Enthalpies of Formation of N-Substituted Pyrazoles and Imidazoles

Abstract

Accurate experimental enthalpies of formation measured using static bomb combustion calorimetry, the “vacuum sublimation” drop calorimetry method, and the Knudsen-effusion method are reported for the first time for four azoles: 1-methylimidazole (1MeIMI), 1-methylpyrazole (1MePYR), 1-benzylimidazole (1BnIMI), and 1-benzylpyrazole (1BnPYR). These values and those corresponding to imidazole (1HIMI), pyrazole (1HPYR), 1-ethylimidazole (1EtIMI), 1-ethylpyrazole (1EtPYR), 1-phenylimidazole (1PhIMI), and 1-phenylpyrazole (1PhPYR) are compared with theoretical values using the G2(MP2) and the B3LYP/6-311*G(3df,2p)//6-31G(d) approaches. In general, there is a very good agreement between calculated and experimental values for the series of N-substituted imidazoles, while the agreement is less good for the series of the N-substituted pyrazoles. Experimentally, the gap between the enthalpies of formation of imidazoles and pyrazoles decreases significantly upon N-substitution, while the theoretical estimates indicate that this decrease is smaller.

Experimental standard molar enthalpies of formation of crystalline 3,5-dimethylpyrazole, 3,5-dimethyl-4-nitrosopyrazole, 1,3,5-trimethyl-4-nitrosopyrazole, and 3,5-dimethyl-1-phenyl-4-nitrosopyrazole

“…All the experimental results are reported for the first time, except the enthalpy of sublimation of 3,5-dimethylpyrazole (table 3) for which there is a previous result in the literature, (83.3 ± 0.2) kJ · mol −1 , (22) obtained from measurements of vapour pressures, at (23) 3,5-dimethylpyrazole, (102.3 ± 2.9) kJ · mol −1 , obtained in this work, N -methylpyrazole, (156.5 ± 2.1) kJ · mol −1 , (24) and N -phenylpyrazole, (291.4 ± 4.5) kJ · mol −1 , (25) we estimate the values of f H o m (Me 3 Pyr, g) = (79 ± 10) kJ · mol −1 and f H o m (Me 2 PhPyr, g) = (214 ± 10) kJ · mol −1 , where the uncertainties are increased to ±10 kJ · mol −1 . The observed f H o m (g) values for 4-nitrosopyrazole derivatives and pyrazole derivatives are listed, as well as the increment for each pair of compounds, in table 4.…”

“…Hexadecane (Aldrich Gold Label, mass fraction w > 0.99) stored under nitrogen was used as an auxiliary combustion aid in measurements of Me 2 PhPyrNO, to produce a convenient temperature rise in the calorimeter, since difficulties in purifying the compound led to very small amounts of purified samples. The standard massic energy of combustion of the hexadecane was measured,− c u o (l) = (47145.8 ± 1.3) J · g −1 , and it is in good agreement with the value of Fraser and Prosen (24) for a sample with w = 0.9996,…”

Enthalpies of combustion, heat capacities,and enthalpies of vaporisation of 1-phenylimidazole and 1-phenylpyrazole

“…According to these calculations, (23) the relative stabilisation results for the pair imidazole/pyrazole and 1-ethylimidazole/1-ethylpyrazole are, respectively, (46.9 and 38.5) kJ · mol −1 . The experimental results for the enthalpies of formation of 1-phenylimidazole and 1-phenylpyrazole obtained in the present work are consistent with this; 1-phenylimidazole being more stable by (26.7 ± 6.2) kJ · mol −1 .…”

Interplay of Thermochemistry and Structural Chemistry, the Journal (Volume 14, 2003) and the Discipline

“…Among the basic species are 1-and 2-methylimidazole [98]. The enthalpies of formation of these two species are known, as is that of the isomeric 1-methylpyrazole [99], and some other corresponding imidazoles and pyrazoles. However, the values for the remaining isomers, the 4-methylimidazole, 3-and 4-methylpyrazole seemingly remain unmeasured.…”