Is the homogeneous thermal dimerization of acetylene a free-radical chain reaction? Kinetic and thermochemical analysis

Abstract

Basic kinetic and thermochemical arguments incorporated into calculations modeling the initial stages of the homogeneous pyrolysis of acetylene reveal that the usual assumption of a free-radical mechanism is flawed. The key findings are that updated thermochemistry for ethynyl, vinyl, and propargyl radicals and the inclusion of falloff corrections in radical reactions lead to (1) exceedingly slow decomposition rates below 1300 K, (2) kinetic inert gas effects, and (3) the formation of benzene rather than vinylacetylene or diacetylene, all predictions at variance with experimental observations. On this basis the involvement of vinylidene is proposed.

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