The results of a study aimed at improvement of the group-contribution methodology for estimation of thermodynamic properties of organic substances are reported. Specific weaknesses where particular groupcontribution terms were unknown, or estimated because of lack of experimental data, are addressed by experimental studies of enthalpies of combustion in the condensed phase, vapor-pressure measurements, and differential scanning calorimetric (dsc) heat-capacity measurements. Ideal-gas enthalpies of formation of cyclohexene, phthalan (2,5-dihydrobenzo-3,4-furan), isoxazole, octylamine, dioctylamine, trioctylamine, phenyl isocyanate, and 1,4,5,6-tetrahydropyrimidine are reported. Two-phase (liquid + vapor) heat capacities were determined for phthalan, isoxazole, the three octylamines, and phenyl isocyanate. Liquidphase densities along the saturation line were measured for phthalan and isoxazole in the temperature range 298 K to 425 K. The critical temperature and critical density of octylamine were determined from the dsc results and a critical pressure derived from the fitting procedures. Fitting procedures were used to derive critical temperatures, critical pressures, and critical densities for cyclohexene (pressure and density only), phthalan, isoxazole, dioctylamine, and phenyl isocyanate. Group-additivity parameters or ring-correction terms useful in the application of the Benson group-contribution correlations are derived.

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This report describes the results obtained in the fifth year of a project whose goal is the improvement of group-contribution methodology for esti'nation of thermodynamic data. It has long been recognized that it is impractical to carry out experimental measurements of the thermodynamic properties of the multitude of organic substances that are of importance in an area such as the processing of alternate fuel sources, or for those chemicals which are of major importance in modern industry. Experimental measurements on a restricted number of carefully selected molecules can be used to relate thermodynamic properties to molecular structure and serve as the foundation for estimation of properties for great numbers of related molecules. The most successful methodology used for this structure/property correlation is the "group-contribution" approach as applied by numerous researchers. The empirical nature of this approach requires data of unassailable quality for the foundation of the correlation. The need for such high-quality data is the basis for this project. Representatives of the Department of Energy (Bartlesville Project Office), the Design Institute for Physical Property Data (DIPPR), and the National Institute for Petroleum and Energy Research (NIPER) agreed on a list of compounds for which the determination of the enthalpy of formation in the ideal-gas state would be of benefit to all the participants. Nine compounds were selected for study in the fifth year of the project, and the ideal-gas enthalpies of formation of cyclohexene, phthalan (2,5-dihydrobenzo-3,4-furan), isoxazole, phenyl isocyanate, 1,4,5,6-tetrahydropyrimidine, n-octylamine, din -octylamine, and tri-n-octylamine are reported here. Liquid-phase densities along the saturation line were measured for phthalan and isoxazole in the temperature range 298 K to 523 K. Values for the group-additivity parameters N-(C)(H)2, N-(C)2(H), and N-(C)3, were confirmed. A value for the N-(Cb)(CO) group-additivity parameter was defined. Ring correction terms were derived for cyclohexene and phthalan ring systems. An estimate for the ideal-gas enthalpy of formation of 2,3-dihydrofuran was derived. Also, the results of isoxazole and 1,4,5,6-tetrahydropyrimidine were used to estimate ideal gas enthalpies of formation for substituted compounds. The results reported continue to establish the validity of the group-contribution approach for the accurate estimation of the enthalpies of formation of organic compounds. They will extend the range of applicability of group-contribution methods open to the chemical engineer concerned with estimation of the energy hazard potential of new chemical processes. Pcs = density calculated using extended corresponding states Ig = IOglo = acentric factor = [-Ig (Px/Pc)-1.000] where Px is the vapor pressure at Tr = 0.700. To avoid listing units in the heat-capacity measurement tables, the heat capacities are reported as divided by the gas constant R. Units of time are s (seconds) or h (hours).

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