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

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.