Experimental and computational thermochemical study of two fluorobenzazoles: 5-fluoro-2-methylbenzoxazole and 5-fluoro-2-methylbenzothiazole

Highlights

• Combustion enthalpies for of two fluorobenzazole derivatives have been determined by rotating-bomb combustion calorimetry.

• Enthalpies of vaporization of two fluorobenzazoles have been determined by the vacuum drop microcalorimetric technique.

• Gas-phase enthalpies of formation of two fluorobenzazoles were estimated computationally (G3(MP2)//B3LYP level of theory).

• Energetic properties of 5-fluoro-2-methylbenzoxazole and of 5-fluoro-2-methylbenzothiazole were determined by calorimetry.

• Comparison experiments performed to characterize the final state of 5-fluoro-2-methylbenzothiazole combustion experiments.

Abstract

An energetic study of 5-fluoro-2-methylbenzoxazole (FMBO) and of 5-fluoro-2-methylbenzothiazole (FMBT), in condensed and gaseous states, has been performed using calorimetric techniques and computational calculations. The standard $(p^{\circ }=0.1\text{MPa})$ molar enthalpies of formation of FMBO and FMBT, in the liquid phase, at T = 298.15 K, were derived from the corresponding standard molar energies of combustion, measured by rotating-bomb combustion calorimetry. At T = 298.15 K, the standard $(p^{\circ }=0.1\text{MPa})$ molar enthalpy of vaporization, for each compound, was determined, by a direct method, using the vacuum drop microcalorimetric technique. For each compound, from this last value and from the enthalpy of formation of the liquid compounds, the corresponding standard $(p^{\circ }=0.1\text{MPa})$ enthalpy of formation in the gaseous phase has been calculated. Additionally, the gas-phase standard molar enthalpies of formation of these two compounds were estimated computationally at the G3(MP2)//B3LYP level of theory, as well as their gas-phase basicities and proton affinities.

Introduction

The present work is part of an extensive study on the thermochemical properties of 1,3-benzazole compounds, containing the pentagonal heterocyclic ring fused to a benzenic ring, in order to evaluate the energetic effect of different substituents on the rings, namely the methyl, the amine, the mercaptan, the chlorine and the fluorine groups [1], [2], [3], [4], [5], [6]. Further, we intend to analyse the energetic effect associated to different heteroatoms (nitrogen, oxygen and sulphur) in position one of the pentagonal ring for this class of molecules. Hence, we report new experimental results for the energies of combustion and the enthalpies of vaporization of two benzazole fluorinated derivatives, the 5-fluoro-2-methylbenzoxazole, FMBO, and the 5-fluoro-2-methylbenzothiazole, FMBT, (Fig. 1), measured by rotating bomb combustion calorimetry, at T = 298.15 K, and high temperature Calvet microcalorimetry, respectively. From these results, the standard (p° = 0.1 MPa) molar enthalpies of formation of both benzazole derivatives, in condensed and gaseous states, were derived. It is important to highlight that the combustion of FMBT, a compound containing fluorine and sulphur atoms, origins a final bomb solution containing a mixture of sulphuric and hydrofluoric acids whose thermodynamic parameters (e.g. enthalpies of dissolution of the gases on the solution) needed for the calculations are unknown. To overcome this difficulty, we designed comparison experiments using standard samples of benzoic acid and thianthrene, jointly with an aqueous solution of hydrofluoric acid, in adequate proportions to guarantee identical qualitative and quantitative compositions of the final states of these comparison experiments and those of the combustion reactions of FMBT [7]. Considering this procedure, corrections to the standard state are required only to the initial states, except a minor correction to the final states since the amounts of gas in the final states were slightly different.

Additionally, the gas-phase standard molar enthalpies of formation of these compounds were estimated computationally at the G3(MP2)//B3LYP level of theory, as well as their gas-phase basicities and proton affinities.

The results obtained allow correlation of the energetic parameters with the structural characteristics of the molecules, which is important to contribute for the knowledge on the reactivity of the species.