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 Δ G ≠o (XH) were obtained using a kinetic equation [Δ G ≠ XH/Y = Δ G ≠o (XH) + Δ G ≠o (Y)]. Bond dissociation free energies Δ G o (XH) were calculated by the iBonD HM method, whose details are available at https://pka.luoszgroup.com/bde_prediction. Intrinsic resistance energies Δ G ≠ XH/X and Δ G ≠o (X) were determined as Δ G ≠o (XH) and Δ G o (XH) were available. Δ G o (XH), Δ G ≠ XH/X , Δ G ≠o (XH) and Δ G ≠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 Δ G ≠o (XH) and Δ G ≠o (X) were examined. The difference between the method of determining Δ G ≠ XH/X mentioned in this study and the dynamic nuclear magnetic method mentioned in the literature was studied. Via this study, not only Δ G o (XH), Δ G ≠ XH/X , Δ G ≠o (XH) and Δ G ≠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.