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

Kinetics and mechanisms of the oxidation of methoxyurea and N-methylhydroxyurea were studied in neutral and basic aqueous solutions. The obtained pH dependences of the oxidation rates indicate that for both hydroxyureas the reactive species are the deprotonated ones. The second order rate constants, the activation enthalpies and the activation entropies for the reactions of methoxyurea (O-methylhydroxyurea) and N-methylhydroxyurea anions with Fe(CN)(6)(3-) at 25 degrees C, I = 2 mol dm(-3) (NaClO4) were determined as (5.06 +/- 0.01) x 10(2) mol(-1) dm(3) s(-1), (1.92 +/- 0.02) x 10(4) mol(-1) dm(3) s(-1), 27 +/- 1 kJ mol(-1), 16 +/- 1 kJ mol(-1), -101 +/- 2 J mol(-1) K-1, and -107 +/- 4 J mol(-1) K-1, respectively. The pK(a) value of methoxyurea at 25 degrees C and 2 mol dm(-3) ionic strength was determined kinetically as 12.7 +/- 0.1 and the thermodynamic parameters for the deprotonation reaction were determined as Delta H-a = 43 +/- 1 kJ mol(-1), and Delta S-a = -96 +/- 4 J mol(-1) K-1. When the kinetic results are compared with the data reported for hydroxyurea, an inverse dependence of the rate constants on the pK(a) of the hydroxyurea derivatives at 25 degrees C is observed. Such unexpected behaviour has been explained by the ab initio calculations and NBO analysis of HOMOs for all three hydroxyureates. (doi: 10.5562/cca1799)