O–H bond strengths and one-electron reduction potentials of multisubstituted phenols and phenoxyl radicals. Predictions using free energy relationships

Abstract

General equations for the empirical determination of relative O–H bond strengths and one-electron reduction potentials of multisubstituted phenols and phenoxyl radicals are derived. The derivations are based on data from direct and indirect measurements of phenolic O–H bond strengths, and one-electron reduction potential measurements using electrochemical and radiation chemical methods. The resulting equations are ΔD(O–H)=–1.97 + 29.87 ∑σ⁺(kJ mol)^–1 and E°= 0.79 + 0.34 ∑σ⁺(V vs. NHE) where ∑σ⁺ is the sum of the Brown substituent values (σ_o ⁺= 0.66 σ_p ⁺, σ_m ⁺ and σ_p ⁺). The conditional relationship σ_o ⁺= 0.66 σ_p ⁺ is also derived in this work.