Photooxidation increasing rate

Photolytic. When a dilute aqueous solution (1-10 mg/L) of bromacil was exposed to sunlight for 4 months, the TV-dealkylated photoproduct, 5-bromo-6-methyluracil, formed in small quantities. This compound is less stable than bromacil and upon further irradiation, the de-brominated product, 6-methyluracil was formed (Moilanen and Crosby, 1974). Acher and Dunkelblum (1979) studied the dye-sensitized photolysis of aerated aqueous solutions of bromacil using sunlight as the irradiation source. After 1 h, a mixture of diastereoisomers of 3-5ec-butyl-5-acetyl-5-hydroxyhydantoin formed in an 83% yield. In a subsequent study, another minor intermediate was identified as a 5,5 -photoproduct of 3-5ec-butyl-6-methyluracil. In this study, the rate of photooxidation increased with pH. The most effective sensitizers were riboflavin (10 ppm) and methylene blue (2-5 ppm) (Acher and Saltzman, 1980). Direct photodegradation of bromacil is not significant (Acher and Dunkelblum, 1979 Ishihara, 1963). 

For measuring the steady-state concentration of organic peroxy radicals (ROO ) produced in sunlit natural waters series of antioxidants, such as poly-methylphenols, have been successfully applied as selective probe compounds (Faust and Hoigne, 1987). The rates of transformation have shown that the steady-state concentration of the apparent photooxidants increases with the amount of light absorbed by the DOM. The sink for ROO has not been identified, but kinetic evidence is that DOM, even when the DOC amounts up to 5 mgL"1, does not control the lifetime of the peroxy radicals. The following reaction scheme summarizes the results of the kinetic analysis for peroxy radical formation ... 

Pandey and Singh [60] have recently shown that polypropylene biodegrades much more rapidly than polyethylene by mass loss in compost. This was carried out on solvent extracted polymers to remove antioxidants and PP lost over 60% mass in 6 months whereas LDPE lost about 10% in the same time. Ethylene-propylene co-polymers biodegraded at rates intermediate between PP and PE. As expected, prior UV irradiation (photooxidation) increased both the rate and extent of the bioassimilation. This is fully in... 

A number of recent studies have been concerned with the effects of commercial heat stabilizers on the photodegradation of PVC. During irradiation at room temperature under air with 253.7-nm light, several dialkyltin dicarboxylates were found to increase the rates of the photooxidation and cross-linking of the polymer (62). However, at 0°C under air in a sunshine weatherometer, photooxidation was shown to be retarded by certain dibutyltin dicarboxylates (63). The latter result was also obtained in experiments involving the use of dibutyltin maleate with irradiation in the (280-400)-nm wavelength region at 38°C under air (50,51,64, 5,66). 

A study of the relative fluorescence intensities at 460 nm of PET and PET-4,4 -SD yarns after receiving identical irradiation intensities reveals an increase in the formation of the hydroxyterephthaloyl moiety with increasing amounts of 4,4 -SD. This indicates that a photooxidative mechanism involving the second monomer may be an explanation of the increasing degradation rates. 

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). 

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

As pointed out above (Section III) the side chain controls the rate of the catalytic reaction. It was subsequently observed that it also controls the conformative response, i.e., the effect of the substrate on the conformation of the enzyme (75). Significantly, the nature of the conformative response could be correlated with the effect on the catalytic activity. Thus, the conformative response to benzylpenicillin and other penicillins with side chains promoting the rate of catalysis is characterized by increased stability of the enzyme molecule. Conversely, penicillins which carry side chains interfering with the catalytic activity labilize the enzyme and facilitate inactivation by heat, urea, proteolysis, iodination, or photooxidation. Such penicillins act as competitive inhibi-... 

Glaze and Kang (1989a,b) presented a model describing the photooxidation of water-soluble hazardous organic waste and also observed the rate increase with a decrease in pH of reaction medium. 

Bhowmick and Semmens (1994) studied the photooxidation kinetics of five halogenated VOCs, including chloroform (CHL), carbon tetrachloride (CTC), trichloroethylene (TCE), tetrachloroethylene (PCE), and 1,1,2-trichloroethane (TCA) by UV/ozone. In the low to intermediate concentration range of 0.07 to 1.11 mg/L, the reactivities of the organics rank in the following order TCE > PCE > CTC > CHL > TCA. This order indicates that the reaction between ozone and saturated hydrocarbons is much slower than that of ozone with unsaturated compounds such as alkenes. For CHL, TCA, and CTC, an increase in ozone concentration had little effect on the rate constant. By comparison, the rates of oxidation for TCE and PCE increased with added... 

Brunow and Sivonen (34) obtained similar results on a lignin model system, ethylguaiacylcarbinol. This compound did not undergo any oxidation when irradiated with near-uv light until p-methoxyacetophenone, a triplet sensitizer, was added to the solution. As the molar ratio of p-methoxyacetophenone to ethylguaiacylcarbinol was increased to 0.32 the rate of photooxidation of this benzyl alcohol increased. This result is consistent with the mechanism of light-induced yellowing shown in Scheme 3. 

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