Thermal and kinetics studies of primary, secondary and tertiary alkanolammonium salts of 4-nitrobenzoic acid

Abstract

Nitrobenzoic derivatives are chemically and biologically significant molecules, recently listed as active ingredients in the medical-pharmaceutic field. A series of p-nitrobenzoic acid salts were synthesized with different substituted alkanolamine (ethanolamine, diethanolamine and triethanolamine) via proton exchange reactions and characterized. Fourier transform infrared spectroscopy—FTIR-UATR, and a combination of thermal techniques (differential scanning calorimetry—DSC, and thermogravimetric analysis—TGA) with hot-stage microscopy were used in order to demonstrate the formation of salts and to analyse thermal stability and phase transitions. The aim of this study is to investigate thermal behaviour and kinetics of this class of compounds, previously poorly examined, which offers interesting phase transformations in the solid state. DSC indicated that the synthesized salts had very distinct melting points. Diethanolamine and triethanolamine used as cation in the formation of multicomponent systems with 4-nitrobenzoic acid lead to melting points near 100 °C, compared to compound based on ethanolamine. Calorimetric and thermogravimetric data indicate the absence of solvate forms in all studied compounds. TGA and kinetic experiments allowed the calculation of the activation energy, revealing that triethanolammonium salt has the highest stability in this studied series of compounds.