Photolysis of Acetone in the Presence of HI and the Decomposition of the Acetyl Radical

Abstract

Acetone over a range of pressures and temperatures (1.2–15 cm Hg; 126–295°C) was photolyzed with light of 3130 A in the presence of small amounts of hydrogen iodide. The primary process was found to be the C–C split to form acetyl and methyl radicals. Hydrogen iodide acted as an excellent radical ``trap'' for these radicals. No spontaneous decomposition of acetyl radicals was observed. Relative rates of methyl radical reactions with iodine and hydrogen iodide were measured. Complications due to surface effects in these reactions are considered. Kinetic constants for the reactions CH3+HI→CH4+I(1); CH3+I2→CH3I+I(2) are E1—E2=1.3±0.5 kcal, A2/A1=2.4, and log(k1/k2) = —0.38–1300/4.575T. Above 200°C the acetyl radical decomposition is measurable and the rate constant is observed to be pressure dependent. The pressure dependence is treated by an approximate method related to the Hinshelwood-Lindemann equation and the high- and low-pressure limiting rate constants are given by logk∞=10.3–15 000/4.575T sec—1, logk0=11.5–12 000/4.575T (l/mole-sec). The number of vibrational modes S contributing to the decomposition is 4.5 and the lifetime of the excited acetyl radical is t∼10—8 sec. The abnormally low frequency factor of the high-pressure rate constant implies a nonadiabatic decomposition of the acetyl radical. From the kinetic constants, the heat of formation of the acetyl radical is found to be ΔHf0 (CH3CO)—6.2 kcal and the values of the appropriate bond dissociation energies in a series of acetyl derivatives are tabulated.