Experimental thermochemical study of 3-acetyl-2-methyl-5-phenylthiophene

The standard (p∘=0.1MPa) massic energy of combustion, in oxygen, of the crystalline 3-acetyl-2-methyl-5-phenylthiophene was measured, at T = 298.15 K, by rotating-bomb combustion calorimetry, from which the standard molar enthalpy of formation, in the condensed phase, was calculated as ΔfHm∘(cr)=-(104.3±3.1)kJ·mol-1. The corresponding standard molar enthalpy of sublimation, at T = 298.15 K, ΔcrgHm∘=(108.9±0.4)kJ·mol-1, was derived by the Clausius–Clapeyron equation, from the temperature dependence of the vapour pressures of this compound, measured by the Knudsen effusion mass-loss technique.From the results presented above, the standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, was derived, ΔfHm∘(g)=(4.6±3.1)kJ·mol-1. This value, in conjunction with the literature values of the experimental enthalpies of formation of thiophene, 2-methylthiophene, and 3-acetylthiophene, was used to predict the enthalpic increment due to the introduction of a phenyl group in the position 2- of the thiophene ring. The calculated increment was compared with the corresponding ones in benzene and pyridine derivatives.