Thermal analysis of new dimethyltin compounds with glycine and DL-valine

Enalapril maleate (EM) is an angiotensin-converting enzyme inhibitor indicated in the hypertension and congestive heart failure treatment. The thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques were used for the thermal characterization of EM. TG analysis revealed that the thermal degradation occurs in two steps, in the 150-340°C range. Each thermal degradation stage results an endothermic peak in the DSC curve, where the melting transition is also observed just before the decomposition starts. The Ozawa-Flynn-Wall isoconvertional method was used for a kinetic analysis on the thermal decomposition of EM. In the lower temperature thermal degradation step a complex pyrolysis mechanism was observed. The calculated apparent activation energy ( E a ) decreases from 200 to 80 kJ mol -1 in this event. In the second stage, for a>0.3 conversions, an E a of 100 kJ mol -1 was obtained.