Accurate Oxidation Potentials of Benzene and Biphenyl Derivatives via Electron-Transfer Equilibria and Transient Kinetics
Accurate Oxidation Potentials of Benzene and Biphenyl Derivatives via Electron-Transfer Equilibria and Transient Kinetics
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DOI：
10.1021/jo9011267
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发表时间：
2009-08-07
影响因子：
3.6
通讯作者：
Farid, Samir
中科院分区：
文献类型：
--
作者：
Merkel, Paul B.;Luo, Pu;Farid, Samir
采用纳秒瞬态吸收法测定了苯及其烷基取代衍生物在乙腈中的准确氧化电位。E-ox值是由苯和联苯衍生物对产生的自由基阳离子的平衡电子转移测量和电子转移动力学相结合得到的，对中的一个成员作为参考。采用氧化还原阶梯法，测定了21种苯和联苯衍生物的热力学氧化势。特别有趣的是，苯和甲苯的E-ox值分别为2.48 +/- 0.03和2.26 +/- 0.02 V vs SCE。由于自由基阳离子的溶剂稳定性随着烷基取代的减少而显著增加，苯的电离电位和氧化电位之间的差异比六甲基苯大0.5 eV。联苯衍生物的氧化电位与取代基sigma（+）值具有良好的相关性，这使得E-ox可以预测其他联苯衍生物。在几种情况下观察到明显的正离子自由基形成，并确定了二聚化的平衡常数。方法描述了确定准确的电子转移平衡常数，甚至当二聚体自由基阳离子形成。在三氟乙酸、二氯甲烷和乙酸乙酯中进行的额外平衡测量表明，溶剂化差异可以大大改变甚至逆转相对E-ox值。
Nanosecond transient absorption methods were used to determine accurate oxidation potentials (E-ox) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. E-ox 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, E-ox 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 similar to 0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent sigma(+) values, which allows E-ox predictions for other biphenyl derivatives. Significant dinner 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 E-ox values.