Formation of radical cations and zwitterions versus demethoxylation in the reaction of OH with a series of methoxylated benzenes and benzoic acids. An example of the electrophilic nature of the OH radical

Abstract

Using spectrophotometric and conductometric pulse radiolysis, in situ e.s.r. and product analysis techniques, the reaction of OH radicals with mono-, di- and tri-methoxylated benzenes and benzoic acids has been further studied in aqueous solution. It is found that, in addition to radical cations or zwitterions, phenoxyl radicals and methanol are formed. With the methoxylated benzenes the yield of phenoxyl radical is the same as the yield of methanol, and the sum of the yields of radical cation and of phenoxyl radical (or methanol) is ∼100% of G(OH). The ratio of the yield of radical cation to the yield of phenoxyl radical depends on the number and on the positions of the methoxyl groups relative to each other. The yields of phenoxyl radical are low when the methoxyl groups are meta to one another, whereas the yields are high for substrates with methoxyl groups in an ortho or para relation. A similar dependence on substrate structure of the distribution of products, i.e., radical zwitterions, phenoxyl radicals and methanol, is found for methoxylated benzoic acids.

The results are quantitatively interpreted in terms of electrophilic addition of OH to the aromatic ring, the positions of attachment being governed by the ortho-para directing effect of the electron-donating methoxyl groups. H-abstraction from the methyl groups is negligible. The radical cations or zwitterions are derived from OH adducts formed by addition of OH to ring positions not occupied by methoxyl groups, whereas the phenoxyl radicals are produced by elimination of methanol (k∼ 4 × 10⁴ s^–1 at pH 7) from OH adducts formed by attachment of OH to ring carbons carrying methoxyl groups.