Solid- vs. Liquid-state Thermal Decomposition: Thermolysis of Halogenated Benzene Derivatives with 2-Nitrodiazene-1-N-oxide Moiety

Abstract

Melting often accompanies the thermolysis of crystalline organic compounds. The presence of melting, in turn, brings additional complexity for the thermokinetic analysis of decomposition process. As a consequence of this, the kinetic data for the thermolysis of particular compounds both in solid- and liquid-state are rarely reported and often contradict each other. Herein, we proposed a strategy of dealing with melting with decomposition case for various types of thermoanalytical experiments (DSC, TGA) and temperature programs (both isothermal and non-isothermal). Furthermore, we proposed to study several related species differing by non-energetic moieties to shift the melting point over a wide temperature range without remarkable changes in the decomposition temperatures. To illustrate the value of the proposed approach, we consider the decomposition kinetics of the three halogenated benzene derivatives bearing a 2-nitrodiazene-1-N-oxide moiety. In all cases the noisothermal experimental data is fitted by a first-order reaction paralleled by the autocatalytic process. The isothermal experiments below the melting point of compounds in ramped heating runs still show the formation of liquid. This observation and other findings are explained by the Bawn kinetic model. The activation energies for the liquid-state decomposition of all compounds were found to be 145 ± 3 kJ mol-1. The experiment was complemented with the highly accurate CCSD(T)-F12 quantum chemical calculations. Theory predicts the primary decomposition pathway to be the radical scission of a nitro radical followed by the fast elimination of nitrous oxide. With the suggested approach, we obtained the acceleration factor of the rate constant when decomposition commences in the solid or liquid state to be 2-4 times, not orders of magnitude, as was proposed in some previous publications.