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 ∆G o ‡ = 59 kJ mol Ϫ1 was calculated. The homolytic bond dissociation energy D for ArH ؒ →Ar ϩ H ؒ was evaluated and the obtained value of D/4 = 51 kJ mol Ϫ1 is in good agreement with the theoretical prediction ∆G o ‡ = (D ϩ 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.

Kinetics of protonation of anthracene radical anions in DMSO by a series of substituted phenols

“…Neglecting the homoconjugation reaction (5) in data treatment means 3-5 that the lowering of the phenol concentration at the reaction layer is not taken into account and thus the bimolecular rate constants k H obtained in this work are underestimated as in the previous work in DMF. 10 Indeed, the rate constants obtained are slightly lower than those reported by Nielsen and Hammerich 6,7 for similar conditions but taking into account the effect of homoconjugation: for phenol and 4-methylphenol the values obtained by us are logk H ¼ 3.15 and 3.0, respectively, whereas the literature values data are logk H ¼ 3.39 or 3.53 for phenol 6,7 and 3.18 for 4-methylphenol. 7 Hence the rate constants determined here, although less accurate, appear reliable for further analysis.…”

“…The DISP1 mechanism has been found for the protonation by phenol and substituted phenols in a number of aprotic solvents. 1,6,7,10 The voltammetric characteristics of the DISP1 and ECE mechanisms are similar 1 and fortunately the k H values can be determined in the same manner for both cases. The DISP1-ECE mechanism was supported for each system by the values of a potential shift with the scan rate close to the theoretical value of @E p /@logv ¼ À29.6 mV per log unit and small deviations predicted recently 3,5 after taking into account reaction (5) were neglected.…”

“…leading to the ECE mechanism. 1,10,11 Moreover, the parallel hydrogen-bonding equilibria, first of all homoconjugation between the proton donor and its anion:…”

“…the barrier for the thermodynamic driving force ÁG 0 ¼ 0. Our recent comparison of experimental ÁG 0 z values and estimated D values indicated that protonation in DMF of anthracene radical anions, forming the C-H acid, occurs really as the concerted electron and H atom transfer, 10 whereas for acridine radical anions, forming the 'normal' acid, 15 i.e. N-H acid, the process should be considered as the real proton transfer, 16 with the value of 0.25 D substantially higher than the experimental ÁG 0 z value.…”

Rate and Driving Force for Protonation of Aryl Radical Anions in Ethanol

“…Absolute ethanol (AAPER Alcohol and Chemical Co) was used as received in most experiments. For the experiments with terphenyl, pyrene, and biphenyl, in which protonations of the anion radicals were slow, ethanol was purified by distillation from CaH 2 and then from NaBH 4 . The solvent quality was judged from the solvated electron half-life; it was 2.5 and 5.5 µs in commercial and purified ethanol, respectively; the latter compares well with the literature value of 6 µs.…”