Thermochemical properties of three 2-thiophenecarboxylic acid derivatives

The standard (p∘ = 0.1 MPa) molar enthalpies of formation, in the crystalline state, of the three thiophenecarboxylic acid derivatives, at T = 298.15 K, were derived from the standard massic energies of combustion, measured by rotating-bomb combustion calorimetry. The Knudsen mass loss effusion method was applied to determine the temperature–vapour pressure dependence, from which the standard molar enthalpies of sublimation, at T = 298.15 K, were derived by fitting data to the Clausius–Clapeyron equation. The values obtained were: Compounds - Δ f H m ∘ (cr)/(kJ · mol−1) Δ cr g H m ∘ /(kJ · mol−1) 3-Methyl-2-thiophenecarboxylic acid 393.6 ± 1.7 98.0 ± 0.4 5-Methyl-2-thiophenecarboxylic acid 395.7 ± 1.6 101.9 ± 0.3 5-Acetyl-2-thiophenecarboxylic acid 547.8 ± 1.8 123.5 ± 0.6 The standard (p∘ = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, were derived from the experimental results and the enthalpy of isomerisation between 3-methyl- and 5-methyl-2-thiophenenecarboxylic acids was calculated. Moreover, the results are compared with the analogue benzene derivatives and discussed in terms of molecular structure.