Standard molar enthalpies of formation of three methyl-pyrazole derivatives

Abstract

The standard ( p ° = 0.1 MPa) molar enthalpies of formation of the crystalline 3-methyl-1-pyrazolecarboxamide; 3-methyl-3-pyrazoline-5-one; and 4-methyl-2-pyrazoline-5-one were derived from the standard massic energies of combustion, in oxygen, to yield CO 2 (g), H 2O (l) and N 2 (g), at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation were calculated from the variation of the vapour pressures of each compound with temperature, measured by the Knudsen effusion technique. These two thermodynamic parameters yielded the standard molar enthalpies of formation of the pyrazole derivatives, in the gaseous phase, at T = 298.15 K. Compound Δ c u °/(J · g −1) Δ f H m ∘ (g)/(kJ · mol −1) 3-Methyl-1-pyrazolecarboxamide (cr) −22475.4 ± 4.0 −62.8 ± 1.9 3-Methyl-3-pyrazoline-5-one (cr) −23248.4 ± 4.2 −39.4 ± 1.2 4-Methyl-2-pyrazoline-5-one (cr) −23304.5 ± 3.4 −33.5 ± 1.9 The derived standard molar enthalpies of formation, in gaseous state, are analyzed in terms of enthalpic increments and interpreted in terms of molecular structure.