Thermal properties of amine hydrochlorides: Part II. Thermolysis and thermochemistry of alkanaminium chlorides

Abstract

Thermoanalytical methods (DTA, TG, and DTG) have been applied to the investigation of the thermal properties of unbranched compounds of general formula [(C n H 2 n+1 ) p NH 4- p ]Cl, with n = 1–4 and p = 2–4, and several other cyclic and open chain branched aliphalic alkanaminium chlorides. Thermal decomposition of all the compounds studied occurs in one step and leads to their total volatilization. The experimental TG curves were used for the examination of the thermodynamics and kinetics of this process. The nature of the thermal decomposition of the salts is also considered. The enthalpies of the thermal dissociation were evaluated on the basis of the Van't Hoff equation. Thus derived values together with the available literature data were used for the estimation of the enthalpy of formation and the energy of the crystal lattice of the salts. Furthermore, this information was used for the evaluation of the enthalpy of formation of quaternary alkanaminium ions and alkane cation affinities. The “thermochemical” radii for alkanaminium ions were also derived from the Kapustinskii-Yatsimirskii equation. The Jacobs and Russell-Jones approach was applied for the description of the kinetics of volatilization of amine hydrochlorides. It was found that this model approximates very well with the experimental TG data and evaluated values of the activation energy are very close to the values of the enthalpy for the process. Moreover, this approach accounts also for the limitation of the volatilization rate at the end of the process as a result of a decrease of the geometric surface area of a condensed phase. On the other hand, the application of the standard methods in the case of quaternary salts did not explain the basic questions regarding the kinetics and mechanism of the thermal decomposition of this group of compounds. The values of parameters characterizing thermal properties of alkanaminium chlorides, namely, temperatures of the thermal effects, as well as thermochemical and kinetic constants of thermolysis depend notably on the number of alkyl substituents and the length and structure of an alkyl group. The influence of the length and number of n-alkyl substituents on the values of both the enthalpy of formation and the crystal lattice energy is thoroughly discussed. The essential thermal and thermochemical characteristics for alkanaminium chlorides have been also reviewed.