The vapour phase kinetics of pyrolysis of cyclopropene have been studied both by product formation and reactant loss relative to an internal standard in the temperature range 466–516 K. The major product is methylacetylene while the minor one is allene; there is evidence for some material loss under the conditions of these experiments (partial pressures of cyclopropene, ⩽ 2 Torr ‡ highly diluted in N₂ or SF₆). Under conditions of minimal material loss methylacetylene is formed by a homogeneous unimolecular reaction whose rate constant is dependent on total pressure. At (70 ± 2) Torr of SF₆, which is close to the high pressure limit, the following Arrhenius equation is found log k/S^–1=(13.09 ± 0.04)–(156.1 ± 0.4 kJ mol^–1)/RTln 10. Allene appears to be formed by two pathways, one molecular the other radical in nature. A rate constant for the former has been measured. A unimolecular “fall-off” curve for methylacetylene formation has also been obtained which spans nearly five orders of magnitude in pressure and approaches closely both high and low pressure limiting behaviour (at 495 K).

The significance of these results for the direct allene to methylacetylene isomerisation is discussed; for this purpose a table of thermodynamic properties of cyclopropene is also included.