Accurate Oxidation Potentials of Benzene and Biphenyl Derivatives via Electron-Transfer Equilibria and Transient Kinetics

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• 作者：Paul B. Merkel ; Pu Luo ; Joseph P. Dinnocenzo ; Samir Farid
• 刊名：Journal of Organic Chemistry
• 出版年：2009
• 出版时间：August 7, 2009
• 年：2009
• 卷：74
• 期：15
• 页码：5163-5173
• 全文大小：996K
• 年卷期：v.74,no.15(August 7, 2009)
• ISSN：1520-6904

文摘

Nanosecond transient absorption methods were used to determine accurate oxidation potentials (Eb>oxb>) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. Eb>oxb> values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, Eb>oxb> values of 2.48 ± 0.03 and 2.26 ± 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is 0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent σ⁺ values, which allows Eb>oxb> predictions for other biphenyl derivatives. Significant dimer radical cation formation was observed in several cases and equilibrium constants for dimerization were determined. Methodologies are described for determining accurate electron-transfer equilibrium constants even when dimer radical cations are formed. Additional equilibrium measurements in trifluoroacetic acid, methylene chloride, and ethyl acetate demonstrated that solvation differences can substantially alter and even reverse relative Eb>oxb> values.