Thermal properties, thermochemistry and kinetics of the thermal dissociation of hydrochlorides of aromatic mono-amines

Abstract

Thermal properties of hydrochlorides of several aromatic mono-amines were examined by thermoanalytical methods (TG, DTG and DTA). The majority of hydrochlorides studied undergo decomposition upon heating with the simultaneous release of HC1 and amines to the gaseous phase. The thermal dissociation of these derivatives proceeds essentially in one step, in which up to 88% of the sample volatilizes, followed by a slow step, seen as a “tail” in TG curves, due to the decreased geometric surface area of the condensed phase. This thermal behaviour is also typical of hydrochlorides of other nitrogen organic bases and can be qualitatively accounted for by the Jacobs and Russell-Jones model for the kinetics of dissociative sublimation processes. Some of the compounds studied, however, decompose in two distinct stages. The first step corresponds to the release of HCl while the second one is the volatilization of free amine. The latter thermal decomposition pattern is characteristic of compounds exhibiting the complex structure of an amminium cation. The thermodynamics and kinetics of thermal dissociation were examined using non-isothermal TG curves. The enthalpies of volatilization were evaluated from the Van't Hoff equation. These derived values, together with information from the literature, were used to estimate the enthalpy of formation and crystal lattice energy of some of the salts. The “thermochemical” radii for amminium ions were also evaluated from the Kapustinsku-Yatsimirskii equation. The Jacobs and Russell-Jones approach was applied to describe the kinetics of volatilization. However, standard phonomenological procedures do not seem to be appropriate for examining the kinetics of dissociative volatilization processes.