Standard molar enthalpies of formation and of sublimation of 2-thiophenecarboxamide and 2-thiopheneacetamide

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Abstract

The standard (p = 0.1 MPa) molar enthalpies of formation of crystalline 2-thiophenecarboxamide and 2-thiopheneacetamide were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g), N2(g) and H2SO4 · 115 H2O(aq), measured by rotating-bomb combustion calorimetry, at T = 298.15 K.

The vapour pressures of these two compounds were measured as functions of temperature by Knudsen-effusion mass loss technique; from the Clausius–Clapeyron equation the standard molar enthalpies of sublimation, at T = 298.15 K, were derived.

The results were as follows:

Compounds-ΔcUm(cr)-ΔfHm(cr)ΔcrgHm
kJ·mol-1kJ·mol-1kJ·mol-1
2-Thiophenecarboxamide3108.7 ± 1.1171.1 ± 1.2107.1 ± 0.4
2-Thiopheneacetamide3760.7 ± 1.6197.2 ± 1.8109.2 ± 0.5

From these values, the standard molar enthalpies of formation of both compounds in their gaseous phase were derived and interpreted in terms of enthalpic increments and molecular structure.

Introduction

Thiophene derivatives, five-membered aromatic sulfur heterocyclic compounds, are important structural fragments that have been incorporated into new pharmaceutical and chemical compounds. They are present in anti-inflammatory [1], [2], [3], anti-viral [4], antibacterial [5], antifungal [6] and anti-cancer [7] drugs. This class of compounds is present in some natural sources like petroleum, coal, and other fossil fuels [8]; thiophenes are removed from the byproducts of petroleum distillation through a chemical process called hydrodesulfurization, over heterogeneous catalysts [9], [10]. They can also be found in some plants, showing biological properties [11], [12]. There has been a great interest in the study of these chemical entities since they are the precursors for oligo- and polythiophenes, an important representative class of conducting conjugated polymers that exhibit a wide range of potential applications. They are used in light emitting diodes (LED’s) [13], field-effect transistors (FET’s) [14], thin-film transistors (TFT’s) [15], sensors [16], batteries [17], in the detection of biological [18] and genetic material [19], etc.

In spite of their importance, available thermochemical data for the family of these organosulfur compounds are still limited. We have been involved in the systematic study of thiophene derivatives, thus contributing to a better understanding of the energetic effects caused by the introduction of different substituent groups in the thiophene ring. Recently, we have reported some thermochemical data for thiophene derivatives, namely, 2- and 3-n-alkylthiophenes [20], 2- and 3-thiophene acetic acid, methyl esters [21], 2- and 3-acetylthiophenes [22], and some substituted thiophenecarbonitrile derivatives [23]. The scope of the present work lies in the experimental thermochemical study of 2-thiophenecarboxamide and 2-thiopheneacetamide, whose structural formulae are represented in figure 1.

The standard (p = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, of the title compounds were derived from the standard molar energies of combustion measured by rotating-bomb combustion calorimetry. The Knudsen-effusion mass loss technique allowed the determination of the standard molar enthalpies of sublimation, at T = 298.15 K, through the application of the Clausius–Clapeyron equation.

Section snippets

Compounds and purity control

The 2-thiophenecarboxamide [CAS 5813-89-8] was obtained commercially from Sigma-Aldrich Chemical Co., with a massic purity of 0.987 (mass fraction) and 2-thiopheneacetamide [CAS 4461-29-4] was purchased from Lancaster Synthesis Ltd., with a massic purity of 0.98 (mass fraction). These two compounds are crystals, at room temperature, and have been purified by successive sublimations under reduced pressure. The purity of each one was checked by g.l.c. and differential scanning calorimetry, being

Results

Detailed results of the combustion experiments of both compounds are given in TABLE 1, TABLE 2: Δm(H2O) is the deviation of the mass of water added to the calorimeter from 3965.0 g, the mass assigned to ε(calor), ΔUΣ is the correction to the standard state and the other symbols have been previously defined [33], [40].

The values of the internal energy associated with the isothermal bomb process, ΔU(IBP), were calculated according to equation (2):ΔU(IBP)=-{ε(calor)+cp(H2O,l)·Δm(H2O)}ΔTad+(Ti-298.15

Discussion

The enthalpic increment for the entrance of a –CONH2 group into the position 2 of the thiophene ring is −(179.0 ± 1.6) kJ · mol−1, as seen in scheme 1, calculated from the literature value of the standard molar enthalpy of formation, in the gaseous phase, of thiophene, ΔfHm(C4H4S, g) = (115.0 ± 1.0) kJ · mol−1 [51], and the value of the standard molar enthalpy of formation, in the gaseous phase, of 2-thiophenecarboxamide, reported in this paper. The same kind of analysis can be made for the analogue

Acknowledgements

Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro de Investigação em Química, University of Porto. A.F.L.O.M.S. thanks FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of a PhD Research Grant (SFRH/BD/12886/2003).

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