Thermodynamics Research Center / ThermoML | Thermochimica Acta

ABSTRACT

This work describes the synthesis of new 4-substituted-pyrazolo[3,4-d]pyridazinones (R = C6H5, 4-F-C6H4, benzofur-2-yl, CF3, 4-NO2-C6H4) from the reaction of dicarbonylpyrazoles with hydrazine hydrate.Solid and solution-state NMR were used to unequivocally assign the structure and identification of thebyproduct formed when R = 4-NO2-C6H4. Geometrical features and the intermolecular interactions ofcompounds were described using single crystal X-ray diffraction. The thermal behavior of the compoundswas studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Kineticparameters, such as the activation energy (Ea) and pre-exponential factor (A), were determined as afunction of the conversion degree for the thermal decomposition of two compounds, by using the iso-conversional methods of Friedman and Ozawa Flynn Wall (OFW). The results showed that the Eaforthermal decomposition of compound with R = CF3is always higher than for compound with R = C6H5,independent of the model used (Friedman or OFW), indicating that the former is more stable than thelatter. Furthermore, the Eavalues for compounds with R = C6H5and CF3are practically constant for all ^2values from 0.1 to 0.9, indicating the existence of a single-step degradation reaction. The kinetics resultsalso reveal the linear dependence of Eaon the pre-exponential factor. In addition, the 4-substituted-pyrazolo[3,4-d]pyridazinones studied have two types of thermal behavior: (i) a compound that passesfrom the glass to a liquid phase, followed by cold crystallization, and a melting transition (R = C6H5, CF3);(ii) A compound that has no melting or freezing points, only glass-transition temperatures (R = 4-NO2-C6H4). Finally, we showed that compounds with R = C6H5and CF3undergo amorphization during theheating cycles, reaching 48% and 74% of amorphous content, respectively.