VAPOR-PRESSURES, HIGH-TEMPERATURE HEAT-CAPACITIES, CRITICAL PROPERTIES, DERIVED THERMODYNAMIC FUNCTIONS, AND BARRIERS TO METHYL-GROUP ROTATION, FOR THE 6 DIMETHYLPYRIDINES

Measurements leading to the calculation of ideal-gas thermodynamic pro perties at p = p degrees = 101.325 kPa are reported for the six dimeth ylpyridines (Chemical Abstract registry numbers: 2,3-dimethylpyridine [583-61-9]; 2,4-dimethylpyridine [108-47-4]; 2,5-dimethylpyridine [589 -93-5]; 2,6-dimethylpyridine [108-48-5]; 3,4-dimethylpyridine [583-58- 4]; and 3,5-dimethylpyridine [591-22-0]). Vapor pressures were measure d for each compound by comparative ebulliometry for the pressure range 2 kPa to 270 kPa. Two-phase (liquid + vapor) heat capacities were mea sured with a differential scanning calorimeter (d.s.c.), and saturatio n heat capacities C-sat,C-m were derived. Densities for the liquid pha se of 2,3-dimethylpyridine and 2,6-dimethylpyridine were measured with a vibrating-tube densitometer. The critical temperature T-c was deter mined experimentally for each compound by d.s.c., and the critical pre ssure and critical density were derived from fitting procedures. Entha lpies of vaporization were calculated from the experimental measuremen ts. Entropies and enthalpy increments by adiabatic heat-capacity calor imetry, published recently by this research group, and literature valu es for the energy of combustion were combined with the present results to derive entropies, enthalpy increments, and Gibbs free energies of formation for the ideal gas at p = p degrees = 101.325 kPa for tempera tures between T = 250 K and T approximate to 0.95.T-c. Barriers to met hyl-group rotation for the ortho-substituted compounds are estimated a nd compared with literature values for 1,2-dimethylbenzene.