Concerted effects of substituents in the reaction of .OH radicals with aromatics: the cresols

The concerted effects of hydroxyl and methyl substituents in controlling the site of .OH radical attack on aromatics in aqueous solutions are explored using the cresols as typical examples. The distributions of dihydroxytoluenes produced in the radiolysis of aqueous solutions of the cresols containing ferricyanide as a radical oxidant were examined by capillary electrophoretic and liquid chromatographic methods. Because .OH is a strong electrophile, it adds preferentially at the electron-rich sites of an aromatic ring. As a result, the observed distributions of dihydroxytoluenes reflect the charge distributions in the cresols. It is shown that in the case of m-cresol the hydroxyl substituent has a dominant ortho-para directing effect similar to that observed for phenol. In o- and p-cresol, this effect is modified, indicating that the methyl substituent has a significant effect on the electronic structure of those cresols. Correlation of the charge distribution in the cresols indicated by the observed distribution of dihydroxytoluenes with the unpaired spin distribution in the corresponding methylphenoxyl radicals demonstrates that the electronic structures of o- and p-cresol and their corresponding phenoxyl radicals are similarly affected by hydroxyl and methyl substitution. Addition of .OH at the methyl-substituted positions of o- and p-cresol to produce o- and p-dienone is also reported. The observation of these dienones demonstrates that addition of .OH at the ipso positions of alkylated aromatics can be of considerable importance. Mass spectrometric studies show that these dienones have relatively higher proton affinities than their isomeric analogues.