Gas-phase oxidation of alkenes: decomposition of hydroxy-substituted peroxyl radicals

Abstract

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

Formation and stability of gas-phase o-benzoquinone from oxidation of ortho-hydroxyphenyl: a combined neutral and distonic radical study

“…The detection of o-BQ (m/z 108) is rationalised by O 2 addition to the o-hydroxyphenyl radical, followed by isomerisation 16,17 and OH loss in the Waddington mechanism for b-hydroxyperoxyl radicals. 23,24 Scheme 1 also includes pathways from o-BQ that lead to CPO and the CPO radical cation that will now be discussed. Included in Fig.…”

“…As is the case for the o-methylphenylperoxyl radical, 16,17 o-hydroxyphenylperoxyl is expected to isomerise and eliminate OH via a phenoxyl QOOH intermediate to produce o-benzoquinone (o-BQ), a known precursor to cyclopentadienone (CPO) + CO. [18][19][20][21] This mechanism was reported for the oxidation of protonated tyrosinyl radicals 22 and has some similarities to the Waddington mechanism for b-hydroxyperoxyl radicals. [23][24][25] Yet, to date, no direct experimental results have validated this mechanism for the o-hydroxyphenyl + O 2 reaction system.…”

Chlorine atom-initiated low-temperature oxidation of prenol and isoprenol: The effect of C C double bonds on the peroxy radical chemistry in alcohol oxidation

“…Many different types of biofuels are under consideration, amongst them short-and mediumchain saturated alcohols, particularly ethanol, a currently widely used biofuel, the butanol isomers and isopentanol. Fundamental studies on their low-temperature oxidation chemistry [2][3][4][5] have revealed a rich QOOH chemistry and have shown that the alcoholic -OH group significantly alters reactivity when compared to alkane oxidation and introduces new pathways, such as the Waddington mechanism [6,7] or water elimination. [8] Detailed chemical kinetics combustion mechanisms have been developed, which include alcohol-specific low-temperature chemistry and which can successfully model global combustion targets, such as ignition delay time measurements.…”

Pressure-dependent kinetics of peroxy radicals formed in isobutanol combustion

“…Since this reaction is unlikely to dominate at any realistic condition, reactions forming both alkenes and methyl radicals were excluded for the other peroxy networks. The other b-scission reaction involves the simultaneous breaking of two bonds, similar to the Waddington mechanism described for b-peroxy radicals, 17 but with a higher barrier. aC4EtherFromc has the lowest decomposition barrier for aQOOHg, but it is still significantly higher than the decomposition reactions of aQOOHb and aQOOH [O].…”

“…16 b-scission pathways are more general and can lead QOOH to form the same products as HO 2 elimination (if a C-O bond breaks) or an alkyl radical and double bond (if a C-C bond breaks). 17 Multiple simultaneous b-scissions are also possible, forming three or more fragments. Cyclic ethers and OH can also result from QOOH decomposition.…”