Photo-oxidation, alkanes

The nitroxide radical (from processes 5 and 6 and attack by other radicals on the parent piperidine) is found in photo-oxidizing PPH samples in concentrations of M. x 10 M (initial piperidine level 5 x 10-3M) up to the embrittlement point of the PPH film (7.). Nitroxides are well known to scavenge carbon centered radicals (but not peroxyl radicals) in both polymers and liquid alkanes (reaction 7) (10, 8). In the liquid phase k7 is... 

The photo-oxidation of a solid branched alkane can be expected to proceed in localized domains, new oxidation chains being generated from the photo-cleavage of -00H products, and chain propagation (reactions 2 and 3) being concentrated close to each initial site in a given domain to produce a zone of high -00H concentration. Thus the distribution of an additive in and around these domains is of special importance. 

Recently, detailed kinetic studies of the hybrid[type II , 02 - type RH] photo-oxidations of cyclohexane and cyclohexane-dn in both NaY and BaY have been reported. A kinetic isotope effect kulko of 5.7 was determined for X > 400 nm in BaY. This substantial isotope effect, which is nearly identical to the isotope effect on the kinetic acidity of cyclohexane, requires that the proton abstraction step, k, in the alkane radical cation superoxide ion pair be smaller than the back-electron transfer, k, to regenerate the charge-transfer complex (Fig. 18). If kpT were larger than k, the rate expression, Eq. (A) in Fig. 18, would be reduced to Eq. (B) and only a small isotope effect on et would be anticipated. 

Figure 1. Photo-oxidation of azoalkanes and mercury-photo-sensitized oxidation of alkanes...

Tarlov and coworkers studied the mechanism of photo-oxidation of the alkanethiol monolayers on the silver surface and found that UV irradiation leads first to the scission of the C—S bond and removal of the alkane portion of the monolayer. Remaining sulphur species are then oxidized to produce sulphite and sulphate ions289. As alkanesulphonate peaks were, however, observed in the aged alkanethiol monolayers on the Au(lll) surface (vide supra), the generality of this mechanism is unclear. 

In these expressions, a = ft3/(ft 3 + ft4), (3 = Kq KCi I is the intercept and S the slope of plots according to eqn. (41b). The fact that linear plots resulted forN = 0.5 provided support for the involvement of dissociatively adsorbed oxygen in the alkane photo-oxidation. In general, the experimental data of Formenti et al. could be adequately accounted for. 

The oxidation of alkanes under light irradiation and in the presence of solid metal oxides (Ti02, M0O3, Iu203 etc.) is very interesting and many investigations have been devoted to this process [45]. Examples of photo-oxidations are given in Table III 3 [45f]. 

Giannotti, C., S. LeGreneur and O. Watts. 1983. Photo-oxidation of alkanes by metal oxide semiconductors. Tetrahedron Lett. 24, 5071-5072. 

Sodium tetrachloroaurate cat yzes the photo-oxidation of hydrocarbons in solution. Typical of this is the irradiation with wavelengths >310 nm of aerated solutions of cyclohexane in acetonitrile or methylene chloride. The principal isolable products formed from this treatment are cyclohexanol and cyclohexanone . A mechanism that might involve the formation of a metal peroxo or metal oxo complex has been suggested. Such complexes are known to react with alkanes to yield hydroxy derivatives . The principal organic intermediate is the unstable cyclohexylhydroperoxide that readily decomposes to afford cyclohexanone and cyclohexanol. Further study has shown that the hydroperoxide accumulates during the oxidation . The influence of wavelength (A. 300, 365 or 436 nm) on the reaction has also been studied . Hexane and ethylbenzene can be similarly oxidized to yield analogous products . 

Mechanism for the OH-radical initiated photo-oxidation of alkanes alkoxy radical reactions,... 

The photo-oxidation and other photoreactions promoted by soluble poly-oxometal clusters are also interesting methodologies to functionalize alkanes (eq. (52)) (78). The reactions proceed via a radical mechanism. 

Figure 4.24. Proposed mechanism for photo-oxidation of alkanes.

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