Biacetyl photooxidation

Porter108 has made a careful study of the photooxidation at 3650 A. at both room temperature and at 160°C. At room temperature the quantum yield of products were very small (e.g., fao = 0.03) and the product ratios were rather different from those given when 2700 A. radiation was used. It seems that, in contrast to biacetyl photooxidation, the main role of oxygen was in deactivating ketene molecules but there was some direct reaction with the electronically excited ketene molecules. At 160°C. there was a chain oxidation similar to that when 2700 A. radiation was used, but the ethylene forming step appeared to be different. 

Epoxldation. Dye-sensitized photooxidation of aliphatic olefins results in formation of altylic hydroperoxides from reaction with singlet oxygen. Photo-IMiidation with -diketone sensitizers follows a different course epoxides are the foain products. Of several a-diketones examined, epoxides are obtained in highest yields with biacetyl, which is oxidized at the same time mainly to acetic acid. In some cases yields of epoxides in biacetyl photooxidation are 90% or more. Both cis- and trans-alkenes yield mainly tran.r-epoxides. Both epoxidatioh and ketone destruction are much slower with benzil. The nature of this epoxidation reaction is discussed, although many points of ambiguity remain. This reaction is Of practical importance. ... 

Photolytic. Major products reported from the photooxidation of butane with nitrogen oxides under atmospheric conditions were acetaldehyde, formaldehyde, and 2-butanone. Minor products included peroxyacyl nitrates and methyl, ethyl and propyl nitrates, carbon monoxide, and carbon dioxide. Biacetyl, tert-butyl nitrate, ethanol, and acetone were reported as trace products (Altshuller, 1983 Bufalini et al, 1971). The amount of sec-butyl nitrate formed was about twice that of n-butyl nitrate. 2-Butanone was the major photooxidation product with a yield of 37% (Evmorfopoulos and Glavas, 1998). Irradiation of butane in the presence of chlorine yielded carbon monoxide, carbon dioxide, hydroperoxides, peroxyacid, and other carbonyl compounds (Hanst and Gay, 1983). Nitrous acid vapor and butane in a smog chamber were irradiated with UV light. Major oxidation products identified included 2-butanone, acetaldehyde, and butanal. Minor products included peroxyacetyl nitrate, methyl nitrate, and unidentified compounds (Cox et al., 1981). 

Photolytic. Glyoxal, methylglyoxal, and biacetyl were produced from the photooxidation of 1,2,3-trimethylbenzene by OH radicals in air at 25 °C (Tuazon et al., 1986a). The rate constant for the reaction of 1,2,3-trimethylbenzene and OH radicals at room temperature was 1.53 x 10 " cmVmolecule-sec (Hansen et al., 1975). A rate constant of 1.49 x 10 L/molecule-sec was reported for the reaction of 1,2,3-trimethylbenzene with OH radicals in the gas phase (Darnall et al., 1976). Similarly, a room temperature rate constant of 3.16 x 10 " cm /molecule-sec was reported for the vapor-phase reaction of 1,2,3-trimethylbenzene with OH radicals (Atkinson, 1985). At 25 °C, a rate constant of 2.69 x lO " cm /molecule-sec was reported for the same reaction (Ohta and Ohyama, 1985). 2,3-Butanedione was the only products identified from the OH radical-initiated reaction of 1,2,4-trimethylbenzene in the presence of nitrogen dioxide. The amount of 2,3-butanedione formed decreased with increased concentration of nitrogen dioxide (Bethel et al., 2000). 

Photolytic. Glyoxal, methylglyoxal, and biacetyl were produced from the photooxidation of 1,2,4-trimethylbenzene by OH radicals in air at 25 °C (Tuazon et al, 1986a). A rate constant of... 

It was hoped that the photooxidation of biacetyl would enable one to study the experimentally difficult reaction between oxygen and acetyl radicals without the complication of other radicals such as occur in acetone photooxidation. The primary photolysis of biacetyl gives mainly acetyl radicals18 14... 

In view of the difficulty observed by all workers on the photooxidation of biacetyl in obtaining reproducible results, it is difficult to write a satisfactory mechanism. The products are much the same whether 3130 or 4358 A. radiation is used and can be summarized as oxidation products of acetyl radicals or carbon monoxide plus the oxidation products of methyl radicals. 

Photooxidation suitable for the epoxidation of aromatic olefins also occurs with a-diketones (benzil, biacetyl), benzophenone, benzoin, and a-ketoacids, Isotopic mechanistic studies point to a reaction via a biradical. Photooxidation with an a-diketone or a-ketoacid recently has been interpreted in terms of a photochemical a-cleavage leading to an acylperoxy radical, which can effectively transfer an oxygen atom to olefins. Vinylallenes have similarly been photooxidized in the presence of biacetyl. ... 

The important role which oxygen may play in dione photoreactions was mentioned in the Introduction where the phenanthrenequinone-ethyl acetate reaction was cited as an example of reduction in quantum yield and change in product composition (c/. Fig. 2) due to oxygen. A number of dione reactions have been investigated with oxygen deliberately present. Products, generally carboxylic acids, have been characterized but mechanistic details are obscure at present. These reactions are summarized in Table 6. The photooxidation of biacetyl has been reviewed by Hoare and Pearson 75>. 

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