Entropic and enthalpic effects of 4-methoxy substitution in phenoxyl radicals

Abstract

Values of ΔH₃=(–12.4 ± 1.6) kJ mol^–1, ΔS₃=(–18.5 ± 5.6) J K^–1 mol^–1 for reaction (3)(see text), corresponding to an O–H bond energy of 322.2 kJ mol^–1 in 1, and to a 14.5 J K^–1 mol^–1 entropy loss for the CH₃O–Ar (Ar = aromatic) libration in 2 relative to 1, are derived from the temperature dependence of the equilibrium constant K₃, determined by EPR spectrometry in benzene–toluene media, between 251 and 304 K. These results allow, for the first time, discrimination between enthalpic and entropic effects on the rates of (O)H-atom abstraction by peroxyl radicals from 4-methoxyphenols and related species.