Initiation of photooxidation

Chemical reactions conducted within microemulsions occur with the same modes of control afforded by micelles. Thus, photoactivity of semiconductors [75] and metal colloids [76] formed within microemulsions are maintained, as is the capacity for initiation of photooxidative polymerization [77],... 

None of these products contribute towards the initiation of photooxidation of the polymer other than having some u.v. antioxidant action. On long-wavelength irradiation (365 nm) photoinitiation occurs via impurity chromophore whereas with light of wavelengths shorter than 300 nm direct excitation of the diphenylcarbonate unit occurs. 

Chien, J. C. W. 1965. On the possible initiation of photooxidation by charge-transfer excitation. J. Phys. Chem. 69, 4317-4325. 

The reaction sequence (5)-(8) leads to the rapid accumulation of hydroperoxides and the attainment of the stationary peroxide concentration. Ionic Cu, Mn, Fe, Cr, and Co are the most effective promoters of peroxidation whereas Ni, Ce, V, Ti, and Zn are less effective. Transition metal ions are also initiators of photooxidation. 

In analogy with the absorption intensity, the initial rate of oxidation depends on the oxygen and the substrate concentrations. Since UV light absorptions have been entirely (alkanes) or partially (alkenes) attributed to charge-transfer transitions, these have been assumed to contribute to the initiation of photooxidation in all the systems studied, particularly in the photooxidation of methylcyclohexane. 

Like most other engineering thermoplastics, acetal resins are susceptible to photooxidation by oxidative radical chain reactions. Carbon—hydrogen bonds in the methylene groups are principal sites for initial attack. Photooxidative degradation is typically first manifested as chalking on the surfaces of parts. 

Surface vs Solution Reactions, Anotliei issue of debate in pliotocatalyzed mineialization of oiganic substrates is whether the initial oxidation occurs on the photocatalyst s surface or in solution. Kinetic data of photooxidations and photoreductions have often been fitted to the simple... 

Initiation by light accelerates oxidation due to the photochemical generation of free radicals, which was noticed by Backstrom [16] and repeated by many others [9,11 — 13], The quantum yield (photooxidation products is sufficiently higher than unity. Here are several examples [12]. 

A number of workers have looked at the effect of photooxidation and photodynamic sensitizers on DNA. Rose Bengal photosensitizes strand breaks in double-stranded, supercoiled, pBR322 DNA the effect follows first-order kinetics with respect to light fluence and dye concentration. The reaction is substantially more efficient in the absence of oxygen, but the quantum yield of strand breaks in air is only 10 8. The results are consistent with the initiation of chain scission by Rose Bengal triplet, with some additional mechanism coming into play in the presence of oxygen. 

The foregoing and, in particular, the reactivity of the 2-oxopyrimidine radical in DMSO, is relevant to the mechanism of photooxidation of the 2-oxopyrimidine dimer to the monomer on irradiation at 254 nm in DMSO 83). The initial step in the latter process would be the formation of a protonated neutral free radical, RH). which is rapidly oxidized (—0.8 V) to RH), followed by deprotonation. 

If the temperature is not too high, the peracid formed is itself the principal product of the reaction, at least in the early stages of oxidation. It may therefore be said that in concentrating their attention on the initial rate of photooxidation of an aldehyde in the liquid phase, the authors have studied the kinetics of the primary reaction—the oxidation of this aldehyde to the corresponding peracid. 

In 1954 McNesby and Davis studied the kinetics of photooxidation (under the action of the 2537 A. line of mercury) of n-heptanal, either as a liquid or in solution in cyclohexane, at a temperature of about 30°C. The rate of oxidation of this aldehyde would, they claimed, be initially proportional to the oxygen pressure Fo and would then become independent of oxygen pressure between 200 and 400 mm. Hg. However, at higher pressures, the rate would once again become proportional to Po,. This latter finding is completely unexpected and has not been observed in any other reaction of aldehyde photooxidation. 

Fig. 2. Influence of the concentration of n-heptanal on its initial rate of photooxidation. Temperature of the liquid phase 3°C. Volume of the liquid phase 5 cm. 7o = 4 X 10 einstein (5 cm. ) min. Solvent w-decane where [ECHO] = 7.45 mole l. corresponds to pure heptanal. (7) P02 = 475 mm. Hg (2) P

Studies of the rate and mechanism of photooxidation of aldehydes in the vapor phase have been particularly concentrated on ethanal and propanal. Here again, to eliminate any possible kinetic influence of the reaction products, the authors have paid particular attention to the initial rate of reaction. 

Up to now, we have only considered the initial phases of kinetics and mechanism of photooxidation of an aldehyde to the corresponding per acid (the main molecular product). If we try to examine these reactions as a function of time, it becomes clear that frequently new processes intervene to complicate the reaction mechanism, as is the case in many chemical reactions. 

According to Melville and his co-workers, the thermal oxidation of n-decanal and benzaldehyde in liquid phase at around 10°C. must also involve a chain mechanism which would differ from that of photooxidation only in respect to the process of initiation in the case of the thermal reaction, the initiation process would be ... 

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