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

Abstract

Values of ΔH_3=(–12.4 ± 1.6) kJ mol^(–1), ΔS_3=(–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 CH_3O–Ar (Ar = aromatic) libration in 2 relative to 1, are derived from the temperature dependence of the equilibrium constant K_3, 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.

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