Thermochemical study of the ethylpyridine and ethylpyrazine isomers

Organic & Biomolecular Chemistry Pub Date: 2003-10-27 DOI: 10.1039/B308097H

Abstract

The standard (p° = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of four liquids: 2-ethylpyridine, 4-ethylpyridine, ethylpyrazine and 2,3-diethylpyrazine were measured by static bomb calorimetry in an oxygen atmosphere. The values of the standard molar enthalpies of vaporization, at T = 298.15 K, were obtained by Calvet microcalorimetry, allowing the calculation of the standard molar enthalpies of formation of the compounds, in the gas phase, at T = 298.15 K: 2-ethylpyridine (79.4 ± 2.6) kJ mol?1; 4-ethylpyridine (81.0 ± 3.4) kJ mol?1; ethylpyrazine (146.9 ± 2.8) kJ mol?1; and 2,3-diethylpyrazine (80.2 ± 2.9) kJ mol?1.

The most stable geometries of all ethylpyridine and ethylpyrazine isomers were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G* and 6-311G**. These calculations were then used to obtain estimates of the enthalpies of formation of all isomers, including those not experimentally studied, through the use of isodesmic reactions. A discussion of the relationship between structure and energetics of the isomers is also presented.

Graphical abstract: Thermochemical study of the ethylpyridine and ethylpyrazine isomers
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