PYROLYSIS OF BIACETYL AND 3-BODY EFFECT ON COMBINATION OF METHYL RADICALS

The kinetics of pyrolysis of biacetyl at 677-776°K and 0.6-45 torr have been investigated by the stirred-flow method. The decomposition (to carbon monoxide, methane, ketene, acetone, ethane, and 2,3-pentanedione) is shown to be a chain reaction initiated by reaction 1, propagated by reactions 3 and 8, and terminated mainly by reaction 6, which is strongly pressure dependent. The following kinetics data were obtained (k and A being in cm3 mol-1 sec-1) Order k (at 730°K) log A E, kcal mol-1 (1) (CH3CO)2 → 2CH3CO 1 7.6 × 10-5 16.0 67.2 ± 3.3 (3) CH3 + (CH3CO)2 → CH4 + CH2COCOCH3 2 1.2 × 109 11.8 9.1 (4) CH3 + CH3 (+M) → C2H6 (+M) 3 1.6 × 1020 16.3 ∼-13 (8) CH3 + (CH3CO)2 → (CH3)2CO + CH3CO 2 2.6 × 108 10.3 6.3 Pressures of biacetyl, at which the second-order rate constant for reaction 6 is half its limiting high-pressure value, increase from 14 torr at 677°K to 30 torr at 776°K and are in satisfactory agreement with values calculated by previous authors on the basis of the RRKM theory assuming a loose" activated complex and strong deactivating collisions. The presence of a trace of 2-hydroxy-2-methylbutan-3-one in the products is regarded as evidence that reaction 8 proceeds through formation of the radical (CH3)2C(O·)COCH3. The heat of formation of the gaseous acetyl radical is estimated to be -5.1 ± 2.0 kcal mol-1 at 298°K."