Thermodynamic properties of sublimation of the ortho and meta isomers of acetoxy and acetamido benzoic acids

Abstract

This paper reports vapour pressures measured at several different temperatures using the Knudsen effusion method of ortho-acetoxybenzoic acid (aspirin) (341.1 to 361.1)K, meta-acetoxybenzoic acid (344.2 to 362.2)K, ortho-acetamidobenzoic acid (367.2 to 389.2)K, and meta-acetamidobenzoic acid (423.2 to 441.1)K. The experimental results enabled the determination of the standard molar enthalpies, entropies and Gibbs energies of sublimation, at T=298.15K, of the four compounds studied. DSC experiments yield results of the temperature and enthalpy of fusion. The experimental results were compared with literature ones for the para isomers of the acids acetoxybenzoic and acetamidobenzoic. Correlations involving temperature of fusion, and standard molar enthalpy and Gibbs energy of sublimation of several substituted benzoic acids were proposed. Those correlation equations allow a good estimative of volatility of benzoic acid derivatives from their enthalpies of sublimation and temperatures of fusion.