Hydrogen photochemical decomposition

Thermal and photochemical decomposition of peroxides (4) and (5) lacking a-hydrogens (those derived from ketones) produces macrocycHc hydrocarbons andlactones (119,152,153). For example, 7,8,15,16,23,24-hexaoxatrispiro [5.2.5.2.5.2] tetracosane (see Table 5) yields cyclopentadecane and oxacycloheptadecan-2-one. 

Electrolysis, and thermochemical and photochemical decomposition of water followed by purification through diffusion methods are expensive processes to produce hydrogen. 

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

The new method of Yoneda s group132 is also a one-pot diazotization-fluoro-de-diazoniation in a liquid-liquid two-phase mixture of pyridine and hydrogen fluoride. Yields for 25 aromatic amines and diamines are 50-100%, except for 2-and 3-fluorobenzoic acid, the three nitroanilines, 3- and 4-diaminobenzene and 4,4 -diaminodiphenyl-oxide (10-50%). In their 1994 paper the authors demonstrate that, in the same system, the photochemical decomposition gives in many cases significantly higher yields than the thermal reaction. The most spectacular increase in yield was found for the fluorination of 2-fluoroaniline where o-difluorobenzene was obtained photochemically in 80.2% yield, but thermally only in 0.6% ... 

A completely different approach was used to probe the reactivity of tert-butyl-carbene, one of Frey s original examples. Table 7.6 shows the varying products of thermal and photochemical decomposition of the diazo compound. It would appear that carbon-hydrogen insertion and carbon-carbon insertion are about equally facile in the carbene presumed to be formed in photolytic reactions. Even in 1964, this observation should have seemed strange (as it clearly did to... 

Photochemical decomposition of trifluoromethanethiol (F3CSH) on irradiation yields hydrogen, sulfur, fluoroform, XLIX, and XLVII, by... 

Newman158 found that photochemical decomposition of 3,5-diphenyloxadiazole occurred in ether to give JV-benzoylbenzamidine in 31% yield. In contrast to thermal decomposition [Eq. (42)] cleavage occurred at the weak N—O bond [Eq. (43)]. Ether furnished the extra hydrogens found in the product. 

The complete mechanism either for the mercury sensitized or for the direct photochemical decomposition of ammonia is complex and there exists reasonable doubt about some of the steps even after many years of intensive work. Under static experimental conditions hydrogen and nitrogen are virtually the only products and they are formed in the ratio of 3 1. Other products must, therefore, be of negligible importance under ordinary conditions. 

Tetrazoles exhibit qualities of acids, bases, acceptors of hydrogen bonds (cf. Section 6.07.4.5), and polydentate ligands (cf. Section 6.07.5.3.4). NH-LJnsubstituted tetrazoles behave both as substrates and intermediates in transacylation processes (cf. Section 6.07.5.4), etc. Tetrazolate anions (tetrazolides) possess high aromaticity and reactivity toward electrophilic reagents (cf. Sections 6.07.4.1 and 6.07.5.3.2). The thermal and photochemical decomposition of tetrazoles involves formation of nitrenes and other intermediates of high reactivity (cf. Sections 6.07.5.2 and 6.07.5.7) These properties provide a possibility of use tetrazoles as catalysts in chemical and biochemical reactions. 

In contrast, photochemical decomposition of (115) broke the ring at the weakest bond, N—O. The extra hydrogen atoms in the product came from the solvent. Oxadiazolidines break at the same N—O bond <76AHC(20)65). 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

Thermal or Photochemical Decomposition of Hydrogen Peroxide Hydrogen peroxide decomposition can occur by homolytic or heterolytic scission... 

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