Kinetics and Mechanism of Oxidation of Hydroxyurea with Hexacyano- ferrate(III) Ions in Aqueous Solution

Hydroxyurea (HU) effectively reduces Fe(CN)6 to Fe(CN)6 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)6 + NH2CONHOH + 1⁄2H2O  2Fe(CN)6 + CO2 + NH3 + 1⁄2N2O + 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)6/ Fe(CN)6 couple. The kinetic of oxidation of HU by Fe(CN)6 was studied using stopped-flow technique, as a function of H, HU, Fe(CN)6 and Fe(CN)6 concentrations, as well as a function of ionic strength and temperature. The kinetic results reveal that oxidation of HU by Fe(CN)6 proceed via an outer-sphere electron-transfer process. The effect of ionic strength on the reaction rate reveals that NaFe(CN)6 is the reacting species rather than Fe(CN)6 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.