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 pyrolys is of acetone at 825-940 K and 10-180 Torr. Yields of products were me asured by gas chromatography. The rate constant, k(1), for the initiat ion reaction, determined from the sum of the yields of the termination products, was observed to be pressure dependent at 928 K. The Arrheni us expression for this reaction at the high-pressure limit (obtained f rom a nonlinear least-squares fit to the experimental data using the T ree 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 b een used to find the high-pressure limits of the cross-combination rat io 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 consta nts for reaction 3, when combined with values reported from photolysis experiments at lower temperatures, were found to exhibit a curved Arr henius plot. A transition state theory model was fitted to the data fo r k(3) to determine the average transitional vibrational term value in the transition state (279 +/- 8 cm(-1)), the effective activation bar rier height (46 +/- 1 kJ mol(-1)), and the full width of the barrier a t half its height (52 +/- 3 pm).