Aromatic chlorination by peroxodisulphate and chloride ions. Cation radical trapping by copper(II) chloride

Abstract

Details are presented of the factors governing chlorination of aromatic molecules via formation of the corresponding cation radicals, produced by peroxodisulphate oxidation. Moderately stable cation radicals (e.g. from naphthalene or anisole) react directly with chloride ion to give quantitative yields of mononuclear chlorinated hydrocarbon. With less reactive substrates (e.g.benzene, toluene) no nuclear chlorination occurs, although with the latter high yields of side-chain chlorinated product are obtained. For polymethylated benzenes (xylenes, mesitylene), nuclear or side chain chlorination occurs according to the relative stabilities of the aromatic cation radical and the corresponding benzylic radical formed by deprotonation. Side-chain chlorination is thought to involve coupling of benzylic radicals with chloride ion. Catalytic amounts of copper(II) chloride dramatically change the reaction course giving entirely ring chlorinated products and it is proposed that copper(II) chloride is a highly efficient scavenger of aromatic cation radicals.