THE MECHANISM OF THE HOMOGENEOUS PYROLYSIS OF ACETYLENE

The experimental literature on the pyrolysis of C2H2 is reviewed and summarized. These observations divide naturally into three temperature regimes: (i) T < 1100 K, where the homogeneous reaction is a molecular polymerization; (ii) 1100 < T < 1800 K, where the process is still dominated by a molecular polymerization, but a fragment radical chain is clearly involved; and (iii) T > 1800 K, where a fragment chain carried by C2H and H drives a polymerization to polyacetylenes. A combined molecular/fragment chain mechanism is developed and applied to the modeling of three shock‐tube studies which span the two higher‐temperature regimes. The core of the mechanism is a set of 5 mainly molecular reactions whose rates are derived from an application of unimolecular rate theory and detailed balance to recent measurements on the decomposition of vinylacetylene. The combination of this core with consequent fragment radical reactions provides a satisfactory description of most features of the chosen experiments including several‐isotope exchange, hydrogenation to C2H4, and C6H2 formation‐which are almost entirely the result of fragment radical chain reaction. Possible detailed paths for the molecular dimerization are suggested and some possible molecular paths for the “Berthelot synthesis” of benzene are also proposed. Copyright © 1990 John Wiley & Sons, Inc.