Thermal and structural properties of ethyl 2- and 3-aminobenzoates: Experimental and computational approaches

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Highlights

Abstract

Calorimetric experiments performed for ethyl 2-aminobenzoate and ethyl 3-aminobenzoate allowed the determination of their standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K. The techniques used were static bomb combustion calorimetry and high temperature Calvet microcalorimetry, which enabled the determination of the standard molar enthalpies of formation in the liquid phase, and the standard molar enthalpies of vaporization, at T = 298.15 K, of the above aminobenzoates.

In addition, computational calculations, through the G4 composite method, were performed to estimate the enthalpies of formation in the gas phase of the title compounds. Boltzmann weighted averages were performed over sets of stable conformers of each compound, using Gibbs energy to compute population weights. The ethyl 2-aminobenzoate presents an intramolecular hydrogen bond, which was confirmed through topological analyses of the electron density.

Furthermore, the energetic effect caused from exchanging the position of the amino substituent was evaluated, and was also compared with similar compounds.

Introduction

Some of us have studied the thermochemical properties of some methyl n- and ethyl n-hydroxybenzoate derivatives in previous work [1], [2], as a part of a broad thermochemical study on benzoate derivatives, which is aimed at improving the understanding of the energetic effects of different substituents on the benzene ring. Following this line of research, the present paper describes a thermochemical study performed on two additional esters of the same family of compounds, namely the ethyl 2-aminobenzoate and ethyl 3-aminobenzoate, whose chemical structures are represented in Fig. 1. Hereafter, the ethyl 2-aminobenzoate and the ethyl 3-aminobenzoate will be denoted as E2AB and E3AB, respectively. The shorthand term “EABs” will be also used to refer to both compounds taken as a set, and “EnAB” to denote generically any of the EABs (n = 2, 3).

Other related work have been presented by Almeida et al. [3], [4], [5], [6], [7], in which the authors have determined the thermal properties of several esters derived from benzoic acid, and studied the changes on such properties as a consequence of the presence of different substituents on the base structure of methyl benzoate.

Among the interesting properties of the EABs derivatives, it is known that the E2AB (aka ethyl anthranilate) is one of the compounds that give intense flavor to Burgundy Pinot Noir wines [8]. Further applications are also associated with these compounds, for example E2AB is used as a bird repellent for crop protection [9], and E3AB is commonly used as an anaesthetic for sedating fish and amphibians [10], [11], [12], [13].

In this work, the experimental standard (p° = 0.1 MPa) molar enthalpies of formation in the liquid phase, and the standard molar enthalpies of vaporization, at T = 298.15 K, of the EABs were obtained, respectively, by static bomb combustion calorimetry and high temperature Calvet microcalorimetry. From these results, the standard molar enthalpies of formation of the EABs, in the gaseous phase, were derived.

Additionally, computational work was performed, in order to support the analysis of the experimental results. Through the Gaussian G4 method and atomization reactions, the gas-phase enthalpies of formation of the EABs were estimated. Topological analyses of the electron density of the two most-stable conformers of E2AB were also performed.

Section snippets

Materials and purity control

Ethyl 2-aminobenzoate (CAS Registry number: 87-25-2) and ethyl 3-aminobenzoate (CAS Registry number: 582-33-2) were commercial products supplied by Aldrich Co. Both liquid compounds at room temperature, were purified by fractional distillation under reduced pressure and stored under nitrogen atmosphere prior to the calorimetric studies.

Purity analyses were performed by gas chromatography (GC), using an HP 4890 instrument equipped with a HP-5 megaborecolumn and a flame ionization detector

Computational details

All the molecular calculations were performed with the Gaussian 09 software package [25] using the G4 composite method [26].

The theoretical enthalpy of formation in gas-phase of each EnAB was determined as a Boltzmann weighted average, for which we closely followed Simmie and Somers procedure [27]. Each average was performed over the Gaussian-4 (G4) enthalpies of formation of all stable conformers of the corresponding molecule. Weights, on the other hand, were determined using the Gibbs energy,

Combustion calorimetry - standard molar enthalpies of formation in the condensed phase

The specific combustion energies of the EABs were experimentally determined by combustion calorimetry. In Table 2, the individual massic energies of combustion obtained for the two isomers are presented, as well as the respective average value. The uncertainties shown in Table 2 are standard deviations of the mean (aka standard uncertainties). The combustion energies correspond to the idealized combustion reaction shown in Eq. (1), below. Further details on each combustion experiment can be

Conclusions

We have determined thermochemical properties of ethyl 2- and 3-aminobenzoates (EABs). The standard molar enthalpies of formation in condensed phase, at T = 298.15 K, were obtained through static bomb combustion calorimetry. Molar enthalpies of vaporization, on the other hand, were obtained through Calvet microcalorimetry. From these, we determined the gas-phase standard molar enthalpies of formation, at T = 298.15 K. In addition we theoretically obtained the EABs' gas-phase enthalpies of

Acknowledgements

Thanks are due to VIEP-BUAP for financial support through 100279411-VIEP2018, 100501044-VIEP2018, 100499288-VIEP2018, and 100525751-VIEP2018 projects. J. M. L. thanks CONACYT for his grant (Registration Number: 268318). The authors thankfully acknowledge computer resources, technical expertise and support provided by Laboratorio Nacional de Supercómputo del Sureste de México, which is part of the CONACYT network of national laboratories. V. L. S. F. and M. D. M. C. R. S. thank to the projects

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