Glyoxal from aromatic oxidation

While the formation of glyoxal from aromatic VOC is likely over-predicted by the MCM code (m part due to the lower re-eva uated glyoxal yields), glyoxal formation is under estimated from alkenes hke d for which glyoxal is not a predicted oxidation product. 

Dicarbonyls. A third area of uncertainty is the treatment of dicarbonyls formed from aromatic or terpene hydrocarbon oxidation. (The simplest is glyoxal, CHOCHO, but a large number have been identified, 47. The yields and subsequent reactions of these compounds represent a major area of uncertainty in urban air photochemistry (186) and since they may be a significant source of HOjj through photolysis, inaccuracies in their portrayal may result in errors in calculated values of HO. and HO2.. 

Reaction rates in the presence of oxygen and nitric oxides [82], in the presence of hydroxyl radicals [89] or in the presence of mixtures of nitric oxides of varying composition [90] have been compared for various aromatics, m-xylene and trimethyl benzenes react substantially faster than the other xylenes, toluene, and in particular benzene. In the presence of nitric oxides, the following products are formed from aromatics with side chains particularly from toluene methyl glyoxal, dimethyl glyoxal, and biacetyl [83,84], larger quantities of m-nitrotoluene, 2-methyl-4-nitro-phenol, 2-methyl-4,6-dinitrophenol, benzaldehyde, benzyl nitrate, o-cresol, and p-nitrophenol [91-93]. 

The reaction of ozone with an aromatic compound is considerably slower than the reaction with an alkene. Complete ozonolysis of one mole of benzene with workup under non-oxidative conditions will yield three moles of glyoxal. The selective ozonolysis of particular bonds in appropriate aromatic compounds is used in organic synthesis, for example in the synthesis of a substituted biphenyl 8 from phenanthrene 7 ... 

One of the most common approaches to pyrazine ring construction is the condensation of diaminoethane and 1,2-dicarbonyI compounds such as 206 to provide pyrazines 207 after aromatization. Aromatization was accomplished by treating the dihydropyrazines with manganese dioxide in the presence of potassium hydroxide <00JCS(P1)381>. The N-protected 1,2-dicarbonyl compounds 206 were prepared from L-amino acids by initial conversion into diazoketones followed by oxidation to the glyoxal. 

Dialdehydes are important, photochemically reactive intermediates, formed from a wide range of precursors, most importantly isoprene and aromatic hydrocarbons. Glyoxal is formed following reaction of OH with hydroxyacetaldehyde, an intermediate in isoprene oxidation, as discussed previously. Methylglyoxal is formed following reaction of O3 with both methyl vinyl ketone and methacrolein, major intermediates in isoprene oxidation, and following reaction of methacrolein with OH. 

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