Kinetics of protonation of anthracene and phenanthrene radical anions in DMF by a series of substituted phenols. Comparison of Brønsted and Hammett plots

Abstract

The rate constants k_H for the proton transfer from a series of substituted phenols to anthracene and phenanthrene radical anions formed in DMF have been measured by the voltammetric method. The quadratic Brønsted dependencies of RT ln k_H against pK_a of phenols for both radical anions were observed. For the conjugate acid ArH˙ of the anthracene radical anion pK_a is known and the intrinsic activation barrier ΔG_o^‡ = 59 kJ mol^–1 was calculated. The homolytic bond dissociation energy D for ArH˙→Ar + H˙ was evaluated and the obtained value of D/4 = 51 kJ mol^–1 is in good agreement with the theoretical prediction ΔG_o^‡ = (D + λ_o)/4 (where λ_o is the solvent reorganization energy) based on a recent theory of the deprotonation of radical cations described as a concerted electron and H atom transfer. However, linear Hammett plots of log k_H against σ were observed for both radical anions and the lack of a quadratic effect was explained as a result of a through-resonance contribution. On the other hand, the application of normalized substituent constants σ^o resulted in the quadratic Hammett plots in accordance with the Reactivity Selectivity Principle.