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

RSC Advances Pub Date: 2023-01-23 DOI: 10.1039/D2RA06877J

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 ΔGo(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 ΔGo(XH) were available. ΔGo(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 ΔGo(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.

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