Aromatic compounds free radical attack

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

Evans et al. (10) have shown that OH radicals produced by the Fenton reagent can initiate the polymerization of vinyl compounds and that the OH radicals, which in the first instance attack the double bond of the monomer, are built into the polymer chain. Stein and Weiss (45) have used the hydroxylation of benzene and of other simple aromatic compounds (e.g., benzoic acid and nitrobenzene) to detect these radicals. In the action of OH radicals on benzene in aqueous systems the formation of phenol and of diphenyl indicated a free radical mechanism of the following type ... 

Numerous publications have appeared within the year which formally describe light-induced substitution of an aromatic compound. As in previous years, the current account is restricted mainly to reactions in which the light-absorbing species is the aromatic compound and the aromatic ring is directly involved. Reactions in which the substituent undergoes chemical change are only briefly mentioned, and accounts of attack of photo-generated free radicals on the aromatic species are not included. 

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