Determination of the energies of combustion and enthalpies of formation of nitrobenzenesulfonamides by rotating-bomb combustion calorimetry

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Abstract

The energies of combustion for 2-nitrobenzenesulfonamide (cr), 3-nitrobenzenesulfonamide (cr), and 4-nitrobenzenesulfonamide (cr) were determined using a recently described rotating-bomb combustion calorimeter. The condensed phase molar energies of combustion obtained were −(3479.2 ± 1.0) kJ · mol−1 for 2-nitrobenzenesulfonamide (cr), −(3454.2 ± 1.1) kJ · mol-1 for 3-nitrobenzenesulfonamide (cr), and −(3450.1 ± 1.9) kJ · mol-1 for 4-nitrobenzenesulfonamide (cr). From these combustion energy values, the standard molar enthalpies of formation in the condensed phase were obtained as: −(341.3 ± 1.3) kJ · mol−1, −(366.3 ± 1.3) kJ · mol−1, and −(370.4 ± 2.1) kJ · mol−1, respectively. Polyethene bags were used as an auxiliary material in the combustion experiments. The heat capacities and purities of the compounds were determined using a differential scanning calorimeter.

Introduction

One of the purposes of thermochemistry is to derive the enthalpy of formation of chemical compounds from their elements and to relate them to structure and chemical binding [1], [2], [3]. For several years, the main task of our research group has been the determination of the enthalpies of formation of different compounds using combustion calorimetry techniques. We then try to relate the experimental values to the respective molecular structures [4], [5], [6].

The study of the structural features of the benzenesulfonamides and its derivatives has received great attention because these compounds have a vast variety of molecular structures and because of their different applications; 2- and 4-nitrobenzenesulfonamides, readily prepared from primary amines, undergo smooth alkylation by the Mitsunobu reaction or by conventional methods to give N-alkylated sulfonamides in near quantitative yields. These sulfonamides can be readily deprotected via Meisenheimer complexes upon treatment with thiolates in DMF at room temperature, giving secondary amines in high yields [7]. In the synthetic routes to polyazapolycarboxylic acids, 2-nitrobenzenesulfonamide chemistry was used to functionalize selectivity the polyamine precursors [8].

In the present work, we report the experimental results of the determination of the energies of combustion and the molar enthalpies of formation of 2-nitrobenzenesulfonamide (cr) (2NBS), 3-nitrobenzenesulfonamide (cr) (3NBS), and 4-nitrobenzenesulfonamide (cr) (4NBS) in their condensed states. The main structural difference among the nitrobenzenesulfonamides (NBSs) studied is related to changes in the position of the nitro group on the aromatic ring. Figure 1 shows the chemical structure of NBSs studied in this work.

The combustion experiments were carried out in an isoperibolic rotating-bomb calorimeter, which has been calibrated and tested recently [9], and polyethene bags were used as an auxiliary combustion material. We present our results for the temperature, enthalpy and entropy of fusion, and the heat capacity of the compounds under study as determined by differential scanning calorimetry (DSC).

Section snippets

Experimental

All the NBSs studied were supplied by Aldrich Chemical. The reported mass fraction purity of 2NBS and 3NBS were greater than 0.99. Since the purity of 4NBS was 0.97, it was further purified by repeated recrystallization from an (acetone + chloroform) mixture. The purities, fusion temperatures, fusion enthalpies, and molar heat capacities at constant pressure were obtained by DSC using a TA Instruments differential scanning calorimeter model 2010. Previous to the melting experiments, the

Results

Table 1 contains the results from the DSC analysis: enthalpies and temperatures of fusion, molar fraction, and heat capacities of the each compound studied. Also shown are the physical properties of the compounds and materials used in the combustion experiments.

In the combustion process of the nitrobenzenesulfonamides in the presence of oxygen, an aqueous solution of sulfuric and nitric acids was obtained. Detailed results of the combustion experiments of each NBS are shown in TABLE 2, TABLE 3,

Acknowledgements

Financial support from VIEP-BUAP through the project FLSH-NAT08-G is acknowledged. E.A.C. thanks CONACyT (México) for her scholarships (registration number 165562).

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