Rate constants for the addition of methyl radicals to propene

Abstract

Measurements have been made over a wide range of mixture composition of the yields of cis- and trans-but-2-ene, but-1-ene, isobutene and methane, when propene is added to slowly reacting mixtures of H2 and O2 at 480 °C. From the initial yields of the butenes relative to that of methane, rate constants have been determined for the overall addition of CH3 radicals to propene and for specific terminal and non-terminal addition. Values of kint= 2.18 × 106, klt= 1.60 × 106 and kln= 5.8 × 105 dm3 mol–1 s–1 at 480 °C are obtained. [graphic omitted] Combination with low-temperature data for the overall addition gives A1nt=(2.57 ± 0.83)× 108 dm3 mol–1 s–1 and E1nt= 30.3 ± 3.2 kJ mol–1 over the range 80–480 °C. Combination of the present results with the very limited low-temperature data for specific addition gives A1t= 108.25 ± 0.25 dm3 mol–1 s–1 and E1t= 29.5 ± 3 kJ mol–1, and A1n= 108.30 ± 0.30 dm3 mol–1 s–1 and E1n= 36.5 ± 5 kJ mol–1. The main cause of the selectivity thus appears to arise from activation-energy differences as suggested for H-atom addition to propene. A maximum value of the rate constant for abstraction at the allyl position in propene by CH3 radicals has been determined at 480 °C and combined with low-temperature data to give maximum values of A= 4.4 × 107 dm3 mol–1 s–1 and E= 31.5 kJ mol–1. Arising from the difference in the A factors and the almost identical activation energies for addition and abstraction, then addition will always be the more important primary process, but at combustion temperatures reverse decomposition of the methyl adducts will reduce the importance of addition and abstraction will predominate.