PYROLYSES OF AROMATIC AZINES - PYRAZINE, PYRIMIDINE, AND PYRIDINE

The thermal decompositions of pyrazine, pyrimidine, and pyridine in sh ock waves have been investigated using the complementary techniques of laser-schlieren (LS) densitometry and time-of-flight (TOF) mass spect rometry (1600-2300 K, 150-350 Torr). A free radical chain reaction wit h initiation by ring C-H fission in the pyrolyses of all three azines is proposed. The measured C-H fission rates are compared and analyzed by RRKM theory. Barriers of 103 +/- 2 kcal/mol for pyrazine, 98 +/- 2 for pyrimidine, and 105 +/- 2 for pyridine have been determined, suppo rting values lower than the barrier for C-H fission in benzene, 112 kc al/mol. The lower barriers for the azines are explained by the additio nal contributions of resonance structures of azyl radicals due to neig hboring N-C interactions, which serve to further stabilize the azyl ra dicals. Detailed chain mechanisms are constructed to model the LS prof iles and the TOF concentration profiles of the major products, hydroge n cyanide, acetylene, cyanoacetylene, and diacetylene. Of particular i nterest are the TOF observations and the mechanistic explanation of te mperature dependent maxima seen in the formation of cyanoacetylene in the presence or absence of excess H-2.