Acetonyl radical, decomposition

It seems to be well-established that methane and ketene are formed in the main reaction, while carbon monoxide, ethylene and other products, produced in trace amounts, are probably the result of the ketene decomposition. Ethane and acetonyl acetone are chain termination products (see later), while the precursor of acetyl acetone is probably the radical formed in the reaction of the acetonyl radical and the ketene. 

Though radicals react with acetone, chains are not propagated below about 450 °C. On the other hand, at higher temperatures where the thermal decomposition of acetone has been generally studied, the acetonyl radical is unstable and decomposes into ketene and methyl radical. Thus, under such conditions, the reaction is a chain process. 

It seems to be well established that reactions (l)-(4) are the chain initiation and propagation steps of the decomposition. The question of chain termination is, however, far from being clear. In their fundamental paper. Rice and Herzfeld suggested that the recombination of the methyl and acetonyl radicals to give methyl ethyl ketone was the probable termination step. This leads to an overall order of one which is in agreement with the observations at high pressures. 

Ketene, which is formed with the highest overall rate around 300 °C, comes from the decomposition of the acetonyl radical, viz. 

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