Kinetics of the Thermal Decomposition of 2,4,6-Triazido-3,5-Difluoropyridine

Abstract

The kinetics and products of the thermal decomposition of 2,4,6-triazido-3,5-difluoropyridine in melt at temperatures of 120–160°C have been studied using pressure measurements, differential thermal analysis, and IR spectroscopy. The reaction occurs in two macroscopic steps, each described by a first-order kinetic equation. In the first, the activation energy and the common logarithm of the pre-exponential factor are 35.6 ± 1.2 kcal/mol and 15.1 ± 0.6 s^–1, respectively. For the studied compound (and certain other heterocyclic azides, such as 2,4,6-triazido-1,3,5-triazine and 2,4,6-triazidopyrimidine), the pre-exponential factor is anomalously high. This is due to the fact that the 2,4,6-triazido-3,5-difluoropyridine molecule contains no hydrogen atoms. For such azides, the usual decomposition mechanism (cleavage of the azide group with subsequent attack of nitrene on a hydrogen atom of a neighboring molecule) is impossible, which favors the stability of the formed nitrene. In this case, the reaction occurs through a complex chain polymerization mechanism, which leads to the formation of specific condensed products: packs of planar polyconjugate carbon–nitrogen networks with a porphyrin-like structure.