Thermochemistry of phenylacetic and monochlorophenylacetic acids

The standard (p∘ = 0.1 MPa) molar enthalpies of formation in the crystalline state of phenylacetic acid and ortho-, meta- and para-chlorophenylacetic acids were derived from the standard molar energies of combustion in oxygen at T = 298.15 K, measured by combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure the dependence of the vapour pressure of the solid isomers of chlorophenylacetic acid with the temperature from which the standard molar enthalpies of sublimation were derived using the Clausius–Clapeyron equation.-ΔcUm∘(cr)kJ·mol-1-ΔfHm∘(cr)kJ·mol-1ΔcrgHm∘kJ·mol-1Phenylacetic acid3887.6 ± 1.6401.3 ± 1.999.0 ± 0.62-Chlorophenylacetic acid3722.7 ± 1.1448.2 ± 1.5110.9 ± 0.53-Chlorophenylacetic acid3725.2 ± 1.1445.7 ± 1.5109.5 ± 0.64-Chlorophenylacetic acid3727.8 ± 1.0443.1 ± 1.4109.7 ± 0.6Full-size tableTable optionsView in workspaceDownload as CSVFrom these values the standard molar enthalpies in the gaseous phase, at T = 298.15 K, were derived, compared with the same parameters estimated from the Cox scheme and interpreted in terms of molecular structure.The standard (p∘ = 0.1 MPa) molar enthalpies, entropies, and Gibbs energies of sublimation, at T = 298.15 K, were derived for the three monochlorophenylacetic acids, using estimated values for the heat capacity differences between the gas and the crystal phases.Moreover, the enthalpies and the temperatures of fusion for the crystalline isomers of chlorophenylacetic acid were measured by differential scanning calorimetry.