Determination of the ortho-, para-, and meta-Interactions in secondary-Alkylphenols from Thermochemical Measurements

The standard molar enthalpies of formation Δ_fH⁰_m, (1 or cr) at the temperature 298.15 K were measured by using combustion calorimetry for 4-ethylphenol, 2-iso-propylphenol, 2-iso-propyl-5-methylphenol, 2-sec-butyl-phenol, 4-iso-propylphenol, 4-sec-butylpnenol, and 4-cyclo-hexylphenol. The enthalpy of formation Δ_fH⁰_m, (1) of 3-iso-propylphenol was obtained from the results of calorimetric and equilibrium studies. The standard molar enthalpies of sublimation (or vaporization) of these compounds, and also of phenol, were obtained from the temperature dependence of the vapour pressure measured in a flow system. Molar enthalpies of fusion Δ¹_crH⁰_m of the solid compounds were measured by d.s.c. Resulting values of Δ_fH⁰_m (g) were obtained at the temperature 298.15 K and used to derive strain enthalpies of alkylphenols. The intra-molecular interactions of the substituents were discussed in terms of deviations of Δ_fH⁰_m (g) from the group additivity rules. Ortho-, para-, and meta-interactions of sec-alkyl substituent with OH-group in the gaseous phase was found to be equal 3.9 kJ·mol⁻¹. These values provided a further improvement on the group-contribution methodology for estimation of thermodynamic properties of organic compounds.