Kinetics of the oxidation of hydroxyurea with vanadium(V) ions in acidic aqueous solution

Hydroxyurea (HU) effectively reduces vanadate (VO 2 + ) into vanadyl (VO 2+ ) species in acidic aqueous solution acting as a two-electron donor. The reaction starts by the formation of a transient complex followed by an electron transfer process that includes the formation and subsequent fading out of a free radical, U • (U • ≡H 2 N–C(=O)N(H)O • ). The rate determining step of the redox reaction is the formation of a free radical either by the inner-sphere one-electron transfer within the formed VO 2 + -complexes, or by an outer sphere one-electron transfer from VO 2 + -complex to the second VO 2 + ion. Assuming a rapid pre-equilibrium for the proton-transfer and complexation reactions, an inner-sphere electron transfer pathway has been proposed (VO 2 -U 2+ VO 2+ -U •2+ , k 1 = 1.4 s −1 ) when HU is in excess. When VO 2 + in excess, an additional reaction possibly proceeds through two outer-sphere electron-transfer pathways: VO 2 -U 2+ + VO 2 + + 2H 2 O VO 2 + + VO 2+ + U • , and VO 2 -U 2+ + VO(OH) 2+ + 2H 2 O VO(OH) 2+ + VO 2+ + U • , characterized by k 4 = 22 s −1 mol −1 dm 3 , and k 5 = 3.8 × 10 3 s −1 mol −1 dm 3 .