Initiation and abstraction reactions in the pyrolysis of acetone

The rates of the reactions, (CH3)(2)CO --> CH3 + CH3CO (1) and CH3 + (CH3)(2)CO --> CH4 + CH2COCH3 (3) have been studied in the flow pyrolysis of acetone at 825-940 K and 10-180 Torr. Yields of products were measured by gas chromatography. The rate constant, k(1), for the initiation reaction, determined from the sum of the yields of the termination products, was observed to be pressure dependent at 928 K. The Arrhenius expression for this reaction at the high-pressure limit (obtained from a nonlinear least-squares fit to the experimental data using the Tree factorization procedure) was found to be k(1 infinity) (S-1) = 10(17.9+/-0.8) exp(-353 +/- 14 kJ mol(-1)/RT). The Tree method has also been used to find the high-pressure limits of the cross-combination ratio for CH3 and CH3COCH2 radicals (1.9 +/- 0.1) and of the quotient k(5)/k(3)(2), where reaction 5 is 2CH(3) --> C2H6. Calculated rate constants for reaction 3, when combined with values reported from photolysis experiments at lower temperatures, were found to exhibit a curved Arrhenius plot. A transition state theory model was fitted to the data for k(3) to determine the average transitional vibrational term value in the transition state (279 +/- 8 cm(-1)), the effective activation barrier height (46 +/- 1 kJ mol(-1)), and the full width of the barrier at half its height (52 +/- 3 pm).