Thermochemistry of phenols: buttress effects in sterically hindered phenols

Abstract

The standard (po= 0.1 MPa) molar enthalpies of formation ΔfHmo(l, or cr) at the temperature 298.15 K were measured by means of combustion calorimetry for 2,3,6-tri-methylphenol, 2,4,6-tri-methylphenol, 2,6-di-iso-propylphenol, 2-tert-butyl-6-methylphenol, 2-tert-butyl-4,6-di-methylphenol, 2,6-di-tert-butylphenol, 2,6-di-tert-buty-4-methylphenol, and 2,4,6-tri-tert-butylphenol. The standard molar enthalpies of sublimation (or vaporization) of these compounds were obtained from the temperature dependence of the vapor pressure measured in a flow system. Molar enthalpies of fusionΔcrlHmo of the solid compounds were measured by d.s.c. Resulting values ofΔfHmo (g) were obtained at the temperature 298.15 K and used to derive strain enthalpies of alkylphenols. The intramolecular interactions were discussed in terms of the enthalpic contribution of the substituents. The buttress interaction of tert-alkyl groups with hydroxyl t-Bu-OH-t-Bu was found to be 5.0 kJ · mol−1. Other buttress effects of methyl, iso-propyl-, and tert-alkyl substituents with the OH-group were found to be negligible within ±(1 to 2)kJ · mol−1. These values provided a further improvement on the group-contribution methodology for estimation of the thermodynamic properties of organic compounds.