Photochemical reactions aromatic side-chain reaction

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

A parallel series of changes in reactivity is found in the side-chains of aromatic compounds. Whereas in thermal reactions, the hydrolysis of phenolic esters and ethers is faster if there is a para Z-substituent, the corresponding photochemical reactions are faster with a meta Z-substituent,1094 1095 as in the m-nitrophenyl phosphate meta-8.14. 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

Most of the papers in this section are concerned with photo-oxidation reactions, but a few papers refer to photochemical reactions in polymer matrices. Thus the kinetics of the photo-oxidation of anthracene and naphthacene in solid polystyrene (PS),170 the photoionization of aromatic hydrocarbons dissolved in PMMA and PS,171 the photoreactions of naphthalene in cellulose triacetate,172 the cis-trans isomerization of stilbene residues in the side-chains of polymers,173 intrachain photodimerization in polymers,174 photoisomerization of 1,2-diphenyl-cyclopropane by peptides containing naphthalene in a side-chain,176 and photochemical transformations of poly(vinyl p-azidobenzoate)176 have been reported. 

Biochemical applications have been made more recently O, ). Here, cyclic photochemical reactions are employed to generate nuclear spin-polarization in biological macromolecules. The information obtained is of a structural nature rather than mechanistic. In the case of proteins aromatic amino acid residues (tyrosine, histidine and tryptophan) can be polarized by reversible hydrogen atom or electron transfer reactions with a photo-excited dye. These reactions require direct contact of the dye with the amino acid side-chains so that they only occur for residues lying at the surface of the protein. 

The mechanism of reactions of the nitrate radical with aromatic and substituted aromatic compounds has been studied in detail using the photolysis of CAN [43, 44]. The photochemical reaction of CAN (Equation 4.4) with toluene derivatives in acetonitrile was found to produce side-chain nitroxidation products in high yields ... 

A radical species can be generated on an aliphatic side chain. Intramolecular addition to the aromatic system then builds up a heterocycle. This strategy was applied to the synthesis of indanone derivatives (Scheme 15). After photochemical excitation, intramolecular electron transfer in compound 87 took place leading to the intermediate U. Radical combination and deprotonation led to the hnal product 88. In the present case, elimination of HBr occurred as a competing reaction and furnished the methacrylamide 89. This compound underwent photochemical electrocyclization to yield 90. 

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