Reduction Potential of the tert-Butylperoxyl Radical in Aqueous Solutions

Rate constants for oxidation of tert -butyl hydroperoxide anion ( t -BuO 2 - ) by several oxidants were determined by pulse radiolysis. Rapid oxidation was found with N 3 • (4.4 × 10 8 L mol -1 s -1 ) and • O - (2.6 × 10 8 L mol -1 s -1 ), but others (Br 2 •-, I 2 •-, ClO 2 •, CO 3 •-, C 6 H 5 N(CH 3 ) 2 •+ ) reacted much more slowly. Since the reduction potentials of these oxidants are much higher than that estimated for t -BuO 2 •, the observed rate constants suggest that the self-exchange rate for t -BuO 2 • / t -BuO 2 - is very slow and that it would be difficult to establish rapid equilibrium by pulse radiolysis with a suitable redox pair. Therefore, to determine the reduction potential for this peroxyl radical, the forward and reverse rate constants for reaction 6, t -BuO 2 • + Fe(CN) 6 4- ⇄ t -BuO 2 - + Fe(CN) 6 3-, were measured independently. The forward rate constant was measured by kinetic spectrophotometric pulse radiolysis in N 2 O-saturated solutions containing high concentrations of N 3 - (1 mol L -1 ), t -BuO 2 - (0.4 mol L -1 ), and varying 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 × 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 -BuO 2 • is very slow compared to those of other peroxyl radicals. The reverse reaction rate constant was measured by mixing the reactants and following the disappearance of Fe(CN) 6 3- in the presence of spin traps to remove the peroxyl radicals from the equilibrium. A rate constant of k - 6 = 1.4 × 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 experiments, we calculate E( t -BuO 2 • / t -BuO 2 - ) = 0.71 V and E( t -BuO 2 • / t -BuO 2 H) = 1.05 V at pH 7 and 1.47 V at pH 0. The rate constants for oxidation of Fe(CN) 6 4- and other compounds by various peroxyl radicals were then utilized for the estimation of the reduction potentials of these peroxyl radicals.