/ Fe(CN)₆4− couple.
The kinetic of oxidation of HU by Fe(CN)₆3− was studied using stopped-flow technique, as a function of H+, HU, Fe(CN)₆3− and Fe(CN)₆4− concentrations, as well as a function of ionic strength and temperature. The kinetic results reveal that oxidation of HU by Fe(CN)₆3− proceed via an outer-sphere electron-transfer process. The effect of ionic strength on the reaction rate reveals that NaFe(CN)₆2− is the reacting species rather than Fe(CN)₆3− ion. The rate of the redox process was found to be first order with respect to both redox reactants while the H+ concentration dependence make clear that U− is about four orders of magnitude more reactive than HU. The formal reduction potentials for U•/U− and HU•/HU couples were estimated from the kinetic results as +0.47 V and +0.84 V, respectively.

Keywords

hydroxyurea; hexacyanoferrate(III); kinetics; redox reaction; oxidation; mechanism

Hrčak ID:

45621

URI

https://hrcak.srce.hr/45621

Publication date:

20.12.2009.

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