Kinetics of thermal decomposition of the diazines: shock-tube pyrolysis of pyrimidine

Abstract

The kinetics of pyrolysis of pyrimidine diluted in argon have been studied behind reflected shock waves over the temperature range 1200–1850 K, at uniform gas residence times of 850–1000 µs and pressures of 13–15 atm. The major products of pyrimidine pyrolysis were found to be acetylene, HCN, acrylonitrile, cyanoacetylene and H₂. Using both end-product analysis and real-time UV spectrometry the kinetics of pyrimidine disappearance were found to be first order with respect to reactant concentration over the concentration range of 0.07–0.3 mol%. The two techniques yielded a first-order rate constant (k_dis) for the disappearance of pyrimidine given by the expression 10^12.3(±0.4) exp[– 275(± 13) kJ mol^–1/RT]s^–1.

A detailed reaction model incorporating a free-radical mechanism for the decomposition of pyrimidine has been developed, and shown to predict the reactant and product concentrations between 1250 and 1600 K. Important radicals in the mechanism were found to be o- and p-pyrimidyl, with H atoms and CN radicals being radical chain carriers. Sensitivity and flux analysis of the kinetic model has shown the most important initiation pathway to be the loss of an H atom from pyrimidine to yield o-pyrimidyl. Optimisation of the Arrhenius parameters for this initiation reaction yields an activation energy consistent with a heat of formation of the o-pyrimidyl radical of 376(± 10) kJ mol^–1.