Thermal properties and thermochemistry of alkanaminium bromides

Abstract

The thermal behaviour of unbranched compounds of the general formula [(C n H 2 n+1 ) p N-H 4− p ]Br, with n = 0 to 4 and p = 1 to 4, was studied by thermoanalytical methods (DTA, TG and DTG). All the compounds examined undergo decomposition upon heating, leading to their total volatilization. In the case of primary, secondary and tertiary amine hydrobromides, the thermal-dissociation process is accompanied by the release of the appropriate amines and HBr to the gaseous phase. Thermogravimetric curves for these derivatives indicate that the process comprises two stages. In both steps thermal dissociation proceeds via the same chemical mechanism; however, each step is determined by different kinetics. Quaternary salts decompose in only one step which is accompanied by the release of the appropriate tertiary amines and bromoalkanes to the gaseous phase. The latter process requires that the remarkable activation barrier be overcome in addition to that resulting from the thermodynamic requirements. On the other hand, the dissociative volatilization of the former derivatives proceeds essentially without any additional barrier over that imposed by the enthalpy change for the reaction. The enthalpies of the thermal dissociation of hydrobromides were evaluated from the non-isothermal thermogravimetric curves, and these values, together with the thermochemical data available in the literature, were used to evaluate the enthalpies of formation and the crystal-lattice energies of the compounds. The crystal-lattice energy was also examined within the Kapustinskii-Yatsimirskii approach, which assumes an additive character of this quantity. The essential thermal and thermochemical characteristics, as well as the influence of the structure of amines on the thermal behaviour of alkanaminium bromides are also reviewed and discussed.