Original scientific paper

https://doi.org/10.5562/cca1799

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

Ana Budimir ; University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovačića 1, 10000 Zagreb, Croatia
Tin Weitner ; University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovačića 1, 10000 Zagreb, Croatia
Ivan Kos ; University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovačića 1, 10000 Zagreb, Croatia
Davor Šakić ; University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovačića 1, 10000 Zagreb, Croatia
Mario Gabričević ; University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovačića 1, 10000 Zagreb, Croatia
Erim Bešić ; University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovačića 1, 10000 Zagreb, Croatia
Mladen Biruš ; University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovačića 1, 10000 Zagreb, Croatia

Abstract

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 oC, I = 2 mol dm−3 (NaClO4) were
determined as (5.06 ± 0.01) x 102 mol−1 dm3 s−1, (1.92 ± 0.02) x 104 mol−1 dm3 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 pKa value of methoxyurea
at 25 oC 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 ΔaH = 43 ± 1 kJ mol−1, and ΔaS = −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 pKa of the hydroxyurea derivatives at 25 oC 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)

Keywords

methoxyurea; N-methylhydroxyurea; hexacyanoferrate; oxidation; kinetics; NBO; quantum chemical study

Hrčak ID:

71976

URI

https://hrcak.srce.hr/71976

Publication date:

3.10.2011.

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