KINETICS OF THERMAL-DECOMPOSITION OF THE DIAZINES - SHOCK-TUBE PYROLYSIS OF PYRIMIDINE

The kinetics of pyrolysis of pyrimidine diluted in argon have been stu died behind reflected shock waves over the temperature range 1200-1850 K, at uniform gas residence times of 850-1000 mus and pressures of 13 -15 atm. The major products of pyrimidine pyrolysis were found to be a cetylene, HCN, acrylonitrile, cyanoacetylene and H-2. Using both end-p roduct analysis and real-time UV spectrometry the kinetics of pyrimidi ne 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 disapp earance of pyrimidine given by the expression 10(12.3(+/-0.4)) exp[-27 5(+/-13) kJ mol-1/RT]s-1. A detailed reaction model incorporating a fr ee-radical mechanism for the decomposition of pyrimidine has been deve loped, and shown to predict the reactant and product concentrations be tween 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 c hain carriers. Sensitivity and flux analysis of the kinetic model has shown the most important initiation pathway to be the loss of an H ato m from pyrimidine to yield o-pyrimidyl. Optimisation of the Arrhenius parameters for this initiation reaction yields an activation energy co nsistent with a heat of formation of the o-pyrimidyl radical of 376(+/ -10) kJ mol-1.