Thermochemical study of chloropyrazines and chloroquinoxalines

Abstract

The standard ( p ∘=0.1 MPa) molar enthalpies of formation for liquid 2-chloropyrazine and crystalline 2,6-dichloropyrazine, 2,3-dichloroquinoxaline and 2,3,6,7-tetrachloroquinoxaline were derived from the standard molar enthalpies of combustion, in oxygen, to yield CO 2(g), N 2(g), and HCl · 600H 2O(l), at the temperature T=298.15 K, measured by rotating-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation, at T=298.15 K, were measured by Calvet microcalorimetry. − Δ c H m ∘ (cr, l)/(kJ · mol −1) Δ cr, l g H m ∘ /(kJ · mol −1) 2-Chloropyrazine, 1 (2-ClPyz) 2149.4 ± 1.0 45.1 ± 1.5 2,6-Dichloropyrazine, cr (2,6-DClPyz) 1981.6 ± 1.7 69.9 ± 2.0 2,3-Dichloroquinoxaline, cr (2,3-DClQ) 3878.1 ± 1.3 91.8 ± 1.1 2,3,6,7-Tetrachloroquinoxaline, cr (2,3,6,7-TClQ) 3564.1 ± 1.3 105.9 ± 2.2 Density functional theory with the B3LYP functional and two different basis sets, 6-31G* and 6-311G**, was used to optimize the geometries of all chloro-substituted pyrazines and some chloro-substituted quinoxalines. The calculation of the energy of isodesmic reactions allowed the estimation of the standard molar enthalpies of formation in the gas phase for all compounds including some not studied experimentally.