Kinetics and mechanism of the low-energy β-α phase transition of the second kind in 2,4-dinitroanisole

Abstract

In this work, comprehensive studies of 2,4-dinitroanisole (2,4DNAN) were carried out using powder thermorentgenography of the internal standard. The time of the complete polymorphic transition in the solid phase β→α in 2,4DNAN under various combinations of conditions has been determined. It has been established that, regardless of the season of manufacture of the substance, when it is stored for 8–9 months, with a change in ambient temperature from minus 30 °C to plus 30 °C, a complete polymorphic transition β→α occurs. When stored in conditions below minus 5 °C, polymorphic transition does not occur. When stored in conditions above plus 30 °C in a closed container, polymorphic transition occurs within 3 weeks. The polymorphic transition is accompanied by a decrease in density by 1.3%–1.5% and an increase in melting temperature by 10–12 °C, depending on the degree of purity of the starting substance. The activation energy of the molecular rearrangement was 68–70 kJ/mol (16.5 ± 3 kcal/mol). The mechanism of polymorphic transition has been evaluated, which is presumably based on internal homodiffusion and energy transfer to the surface of the mass of powder particles and the product. The average activation energy of the polymorphic transition process was 110 ± 6.2 kJ/mol (26.2 kcal/mol). In an open container, reactions proceed by a homogeneous mechanism, and in a closed container by a heterogeneous mechanism involving the gas phase.