Thermodynamic study of phase transitions in methyl esters of ortho- meta- and para-aminobenzoic acids

A static method based on capacitance gauges was used to measure the vapor pressures of the condensed phases of the methyl esters of the three aminobenzoic acids. For methyl o-aminobenzoate the vapor pressures of the liquid phase were measured in the range (285.4 to 369.5) K. For the meta and para isomers vapor pressures of both crystalline and liquid phases were measured in the ranges (308.9 to 376.6) K, and (332.9 to 428.0) K, respectively. Vapor pressures of the latter compound were also measured using the Knudsen effusion method in the temperature range (319.1 to 341.2) K.From the dependence of the vapor pressures on the temperature, the standard molar enthalpies and entropies of sublimation and of vaporization were derived. Differential scanning calorimetry was used to measure the temperatures and molar enthalpies of fusion of the three isomers. The results enabled the estimation of the enthalpy of the intermolecular (N−H…O) hydrogen bond in the crystalline methyl p-aminobenzoate. A correlation relating the temperature of fusion and the enthalpy and Gibbs energy of sublimation of benzene, methyl benzoates and benzoic acids was derived.

► Vapor pressures of liquid and crystalline phases of methyl esters of the aminobenzoic acids were measured. ► Accurate values of enthalpies of sublimation, vaporization, and fusion were derived. ► The enthalpy of intermolecular NH–O hydrogen bonds in methyl p-aminobenzoate was determined. ► The volatility of the methyl benzoates was compared with the volatility of the parent acids.