Benzene, photo-oxidation

Kemula and Grabowska [334] on the other hand found o-benzoquinone and phenol in the products of benzene photo-oxidation by oxygen. These workers consider that the new band at 2780 A (Fig. 18) belongs to the... 

Has been purified by co-distillation with ethylene glycol (boils at 197.5°), from which it can be recovered by additn of water, followed by crysm from 95% EtOH, benzene, toluene, a mixture of benzene/xylene (4 1), or EtjO. It has also been chromatographed on alumina with pet ether in a dark room (to avoid photo-oxidation of adsorbed anthracene to anthraquinone). Other purification methods include sublimation in a N2 atmosphere (in some cases after refluxing with sodium), and recrystd from toluene [Gorman et al. J Am Chem Soc 107 4404 1985]. 

Similar photo-oxidation occurs with increasing readiness as the number of benzene rings in the tin (rings joined successively in the same line) hydrocarbon increases, i.e. as its overall aromatic character decreases this occurs so readily with, for example, the very dark green hydrocarbon hexacene (105),... 

The deficiency of tlie LH equation was demonstrated also for pentachlorophenol [25] and by Cunningham [26-28]. The quantum yields for photodegradation of salicylate and other strongly sorbing substituted benzenes were less than those measured for nonsorbing chlorophenols. For salicylic acid, which chemisorbs, the photo-oxidation rates in air-saturated solutions are independent of the salicylic acid concentration, although the surface excess increases with the concentration [29]. 

Surprisingly, that may work. In order to establish the possible stability of the biimidazoles in a window environment, we poured a benzene solution of the dimer of 2-(p-methoxyphenyl)-4,5-diphenylimidazole into a hollow glass brick, and put in a rubber stopper to seal the system. We positioned the glass brick in a window, and watched it turn blue during the day and less blue when the sun set. It worked for several months but by then, we had turned our attention to photo-oxidation. 

As research into gaseous photocatalysis progressed a potential major disadvantage was the possibility of catalyst deactivation. Einaga et al. [208] concluded that the key factors which influenced catalyst deactivation were the formation of carbon deposits on the photocatalyst and their decomposition to CO. The photo oxidation rate of benzene decreased with decreasing humidity due to the increasing amount of carbon deposits on the catalyst, however, photo irradiation in humidified air decomposed the deposits and regenerated the catalyst [208]. 

Cycloaddition is the major mode of photoreaction between ethylenes and naphthalenes, and the product (58) from (3-naphthol and acrylonitrile has been photo-oxidized in benzene solution in the presence of mercuric oxide and iodine to give (59) in 40% yield." Formation of (59) involves a novel rearrangement and provides a convenient two-step entry into homotropones. 

Intramolecular Cyclization Reactions This Section is concerned with photo-induced cyclization reactions of aromatic compounds in which the final product is also an arene. Examples of intramolecular cyclization in which the product is a stable aliphatic system (e.g. intramolecular meta photocycloaddition of ethylenes to benzenes) are considered in Section 3 of this Chapter. The present type of process occurs for a wide variety of systems and may involve arene-arene and arene-ethylene photo-oxidative cyclization or reactions proceeding by loss of halogen acid. For several years now one of the major interests in these processes has been in their use as synthetic procedures and again within the review period there have appeared a number of accounts which illustrate this feature. Stilbene-... 

Sudhakar and Katz have devised an improved route to [7] helicene. The bromine atom is found to direct the stilbene cyclization away from occupied positions and those ortho to the substituent so that with (153) the bromo [7] helicene (154) is formed in 75% yield on irradiation of benzene solutions containing iodine, and less than 10% of the planar arene (155) is observed. Such results compare very favourably with those from irradiation of the hydrocarbon (156) when equal quantities of the [7] helicene and (157) are formed.Irradiation of (158), the meta isomer of (156), cyclizes selectively without bromine direction and (159) is formed in 75% from xylene solution in the presence of oxygen and iodine The origins of this selectivity may not be readily evident and it is difficult to appreciate why of the four possible conformers of the first intramolecular photo-oxidative cyclization product only (160) undergoes further reaction. The authors suggest that this feature reflects that of the four conformers (160) has the smallest degree of steric interactions between the two aryl moieties. The photo-cyclizations of halostilbenes have been examined by other workers who report that various o-chloro and bromo-compounds (161) undergo... 

By concluding, the photo-oxidation of toluene mainly to benzeldehyde and benzene (as a transient product) was proved to occur in mild conditions in gas-soHd regime. The presence of sites with different oxidant properties is proposed and a preliminary FTIR investigation indicates a weak interaction between toluene molecules and Ti02 surface. 

Hamilton, J.F., A.C. Lewis, C. Bloss, V. Wagner, A.P. Henderson, B.T. Golding, K. Wirtz, K, M. Martin-Reviejo and M.J. Pilling Measurements of photo-oxidation products from the reaction of a series of alkyl-benzenes with hydroxyl radicals during EXACT using comprehensive gas chromatography. Atmos. Chem. PAys., 3, (2003) 1999-2014. 

Irradiation of 2-phenylbenztriazole in aerated solvents leads remarkably to cleavage of the benzene ring and formation of 2-phenyl-l,2,3-triazole-4,5-dicaiboxylic acid (Amiet et al.). Ylidions, a new class of reactive intermediates, have been generated by photo-oxidation of phenacylsulphonium ylides using 9,10-dicyanoanthracene as sensitizer (Zhang and Schuster). 

The observed photo-oxidative degradation of ci s-l,4-poIyisoprene in benzene solution sensitized by benzophenone may be represented as... 

Rabek [498, 505] examined the rapid degradation of cis-1,4-polyisoprene on exposure to light in the presence of p-quinones p-quinone, chloranil, 1,4-naphthoquinone, anthraquinone, phenanthrene-quinone and 1,2-benzanthraquinone (Fig. 20). Later, Rabek [504] also found that p-quinone sensitizes the photo-oxidation of polystyrene in benzene in the presence of UV radiation. 

Rabek [504] found that anthracene sensitizes the degradation and photo-oxidation of polystyrene in solution. It also sensitizes the degradation of cis-polybutadiene in benzene solution. 

In the presence of FeClj, photo-oxidation of trisubstituted cyclic olefins in pyridine-benzene has been reported to give gem-dichloroketones from which methyl ketones possessing a terminal triple bond can be obtained by dehydrochlorination. For example, under these conditions 1-methylcyclohexene has been converted into (43), an important intermediate in the preparation of brevi-comin (44). These reactions have been interpreted in terms of a long-range... 

Irradiation of o-nitrobenzaldehyde in benzene gives o-nitrosobenzoic acid ((/) 0.5) in a process that occurs via a triplet state having a lifetime of 0.6 ns, and the transient enol (49). A mechanism is proposed and this is outlined in Scheme 9. The kinetics of the photo-oxidation of benzaldehyde have been studied in the liquid phase and the rate constants extracted found to be in close agreement with those obtained from the photodecomposition of PhC020H. ... 

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