Substituent Effects on the Benzene Ring. Determination of the Intramolecular Interactions of Substituents in tert-Alkyl-Substituted Catechols from Thermochemical Measurements

Chemical equilibria of reactions of transalkylation among tert -butylphenols and tert -butylcatechols in the liquid phase were investigated in the temperature range 373−483 K. The molar enthalpies of fusion of the catechol, 4- tert -butylcatechol, and 3,5-di- tert -butylcatechol were measured by DSC. The standard ( p ° = 0.1 MPa) molar enthalpies of formation Δ f (cr) at the temperature T = 298.15 K were measured by means of combustion calorimetry for 4- tert -butylcatechol and 3,5-di- tert -butylcatechol. The standard molar enthalpies of sublimation of these compounds, and also the enthalpy of vaporization of 3- tert -butylcatechol, were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. The measured reaction enthalpies were utilized for reconciliation of calorimetrically derived standard molar enthalpies of formation of tert -butyl-substituted phenols and catechols, as a stringent test of thermodynamic consistency of results derived from the diverse techniques employed in this work. Resulting values of Δ f (g) of tert -butylcatechols were obtained at the temperature T = 298.15 K and used to derive their strain enthalpies. The intramolecular interactions of the substituents were discussed in terms of deviations of Δ f (g) from the group additivity rules. No peculiarities in the interaction energy among alkyl groups and the hydroxyl groups in the ortho-, para-, and meta-positions of alkylcatechols in comparison with those of alkyl-substituted phenols were detected. Thus, no new parameters are needed for the prediction of the Δ f (g) values of alkylcatechols by using the group-additive procedure.