Benzene photosensitized decomposition

A different pathway appears to operate in the benzene-photosensitized decomposition of the sulfolene (36)38 formation of the diene (37) as the major product is in accord with the prediction of concerted conrotatory fragmentation in the excited state. Fragmentation in a vibrationally excited state would yield the alternative diene (38) as the major product. Similar observations38 in related sulfolenes support this conclusion. 

Triplet-benzene-photosensitized decomposition for triplet yield (70,134). 

The tests for a triplet intermediate in the photolysis of cyclobutanone at 2537 A by Schlag et gave negative results. However, in the direct photolysis of cyclobutanone at 2650 A, cw-butene-2 was found to isomerize . Quenching by cis-butene-2 of product formation in the benzene-photosensitized decomposition of cyclobutanone also supports the role of the triplet state in the photochemistry of cyclobutanone. 

Photosensitized decomposition of 9b in substituted benzenes led to similar results [83DIS(B)(44)1113]. Therefore, either spin inversion from the triplet to the singlet form of 4-diazoimidazole is faster than the decomposition of the excited diazo compound, or intersystem crossing from the triplet to singlet carbene is easier and faster than the reaction of the triplet state with substrates. 

The mercury-photosensitized decomposition of benzene has been investigated by Scott and Steacie . The main initiation step appears to involve energy transfer, viz. 

The main conclusion to be drawn from the application of the benzene photosensitization method to the decomposition of cyclobutanone is that the Cj-hydrocarbons originate from the low-lying triplet state of the ketone. However, use of this method in the investigation of cyclopentanone decomposition indicated that reactions I, II and III (if it is a separate primary process) occur from the first excited state of the ketone. This conclusion was based on the quantitative agreement found between the pressure dependence of the decarbonylation-product formation and the fluorescence quenching by cyclopentanone. 

By competitive methods Ishikawa and Noyes (226) (sensitized biacetyl phosphorescence) and Cundall and Davies (159) (butene-2 isomerization) both estimated the triplet lifetime to be of the order of 10 ys. In a reexamination of the butene-2 system, Lee (227) estimated a value of 100 ys, a finding confirmed by Cundall and Dunnicliff (105). An examination of the kinetics of the benzene photosensitized composition of cyclopentanone decomposition allowed a value of longer than 3 ys to be deduced. This type of experiment is far from satisfactory since photochemical processes can Intervene at low pressures, and impurities and the quenching effects of photoproducts can affect the results. These problems can only be overcome by some form of direct measurement. Parmenter and Ring (228) used a flash method in which 20 torrs of benzene and 0.01 torr of biacetyl were submitted to a 20 J,... 

The 1,3-cycloaddition of ethylenes to benzene and its derivatives, giving rise to dihydrosemibullvalenes, has been known for a number of years and the reaction has recently been reviewed.84 The reaction with cyclopentene as olefin has been the subject of several earlier reports.10 Its light-induced addition to anisole can be used as a stereospecific synthesis of perhydroazulenes 35 the adduct (18) is formed in 85% yield. Its consumption of only one mole of hydrogen on catalytic hydrogenation and its failure to undergo photosensitized decomposition have been... 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

The addition of absolute benzene to solutions of silver nitrate in DMF gave a fine white precipitate with the composition AgN03-2DMF.276 The adduct was hygroscopic and photosensitive. The IR spectrum showed a series of strong bands (Table 40) and indicated that coordination occurred via the O atom. The adduct melted at 210 2°C after which decomposition occurred in the liquid phase. Further heating to 500 °C eventually led to metallic silver being produced. 

R. Taylor. In early experiments we uv irradiated C60 in hexane, but the amount of decomposition was dependent upon the particular batch of C60. Some decomposed completely within 4 h others showed little decomposition after 24 h. Therefore we keep C60 in the dark. Others have reported that the decomposition of C60 may be triggered by an impurity. Indeed deliberate initiation of decomposition has been reported. We have tried halogenation in the light and obtained a product which is different from that isolated under conventional conditions. C70 is also photosensitive it decomposes faster in hexane than in benzene. 

Benzophenone was used as photosensitizer in a study of the photo-decomposition of benzene solutions of polystyrene and poly(a-methyl styrene). Laser photolysis (A 265 nm) of poIy(a-methyl styrene) in chlorofonn and carbon tetrachloride proceeded by attack of solvent radicals in the polymer molecules resulting in main chain cleavage. " A series of papers have been devoted to the study of the photo-induced decomposition of poly(a-methyl styrene) in benzene solutions, using a,a -bisisobutyronitrile, benzophenone, and triphenyl-methyl cations as sensitizers. Spin-trapping techniques were used to detect transient radicals formed during benzophenone-sensitized degradation of poly-(a-methyl styrene). The rate constant for intramolecular reaction of the macroradical was estimated. 

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