Kinetic study of the promazine oxidation to promazine 5-oxide by trisoxalatocobaltate(III) in basic aqueous media

Abstract

The kinetics of the oxidation of promazine by trisoxalatocobaltate(III) were studied in the presence of a large excess of the cobalt(III) in tris buffer solution using u.v.-vis spectroscopy ([Co III ] = (0.6 ) 2) Â 10 )3 M, [ptz] = 6 Â 10 )5 M, pH = 6.6-7.8, I = 0.1 M (NaCl), T = 288 ) 308 K, l = 1 cm). The reaction proceeds via two consecutive reversible steps. In the first step, the reaction leads to formation of cobalt(II) species and a stable cationic radical. In the second step, cobalt(III) is reduced to cobalt(II) ion and a promazine radical is oxidized to the promazine 5-oxide. Linear dependences of the pseudo-first-order rate constants (k 1 and k 2 ) on [Co III ] with a non-zero intercept were established for both redox processes. Rates of reactions decreased with increasing concentration of the H + ion indicating that the promazine and its radical exist in equilibrium with their deprotonated forms, which are reactive reducing species. The activation parameters for reactions studied were as follows: DH " = 44 ± 1 kJ mol )1 , DS " = ) 100 ± 4 JK )1 mol )1 for the first step and DH " = 25 ± 1 kJ mol )1 , DS " = )169 ± 4 J K )1 mol )1 for the second step, respectively. Mechanistic consequences of all the results are discussed.