Carbonyl compounds are the main products of the low-temperature gas-phase oxidation of alkenes. It is suggested that they are formed by consecutive addition of a hydroxyl radical and an oxygen molecule to the alkene, the hydroxy-substituted peroxyl radical subsequently decomposing to yield two molecules of carbonyl compounds and the chain carrier. A model system has been investigated, the fuel chosen being 2,3-dimethylbutan-2-01 which gives acetone as the major product at 578 K. In reactions with [180]-2,3-dimethylbutan-2-ol and oxygen, the ketone is enriched by ca. 50%. and this result is consistent with the decomposition of the radical (11).CARBONYL compounds are the principal products of the low-temperature oxidation of aliphatic alkenes in the gas p h a ~e . l -~ For example, acetaldehyde, acetone and acetaldehyde, acetone, 2-methylpropanal and formaldehyde, and acetone and formaldehyde are the main products from the oxidation of tram-but-2-ene,l2-methyl-b~t-2-ene,~J 3-methylbut-l-ene? and 2-meth~lpropene,~ respectively. Where different carbonyl compounds are formed from the alkene, it is apparent that one molecule of alkene decomposes to yield two molecules of carbonyl and the overall reaction may be expressed as (1)-The formation of the carbonyl compounds via a dioxetan is ruled out as the reaction is highly endothermic (AHo = 290 kJ mol-1),6 involving a spin-forbidden transition, as oxygen is in its ground state. Knox, on the other hand, suggested that the alkene first undergoes consecutive addition reactions with a hydroxyl or a hydroperoxyl radical and oxygen. ' We have suggested that it is the hydroxyl radical that is the

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