Rate constants photochemical degradation

The oxidative degradations of binuclear azaarenes (quinoline, isoquinoline, and benzodrazines) by hydroxyl and sulfate radicals and halogen radicals have been studied under both photochemical and dark-reaction conditions. A shift from oxidation of the benzene moiety to the pyridine moiety was observed in the quinoline and isoquinoline systems upon changing the reaction from the dark to photochemical conditions. The results were interpreted using frontier-orbital calculations. The reaction of OH with the dye 3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-(l,8)(2//,5//)-acridinedione has been studied, and the transient absorption bands assigned in neutral solution.The redox potential (and also the pA a of the transient species) was determined. Hydroxyl radicals have been found to react with thioanisole via both electron transfer to give radical cations (73%) and OH-adduct formation (23%). The bimolec-ular rate constant was determined (3.5 x lO lmoU s ). " ... 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

N2O can be utilized as an indirect marker of HNO formation. The dimerization of HNO has been studied both experimentally and theoretically since the 1960s, and several values for the rate constant have been offered [reviewed in (104,105)]. The current accepted value, determined by flash photolysis techniques at room temperature, is 8 x 106 M 1 s-1 (106), recently revised from 2 x 109 M-1 s-1 (107). Thus, the rate of this reaction requires that HNO be produced in situ by degradative, reductive, or photochemical methods. 

Ascorbic acid (0.8% w/v) in aqueous solution degraded according to apparent first order kinetics, with a rate constant of 2.34 x 10 2/hour, when irradiated by artificial sunlight [40]. The presence of 5% aspartame in the solution decreased the rate constant to 1.48 x 10 2/hour, thus stabilizing ascorbic acid to photochemical degradation by about 37%. Similar effects were also seen with some carbohydrate sweeteners. 

Photolysis degraded photolytically on soil thin films, t,/2 = 13-57 d in artificial sunlight (Tomlin 1994). Oxidation photooxidation t,/2 = 4.2 h in air, based on an estimated rate constant for the vapor-phase reaction with photochemically produced hydroxyl radicals in the atmosphere (Atkinson 1985 quoted, Howard 1991). Hydrolysis neutral hydrolysis rate constant k < 1.5 x lO 5 h 1 with a calculated t,/2 > 700 d in neutral solution and with faster hydrolysis rates in acidic and basic solutions to be expected (Ellington et al. 1987, 1988 quoted, Howard 1991). 

Benzene in the atmosphere exists predominantly in the vapor phase (Eisenreich et al. 1981). The most significant degradation process for benzene is its reaction with atmospheric hydroxyl radicals. The rate constant for the vapor phase reaction of benzene with photochemically produced hydroxyl radicals has been determined to be 1.3 10"12 cm3/molecule-second, which corresponds to a residence time of 8 days at an atmospheric hydroxyl radical concentration of 1.1 x 106 molecules/cm3 (Gaffney and Levine 1979 Lyman 1982). With a hydroxyl radical concentration of 1 x 108 molecules/cm3, corresponding to a polluted atmosphere, the estimated residence time is shortened to 2.1 hours (Lyman 1982). Residence times of 472 years for rural atmospheres and 152 years for urban atmospheres were calculated for the reaction of benzene with ozone (03) using a rate constant for 03 of 7 /1 O 23 cm3/molecule-second (Pate et al. 1976) and atmospheric concentrations for 03 of 9.6/1011 molecules/cm3 (rural) and 3/ 1012 molecules/cm3 (urban) (Lyman 1982). 

It must be emphasized that the first- or zero-order rate constant obtained as described above by analysis of a drug photodegradation system is applicable only to that experimental arrangement. For a "true" constant to describe the photochemical reaction of drug D degrading to products PI, P2, etc.. 

Mokrosz J, Bojarski J. Photochemical degradation of barbituric acid derivatives. Part 3 rate constants of photolysis of barbituric acid and thiobarbituric acid derivatives. Pharmazie 1987 37(ll) 768-773. 

Reactions 1 and 2 describe reversible photochromism, and 3 and 5 irreversible photochemical degradation processes having rate constants of Pad and pDB - The thermal degradation processes are reactions 4 and 6, with the latter having a negligibly small rate constant as SPs do not degrade appreciably in the absence of UV light. 

The dominant atmospheric fate process for 1,1,1-trichloroethane is predicted to be degradation by interaction with photochemically-produced hydroxyl radicals. Earlier experimental rate constants for this gas-phase reaction ranged from 2.8x1 O to 1.06x10 cm /mol-sec (20-30 °C) (Butler et al. 

The half lifetime in air is related to the atmospheric degradation ability of a chemical, measured by the rate constant of its reactions with free radicals (e.g., OH NO3) and ozone O3 or of photochemical reactions [Gramatica and Papa, 2007 Gramatica, Pilutti et al., 2003b]. 

When the drug has a substantial absorption in the UV and visible regions (maximum absorbance > 2), solutions of the compound will absorb most of the incident irradiation (see Chapter 3). The drug acts as a filter and prevents transmission of photons through the formulation. The rate limiting factor is then the intensity of the incident irradiation, and the photochemical degradation rate will follow zero-order degradation kinetics. Photochemical reactions can take place at the surface of the product the decomposition proceeds at a constant rate independent of the concentration of the reactant (Connors, 1990) ... 

As discussed in Chapter 3, the use of rate constants can be helpful for comparative purposes under the same irradiation conditions however, the quantitative expression of photochemical rate should be given in the terms of the quantum yield of the reaction. To determine the quantum yield in the solid state is unfortunately not a straightforward process. As a result, most studies in the literature refer to an apparent reaction order calculated from the degradation of a minor fraction of the sample. Such data should preferentially be supported by some detail of the reaction mechanism. 

The photo-oxidative degradation of polypropylene and stabilization by hindered amines has been reviewed. A study has appeared of the effect of P-carotene on the photoreactivity of anthracene in hexane solution and a kinetic scheme has been proposed to account for the photochemical and photophysical processes that occur on irradiation at 365 nm. Quenching rate constants have been determined between /S-carotene and singlet oxygen. Some characteristics have been communicated of the sensitized photo-oxidation of abietic acid contained in a vinyl butyl ether-butyl methacrylate-methacrylic acid copolymer. At 400 nm... 

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