Quantitative evaluation of the actual hydrogen atom donating activities of O–H bonds in phenols: structure–activity relationship

Abstract

The H-donating activity of phenol and the H-abstraction activity of phenol radicals have been extensively studied. In this article, the second-order rate constants of 25 hydrogen atom transfer (HAT) reactions between phenols and PINO and DPPH radicals in acetonitrile at 298 K were studied. Thermo-kinetic parameters ΔG^≠o(XH) were obtained using a kinetic equation [ΔG^≠_XH/Y = ΔG^≠o(XH) + ΔG^≠o(Y)]. Bond dissociation free energies ΔG^o(XH) were calculated by the iBonD HM method, whose details are available at https://pka.luoszgroup.com/bde_prediction. Intrinsic resistance energies ΔG^≠_XH/X and ΔG^≠o(X) were determined as ΔG^≠o(XH) and ΔG^o(XH) were available. ΔG^o(XH), ΔG^≠_XH/X, ΔG^≠o(XH) and ΔG^≠o(X) were used to assess the H-donating abilities of the studied phenols and the H-abstraction abilities of phenol radicals in thermodynamics, kinetics and actual HAT reactions. The effect of structures on these four parameters was discussed. The reliabilities of ΔG^≠o(XH) and ΔG^≠o(X) were examined. The difference between the method of determining ΔG^≠_XH/X mentioned in this study and the dynamic nuclear magnetic method mentioned in the literature was studied. Via this study, not only ΔG^o(XH), ΔG^≠_XH/X, ΔG^≠o(XH) and ΔG^≠o(X) of phenols could be quantitatively evaluated, but also the structure–activity relationship of phenols is clearly demonstrated. Moreover, it lays the foundation for designing and synthesizing more antioxidants and radicals.