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Application of the Marcus Theory on the Reaction of Substituted Dibenzoyl Peroxides with Hydroquinones: Evidence for an Inner‐Sphere Electron Transfer (ET) Mechanism
Author(s) -
Adam Waldemar,
Schönberger André
Publication year - 1992
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19921250925
Subject(s) - chemistry , marcus theory , hammett equation , redox , electron transfer , reaction rate constant , hydroquinone , inner sphere electron transfer , reactivity (psychology) , medicinal chemistry , kinetics , photochemistry , inorganic chemistry , organic chemistry , ion , medicine , physics , alternative medicine , pathology , quantum mechanics
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 oneelectron oxidation potentials of the hydroquinones E 0 (HQ . /H 2 Q). A Hammett plot gives ϱ = +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 log k 2 versus E 0 (HQ . /H 2 Q) 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 λ for the redox process. The E 0 (ROOR/RO . RO ‐ ) values range between 150 mV for the 4‐NO 2 to 47 mV for the 4‐MeO derivatives. A well‐fitting ( r = 0.996) Hammett plot of E 0 (ROOR/RO . RO ‐ ) versus s̀ values provides a slope of 100 mV per s̀ unit. The reorganization energy of ca. 17 ± 3 kcal mol ‐1 speaks for an inner‐sphere electron transfer (ET).