Kinetics of phenyl radical reactions with ethane and neopentane: reactivity of C₆H₅ toward the primary C-H bond of alkanes

The kinetics of C₆H₅ reactions with C₂H₆ (1) and neo-C₅H₁₂ (2) have been studied by the pulsed laser photolysis/mass spectrometric method using C₆H₅COCH₃ as the phenyl precursor at temperatures between 565 and 1000 K. The rate constants were determined by kinetic modeling of the absolute yields of C₆H₆ at each temperature. Another major product, C₆H₅CH₃, formed by the recombination of C₆H₅ and CH₃, could also be quantitatively modeled using the known rate constant for the reaction. A weighted least-squares analysis of the two sets of data gave k₁ = 10^11.32±0.05 exp[-(2236 ± 91)/T] cm³ mol^-1 s^-1 and k₂ = 10^11.37±0.03 exp[-(1925 ± 48)/T] cm³ mol^-1 s^-1 for the temperature range studied. The result of our sensitivity analysis clearly supports that the yields of C₆H₆ and C₆H₅CH₃ depend primarily on the abstraction reactions and C₆H₅ + CH₃, respectively. From the absolute rate constants for the two reactions we obtained the value for the H-abstraction from a primary C-H bond, k_p-CH = 10^10.40±0.06 exp(-1790 ± 102/T) cm³ mol^-1 s^-1. This result is utilized for analysis of other kinetic data measured for C₆H₅ reactions with alkanes in solution as well as in the gas phase.