Photodegradation radical reactions

Radiation initiated or photodegradation - free radical reactions (see Chapter 18 on photodegradation and photostabilization of poly (ethylene terephthalate)). 

Hodson J. 1988. The estimation of the photodegradation of organic compoundsby hydroxyl radical reaction rate constants obtained from nuclear magneticresonance spectroscopy chemical shift data. Chemosphere 17 2339- 2348. 

Hodson, J., The Estimation of the Photodegradation of Organic Compounds by Hydroxyl Radical Reaction Rate Constants Obtained from Nuclear Magnetic Resonance Spectroscopy Chemical Shift Data, Chemosphere, 17, 2339-2348 (1988). 

In the absence of cosolutes, the photodegradation rates depended on the orientation of the nitro group. Thus 1-nitropyrene decayed relatively fast by the nitro-nitrite primary intramolecular photorearrangement process, followed by secondary radical reactions. However, 2-nitropyrene and 2-nitrofluoranthene were stable toward photolysis, consistent with the N02 group being in the same plane as the aromatic rings. 

There are a number of reports in the literature on the photochemistry of amides, including the photodegradation of serum albumin,224 and of polypeptides.1 Photodegradation involves reactions of acyl and imine free radicals generated by photolytic scission of the amide group.23 33 The photooxidation of N-pentylhexanamide led to the formation of n-valeraldehyde and valeric acid from the amine part of the molecule... 

Tetrahydrofuran is a liquid at room temperature and boils at 66 °C. The fugacity model predicts that tetrahydrofuran will be found in the environment where it is released. Photodegradation by hydroxyl radicals in air is estimated to be rapid and the hydroxyl radical reaction half-life is estimated at 7.3 h. Tetrahydrofuran released to water could partition to the water compartment and readily biodegrade, but not hydrolyze. Tetrahydrofuran has a very low bioaccumulation potential as evidenced by its low octanol/water partition coefficient. 

Theoretically the photodecay rates of dyes in an alkaline buffered environment should be increased due to the neutralization of the generated FT " ions in the photodegradation reaction. However, from Figure 22, there is no significant difference of rate constants between buffered and un-buffered solution at pH 7 and 10. The result suggests that the reaction rates are controlled mainly by the initial hydroxide ion concentration once the hydroxide ions initiate a free radical reaction, the photodegradation will be continued until stable acidic products are formed. 

As has already been shown, polyolefins are subjected to photodegradation upon exposure to sunlight. Additives and impurities eatalyze the breaking of the polymer chain by a series of UV-initiated free radical reactions. Some efforts foeus on the addition of photosensitive species, such as carbonyl groups or metal complexes, to accelerate these proeesses. 

The extent of the photodegradation reaction is measured by the photodegradation quantum yield, pd, which is defined as the fraction of molecules degraded in relation to how many photons were absorbed, and quantifies the light sensitivity of the molecule (Turro 1978). Usually, pd < 1 but values higher than unity can indicate more complex processes, such as radical chain reactions. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

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