SHOCK-TUBE PYROLYSIS OF PYRIDINE

The kinetics of pyrolysis of pyridine dilute in argon have been studied in a single-pulse shock tube, using capillary column GC together with GC/MS and FTIR spectroscopy for product determination, over the temperature range of 1300-1800 K and total pressures of 7-11 atm. At the lower end of the studied temperatures, cyanoacetylene was found to be the principal nitrogen-containing product. At elevated temperature hydrogen cyanide predominated. Other major products were acetylene and hydrogen. Thermochemical estimates of the isomeric cyclic pyridyls produced in the pyrolysis indicate that the ortho isomer is unique in being able to undergo facile cleavage to an open-chain cyano radical from which cyanoacetylene is produced. Several sources of HCN were identified in the system. The m- and p-pyridyls may eliminate HCN in a molecular process. An important source of HCN at high temperatures is the addition of H atoms to cyano compounds, especially cyanoacetylene, but also acetonitrile and acrylonitrile which are produced in the pyrolysis. The pyrolysis is a chain process initiated principally by C-H bond fission to form o-pyridyl. A 58-step reaction model is presented and shown to substantially fit the observed profiles of the major product species. From this model we derive a value for the rate constant of the principal initiation reaction, C5H5N → o-C5H4N + H (1), of k1 = 1015.9±0.4 exp(-98 ± 3 kcal mol-1/RT) s-1 between 1300 and 1800 K and at a total pressure of about 10 atm. © 1990 American Chemical Society.