Pulse Radiolysis and Electron Spin Resonance Studies on the Formation of Phenoxyl Radicals by Reaction of OH Radicals with Methoxylated Phenols and Hydroxybenzoic Acids

Abstract

AbstractOH radicals react with methoxylated phenols and methoxylated hydroxybenzoic acids by addition to the aromatic ring to yield hydroxycyclohexadienyl radicals (OH adducts) (k = (1—2) · 1010 M−1 s−1). With respect to the sites of attachment, OH shows a preference for ring positions activated by the electron donating OH or OCH3 groups. Only those hydroxycyclohexadienyl radicals which are produced by addition of OH to ring positions not occupied by methoxyl groups yield phenoxyl type radicals. Formation of phenoxyl radicals is enhanced in the presence of acid or base. Acid catalysis of phenoxyl formation is suggested to proceed via cationic intermediates similar to those proposed for formation of radical cations. In alkaline solution, OH− is eliminated from deprotonated hydroxycyclohexadienyl radicals to yield phenoxyl radicals (k > 106 s−1).—The phenoxyl radicals undergo reaction with a) each other (2k ∼ 1 · 109 M−1 s−1), b) OH adducts (k ∼ 1.3 · 109 M−1 s−1), and c) H adducts (k ∼ 4 · 109 M−1 s−1). The absorption maxima of the isomeric methoxyphenoxyl radicals are blue‐shifted relative to those of radical cations from structurally analogous di‐ and trimethoxybenzenes.