Oxygen in photooxidation

Gerischer, H. Heller, A. The role of oxygen in photooxidation of organic molecules on semiconductor particles, J. Phys. Chem 1991, 95, 5261. 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Okada and Mukai [48] showed a preference for a contrasteric approach of singlet oxygen to anti face of 7-isopropylidene double bond in photooxidation of 7-isopropylidenenorbomene followed by reduction with dimethyl sulfide (Scheme 32). They explained the stereoselectivity by applying the orbital mixing rules (Scheme 33). The r orbital of the exocyclic double bond enlarges its extention in the anti face. 

Oxidation photooxidation t,/2 = 0.32-3.2 h estimated, based on estimated rate constant for the reaction with hydroxyl radicals in air (Atkinson 1987 quoted, Howard et al. 1991) photooxidation t,/2(aq.) = 1,57 - 157 yr estimated, based on measured rate constant for the reaction with singlet oxygen in benzene (Stevens et al. 1974 quoted, Howard et al. 1991). 

The 02 ion appears to play an important role in a number of photooxidation reactions (see Section VI,C) for example, the photo-oxidation of alkenes over TiOz. However, it seems likely that OJ is not, in many cases, active in the oxidation step but further conversion occurs to give a mononuclear species, not detected directly, which then oxidizes the adsorbed hydrocarbons. Photo-oxidation of lattice oxygen in the M=0 systems (e.g., V2Os supported on PVG) gives rise to an excited charge transfer state such as V4 + -0 . This excited state can react as O- either by addition to a reactant molecule or by an abstraction reaction (see Section V of Ref. /). In the presence of oxygen, 03 is formed which then reacts further with organic molecules. 

Only one other photooxide of a benzo[c]furan has been described. Hydrocarbon 232 on treatment with oxygen in methanol gives 233 in 96% yield further treatment with oxygen in benzene or tetrahydrofuran leads to 234 probably the corresponding benzo[c]furan is involved. Whereas reduction with triphenylphosphine in benzene or KI in acetic acid gives 235, 2 hours reflux in benzene leads to 235, 236, and 237 in 7, 48, and 3% yield, respectively. 

Pteridines can also function as catalysts in photooxidations whereby the protonated species of lumazine and aminopterin, respectively, catalyze the substrate-selective photooxidation of benzyl alcohol derivatives by oxygen in the presence of perchloric acid in acetonitrile <89CC816>. 

When Srinivasan and Noyes116 photooxidized diethyl ketone in the presence of 98% oxygen-18, they found the quantum yield of diethyl ketone-18 was 0.7 and, as in the case of the acetone-oxygen-18 photooxidation, no isotopic change was detected in the oxygen composition. 

Matsushita, S., Terao, J. 1980. Singlet oxygen-initiated photooxidation of unsaturated fatty acids and esters and inhibitory effect of tocopherols and (3-carotene. In Auto-oxidation in Food and Biological Systems (M.G. Simic, M. Karel, eds.), pp. 27 44, Plenum Press, New York. 

For the dimers Dt 3 the electron and proton acceptor in the oxidation reaction is oxygen, in the absence of which photooxidation is inhibited. It should be noted that the maximum concentration of dimer employed in these reactions was 0.5 x 10-4 M, as compared to the oxygen concentration in an aerated solution of 3 x 10 4 M, so that an excess of oxygen prevailed in all instances. 

Af,Af-dimethylaniline, which becomes yellow in air or oxygen but colorless again when the oxygen is removed. Such weak charge-transfer complexes make certain electronic transitions in molecules more intense they are also a plausible first stage in photooxidations. 

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