Phenols, from photooxidation

Carotenoids are singlet oxygen quenchers and protect the oil from photooxidation. Their role in the oxidative stability of olive oil has not yet been fully eludicated. There is probably a relation between carotenoids and the mode of action of polar phenols and a-tocopherol (Psomiadou and Tsimidou 1998). 

Tetrahydrofuran has been reported to exhibit an absorption maximum at 280 nm (52,56), but several workers have shown that this band is not produced by the purified solvent (30,41,57). Oxidation products from THF have been invoked in order to account for the appearance of the 280-nm band in PVC films that are solvent-cast from THF in air (57. 581. However, in some reported cases (56,59), this band was undoubtedly produced, at least in part, by a phenolic antioxidant (2.6-di-tert-butyl-p-cresol)(59) in the solvent. Since certain -alkylphenols have now been shown to be powerful photosensitizers for the dehydrochlorination of PVC (60), it is clear that antioxidant photosensitization might well have been responsible for some of the effects attributed previously (56) to THF alone. On the other hand, enhanced rates of photodegradation under air have also been observed for PVC films cast from purified THF (57), a result which has been ascribed to radical formation during the photooxidation of residual solvent (57,61). Rabek et al. (61) have shown that this photooxidation produces a-HOO-THF, a-HO-THF, and y-butyro-lactone, and they have found that the hydroperoxide product is an effective sensitizer for the photodehydrochlorination of PVC at X = 254 nm (61). 

Photolytic. Based on data for phenol, a structurally related compound, an aqueous solution containing 1-naphthoxide ion (3 x 10 M) in room light would be expected to photooxidize to give 2-hydroxy-1,4-naphthoquinone (Tomkiewicz et al., 1971). 1-Naphthol, methyl isocyanate, and other unidentified cholinesterase inhibitors were reported as products formed from the direct photolysis of carbaryl by sunlight (Wolfe et al., 1976). In an aqueous solution at 25 °C, the photolysis half-life of carbaryl by natural sunlight or UV light (X = 313 nm) is 6.6 d (Wolfe et al, 1978a). 

From the half-lives indicated in Fig. 16.5 it can be seen that for most pollutants, the assumption of a well-mixed epilimnion (typical mixing rates 1-10 d-1) with respect to indirect photolysis with 02 is a reasonable assumption. Furthermore, for compounds exhibiting iq2 values [or (1 - aia) k ptQ values for phenolate species] greater than 107 M l s-1, during the summer, photooxidation by 02 is equal to, or more important than, depletion of the concentration by dilution with inflowing water [f,/2(dilution) in the epilimnion of Greifensee on the order of 70 days]. We should recall, however, that only a few compound classes exhibit such large kplQ values, and that, therefore, 02 must be considered to be a rather selective photooxidant. 

Phenolic antioxidants do not protect fats from oxidation by singlet oxidation (Yasaei et al. 1996). However, the antioxidant ascor-byl palmitate is an effective singlet oxygen quencher (Lee et al. 1997). Carotenoids are widely used as quenchers. Rahmani and Csallany (1998) reported that in the photooxidation of virgin olive oil, pheophytin A functioned as sensitizer, while P-carotene acted as a quencher. 

The flash photolysis of hexaarylbiimidazole produces imidazolyl radicals " which have been shown to be more nearly planar than the parent dimers ort/io-substituents in the aryl rings decrease the radical stability. The radicals oxidize electron-rich substrates by rapid electron abstraction from tertiary amines, iodide, and metal ions, and by hydrogen abstraction from phenols, mercaptans, secondary amines, and active methylene com-pounds." " Studies have been made of the photooxidation of /euco-triphenyl-methane dyes by these radicals." " ... 

Oxygen and suitable catalysts can also be used for the conversion of phenol to ben-zoquinone. Thus, irradiation of phenol in the presence of [Cu(bpy)2] or [Cu(l,10-phenanthroline)] + brings about degradation by a path that shows both pH and solvent dependency . Thus in acetonitrile benzoquinone predominates, but in water carbon dioxide is the sole product. Benzoquinone can also be formed from phenol by continuous irradiation in the presence of the catalysts [Crfbpyfs] or less effectively with [Ru(bpz)3] + and [Ru(bpy)3] +. The reaction path involves 02( A ) as the oxidant . Porphyrins such as 5,10,15,20- tetrakis(2,6-dichlorophenyl)porphyrin and chlorins can also be used to convert naphthols and phenols to the corresponding quinones (Scheme 28) . Phthalocyanines immobilized on polymers have also been used as the catalyst system to effect photooxidation. ... 

The photoreactivity results showed that the reactor reaches steady state conditions after a long period of time (ca. 70 h) from the beginning of the irradiation. At steady state conditions the main photooxidation product was benzaldehyde but also benzyl alcohol and traces of benzoic acid and phenol were detected at all the experimental conditions used. During the transient period, benzene together with CO2 were also produced. No significant evidence of CO2 production was observed at steady state conditions. These results obviously indicate that at the experimental conditions used the photoprocess does not give rise to a complete degradation of toluene. 

According to reactions (12)-(16), CO2 results from the oxidation of toluene to benzoic acid whose traces were found in our experiments and its subsequent photodecarboxylation. It must be reported that some runs carried out by using benzoic acid at the same experimental conditions used for toluene photooxidation, afforded benzene and CO2 in large amounts. It is well known [7, 8] that ethanoic acid is easily photodecarboxylated in a gas-sohd regime in the presence of irradiated polycrystaUine semiconductor oxides. The small amounts of phenol are probably due to an attack of benzene by OH radicals (see eqn.(16)). 

Nitrated hydroxyaromatics may enter into the atmosphere from both primary and secondary sources. The formation of nitrophenols and nitrocresols in die combustion processes of motor vehicles has been reported by Tremp et al. (1993). Others primary sources may be combustion of coal, wood, manufacture of phenol-formaldehyde resins, pharmaceuticals disinfectants, dyes and explosives (Harrison et al., 2005). Studies in our and other laboratories have shown that an additional important source of diese compounds in the atmosphere could be the gas-phase OH-radical initiated photooxidation of aromatic hydrocarbons such as benzene, toluene, phenol, cresols and dihydroxybenzenes in the presence of NOx during the daytime as well as the reaction of NO3 radicals widi these aromatics during the night time (Atkinson et al., 1992 Olariu et al., 2002). Once released or... 

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