ELECTRON-TRANSFER REACTIONS OF ALKYL PEROXY-RADICALS

One-electron-transfer reactions of alkyl peroxy radicals were studied by pulse radiolysis of aqueous solutions. At pH 13, the methyl peroxy radical was found to rapidly, k = 1 X 10(5)-4.9 X 10(7) M-1 s-1, and quantitatively oxidize various organic substrates with E13 = 0.13-0.76 V vs NHE. On the other hand, this radical was unreactive with compounds with E13 greater-than-or-equal-to 0.85 V. Consequently, E13 of the methyl peroxy radical is higher than 0.76 V and lower than 0.85 V, which means that E7 is in the range 1.02-1.11 V. At pH 8, the rate constants of the oxidation of four ferrocene derivatives by the alkyl peroxy radicals ranged from 7.1 X 10(4) M-1 s-1 for ferrocenedicarboxylate (E8 = 0.66 V) to 2.3 X 10(6) M-1 s-1 for (hydroxymethyl) ferrocene (E8 = 0.42 V). These rate constants were used to evaluate the reduction potential and self-exchange rate of alkyl peroxy radicals in neutral media from the Marcus equation. The calculated E7 = 1.05 V is in excellent agreement with the estimated E7 = 1.02-1.11 V and with one of the previously published values E7 = 1.0 V, but the value is considerably higher than the other E7 approximately 0.6 V. It is suggested that the high reorganization energy, lambda = 72 kcal mol-1, of the peroxide redox couple originates from the requirement for solvent reorganization due to the solvation of hydroperoxide anion in the transition state. In support of this are the activation parameters of the reaction of the methyl peroxy radical with uric acid. The activation entropy is 9 eu lower at pH 7.3 than it is at pH 13.2, whereas the activation enthalpies are unchanged. The importance of entropy control was verified in the reactions of cyclohexyl peroxy radicals with alpha- and delta-tocopherol in aerated cyclohexane (DELTAH+ almost-equal-to 0 kcal/mol, and DELTAS+ = -25 and -26 eu). The implications of these findings on the inactivation of alkyl peroxy radicals in general are discussed.