The influence of the halogen size in the volatility and melting of methyl p-halobenzoic esters and of their parent acids

The vapour pressures of the condensed phases of the methyl esters of p-halobenzoic acids were measured over wide temperature ranges. The results enabled the determination of the standard molar entropies and enthalpies of sublimation and of vapourisation, at T = 298.15 K, of these four compounds. The temperatures and molar enthalpies of fusion were determined using differential scanning calorimetry and were compared with the values derived from the vapour pressure measurements. Temperature and enthalpy of fusion of p-chlorobenzoic acid were also measured using DSC. The enthalpies of the intermolecular hydrogen bonds O–H⋯O formed in the crystals of the parent halobenzoic acids were calculated. From the available results for these compounds and from the ones determined in this work, several correlations involving thermodynamic properties of sublimation and fusion as well as the volume of the halogen substituent were derived.

► Vapour pressures of the condensed phases of methyl p-halobenzoates were measured. ► Enthalpies and entropies of sublimation and vapourisation were derived. ► Temperature and enthalpy of fusion were determined indirectly and using DSC. ► Enthalpies of intermolecular OH−O hydrogen bonds of the parent acids were determined. ► Correlations involving thermodynamic properties and halogen’s volume are presented.