Thermal degradation of N-(o-carboxybenzoyl)-l-amino acids

The thermal degradation of N-(o-carboxybenzoyl)-l-phenylalanine (NCBPh), N-(o-carboxybenzoyl)-l-leucine (NCBL) and N-(o-carboxybenzoyl)-l-valine (NCBV) were studied by thermogravimetry and infrared spectroscopy. The results suggested that the loss of mass occurred by two main events which were defined as the imide formation and sublimation. The values of the activation energy (E) for the imide formation, considering both systems, were at the range 170–60 kJ mol−1 and the results indicate that thermal stability obeys the order: NCBPh>NCBV>NCBL. The apparent activation energies obtained for the sublimation process are consistent with the structure of the imides N-Phthaloylphenylalanine (PhtPh), N-phthaloylvaline (PhtV) and N-phthaloylleucine (PhtL). The values of E are in the order PhtPh>PhtV>PhtL. This higher energy for the sublimation of the phenylalanine derivative, may be associated to the presence of the phenyl group, which favours the crystalline packing. In a sealed tube at higher temperature, we detected the degradation of the imide, with the concurrent appearance of CO2 (2370 and 2340 cm−1), CO (2200–2100 cm−1).