Entropy and enthalpy effects of 4-(phenylthio)-substituted phenols

Abstract

Electron paramagnetic monitoring of free phenoxyl radicals in benzene–toluene (1∶1) solution has been used to study the temperature dependence of the equilibrium (a), for which the equilibrium constant, K₁ is given by eqn. (b).

The fit of experimental data to eqn. (c) yields ΔH₁ = 28.80 ± 3.39 kJ mol^–1 and ΔS₁ = 116.9 ± 12.7 J K^–1 mol^–1. The entropy change value is about 30 J K^–1 mol^–1 in excess of the absolute entropy of the molecular rotor evolving about each of the C–S bonds in the thiobis(phenol) 1, thus denoting the enhancement by solute–solvent interactions. As well as using the enthalpy change to account for the stability of the generated phenoxyls, and/or the ability of hydrogen cession by phenols, the entropy change must also be measured when evaluating the antioxidant behaviour of this type of molecule, as its influence on the Gibbs free energy change is shown to be relevant.