Thermochemical studies of 3-methylpyrazole and 1,3,5-trimethylpyrazole
Introduction
The wide range of applications of the pyrazole type of compounds explains the great interest in the study of their properties. Pyrazole derivatives can be used as anti-cancer [1] and cytological [2] agents, they were shown to have anti-bacterial [3] and anti inflammatory [4] properties and they can be found in various pharmacological products, and also in herbicides, pesticides [5], dyes and explosives [6]. Prevention of poisoning by methanol and of damaging effects of ethanol is a potential application of pyrazole derivates because they are potent inhibitors of the ethanol metabolism [7]. These compounds are also used as substrates in the generation of laser emission [8], can act as photo-protectors [9] and play an important role as ligands in coordination chemistry [10], [11] and as bifunctional catalysts [12].
The first studies on thermochemical studies of pyrazole derivatives were published by Bedford and Edmonson, in 1962, when they calculated the standard molar enthalpy of combustion of pyrazole, , and the respective standard molar enthalpy of formation in gaseous phase, [13]. Later, this value was recalculated by Cox and Pilcher, and they suggested the value of for the standard molar enthalpy of combustion [14]. More recently Jiménez et al. [15] published the value for the standard molar enthalpy of formation, in gaseous phase, of pyrazole. Since then, other pyrazoles have been subject of study. Our research group has already been interested for several years in this of type compound and measured the standard molar enthalpies of formation, in the gaseous phase [16], of 1-methyl, 1-benzyl, and 1-ethylpyrazole, respectively as and and lately determined the standard molar enthalpies of formation of 1-phenylpyrazole [17], and of 3,5-dimethylpyrazole [18], .
Section snippets
Compounds
The compounds under study (figure 1), 3-methylpyrazole (A), [CAS RN 1453-58-3] and 1,3,5-trimethylpyrazole (B) [CAS RN 1072-91-9] were all supplied by Aldrich Chemical, both with nominal purity of 0.97 mass fraction. The liquid 3-methylpyrazole was purified by successive distillations and the solid 1,3,5-trimethylpyrazole by repeated sublimations under reduced pressure. The final purity of the compounds was determined recovering the carbon dioxide produced in the combustion experiments and also
Experimental results
TABLE 1, TABLE 2 list the details of all combustion calorimetry experiments of each compound, where is the deviation of the mass of water added to the calorimeter from 3119.6 g and is the energy change for the isothermal combustion reaction under bomb conditions, with is the calorimeter temperature change corrected for the heat exchange and the work of stirring, is the correction to the standard state and the remaining
Discussion
The standard molar enthalpies of formation obtained in this work, together with the literature values of [15], [16] and [18] are used to calculate the enthalpic increments for the introduction of methyl groups in different positions of the pyrazole heterocycle.
As it shown in Scheme 1, the enthalpic increment due to the introduction of a methyl group in position 3
Acknowledgements
Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro de Investigação em Quı´mica, University of Porto. J.I.T.A.C. thanks FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for the award of a Post-Doc research grant (BPD/27140/2006).
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