APPLICATION OF THE MARCUS THEORY ON THE REACTION OF SUBSTITUTED DIBENZOYL PEROXIDES WITH HYDROQUINONES - EVIDENCE FOR AN INNER-SPHERE ELECTRON-TRANSFER (ET) MECHANISM

The reaction of several substituted dibenzoyl peroxides 1 with methyl-substituted hydroquinones 2 gives exclusively the corresponding benzoic acids 3 and quinones 4 as the two-electron redox products. The spectrophotometrically determined rate constants of this reaction exhibit a small solvent effect, i.e. rate acceleration with increasing solvent polarity. Furthermore, it has been shown that the kinetics depend strongly on the substituents of the dibenzoyl peroxides as well as on the one-electron oxidation potentials of the hydroquinones E0(HQ./H2Q). A Hammett plot gives theta = +1.8 +/- 0.2 (r > 0.97), which indicates accumulation of negative charge on the dibenzoyl peroxide in the transition state. Good linear semilogarithmic plots of logk2 versus E0(HQ./H2Q) with slopes of 17 eV-1 were observed for this redox process. A nonlinear statistical treatment of the kinetic data against the hydroquinone one-electron oxidation potentials according to the Marcus theory affords a consistent set of one-electron reduction potentials for the substituted dibenzoyl peroxides and the reorganization energies lambda for the redox process. The E0(ROOR/RO.RO-) values range between 150 mV for the 4-NO2 to 47 mV for the 4-MeO derivatives. A well-fitting (r = 0,996) Hammett plot of E0(ROOR/RO.RO-) versus sigma values provides a slope of 100 mV per sigma unit. The reorganization energy of ca. 17 +/- 3 kcal mol-1 speaks for an inner-sphere electron transfer (ET).