Studies of the kinetics of thermal unimolecular decomposition of methylcyclopentane, methylcyclohexane, ethynylcyclopentane, and ethynylcyclohexane have been carried out at temperatures in the range 861-1218 K using the technique of very lowpressure pyrolysis (VLPP). Multiple reaction pathways and secondary decomposition of primary products results in a complex array of reaction products. VLPP rate data (falloff regime) were obtained for the overall decompositions and interpreted via the application of RRKM theory. The data for methylcyclopentane and methylcyclohexane were interpreted in terms of ring-opening bond fission pathways and bond fission to methyl and cycloalkyl radicals. By selecting Arrhenius parameters consistent with the analogous pathways in open-chain alkanes, a good fit to the overall decomposition is obtained. The data for ethynylcyclopentane and ethynylcyclohexane were interpreted in terms of ring-opening bond fission and alkyne to allene isomerization. The A factors for ring opening were based on known values for C-C fission in open-chain alkynes and the Arrhenius parameters for isomerization were chosen to be consistent with previously reported alkyne to allene isomerizations. The VLPP data are consistent with the following high-pressure rate expressions (at < T > = 1100 K) for the dominant primary reaction channel of ring opening adjacent to the substituent group: log(k/s-') = (16.4 L 0.3) -(341 t lO)/O for methylcyclopentane, log(k/s ') = (16.4 ? 0.3) -(345 f lO)/O for methylcyclohexane, log(k/s-') = (16.0 t 0.3) -(304 ? lO)/O for ethynylcyclopentane, and log(k/s-') = (16.0 L 0.3) -(303 2 lO)/O for ethynylcyclohexane,
where 0 = 2.303RT k J mol-'. Comparison of the activation energies for the ethynylcycloalkanes with those for the methyl-cycloalkanes shows that the effect of the ethynyl substituent is consistent with the propargyl resonance energy. This evidence supports the assumption of a biradical mechanism for ring opening in these cycloalkanes.