REDUCTION POTENTIAL OF THE TERT-BUTYLPEROXYL RADICAL IN AQUEOUS-SOLUTIONS

Rate constants for oxidation of tert-butyl hydroperoxide anion (t-BuO2 -) by several oxidants were determined by pulse radiolysis. Rapid oxid ation was found with N-3 . (4.4 x 10(8) L mol(-1) s(-1)) and . O- (2.6 x 10(8) L mol(-1) s(-1)), but others (Br-2(.-), I-2(.-), ClO2., CO3.- , C6H5N(CH3)(2)(.+)) reacted much more slowly. Since the reduction pot entials of these oxidants are much higher than that estimated for t-Bu O2., the observed rate constants suggest that the self-exchange rate f or t-BuO2./t-BuO2- is very slow and that it would be difficult to esta blish rapid equilibrium by pulse radiolysis with a suitable redox pair . Therefore, to determine the reduction potential for this peroxyl rad ical, the forward and reverse rate constants for reaction 6, t-BuO2. Fe(CN)(6)(4-)reversible arrow t-BuO2- + Fe(CN)(6)(3-), were measured independently. The forward rate constant was measured by kinetic spect rophotometric pulse radiolysis in N2O-saturated solutions containing h igh concentrations of N-3(-) (1 mol L-1), t-BuO2- (0.4 mol L-1), and v arying concentrations of Fe(CN)(6)(4-) (0.012-0.036 mol L-1) at pH 13. 7. The rate constant k(6) was found to be 2.5 x 10(2) L mol(-1) s(-1). Such a low rate constant was possible to measure by pulse radiolysis only because the self-decay of t-BuO2. is very slow compared to those of other peroxyl radicals. The reverse reaction rate constant was meas ured by mixing the reactants and following the disappearance of Fe(CN) (6)(3-) in the presence of spin traps to remove the peroxyl radicals f rom the equilibrium. A rate constant of k(-6) = 1.4 x 10(-2) L mol(-1) s(-1) was derived. By taking the reduction potential of Fe(CN)(6)(3-) /Fe(CN)(6)(4-) as 0.459 V vs NHE for the conditions used in these expe riments, we calculate E(t-BuO2./t-BuO2-) = 0.71 V and E(t-BuO2./t-BuO2 H) = 1.05 V at pH 7 and 1.47 V at pH 0. The rate constants for oxidati on of Fe(CN)(6)(4-) and other compounds by various peroxyl radicals we re then utilized for the estimation of the reduction potentials of the se peroxyl radicals.