Substituent effects on the benzene ring. Prediction of the thermochemical properties of alkyl substituted hydroquinones

Abstract

The standard molar enthalpies of formation Δ_fH_m^°(cr) at 298.15 K for hydroquinone, 2-methylhydroquinone, 2-tert-butylhydroquinone, 2,5-di-tert-butylhydroquinone, and 2,3-dicyanohydroquinone were measured by combustion calorimetry. The standard molar enthalpies of sublimation of 2-methylhydroquinone, 2-tert-butylhydroquinone, and 2,5-di-tert-butylhydroquinone were obtained from the temperature dependence of the vapour pressure measured in a flow system. Molar enthalpies of fusion Δ_cr^lH_m^° of the latter compounds were measured by differential scanning calorimetry (DSC). Resulting values of Δ_fH_m^°(g) were obtained at 298.15 K and were used to derive the strain enthalpies of alkylhydroquinones. The intramolecular interactions of the substituents are discussed in terms of the deviations of Δ_fH_m^°(g) from the group additivity rules. No peculiarities in the interaction energies among the alkyl and the hydroxy groups in the ortho- and meta-positions of alkylhydroquinones were detected in comparison with alkyl-substituted phenols. Thus, no new parameters are needed for the prediction of the Δ_fH_m^°(g) of similar shaped alkylhydroquinones by using the group-additive procedure.