Oxidations of organic compounds by cobaltic salts. Part X. The oxidation of aromatic hydrocarbons

Abstract

The oxidations of a series of aromatic hydrocarbons and of cyclohexane by cobaltic perchlorate in aqueous methy cyanide solution are essentially of the first order with respect to both substrate and oxidant. The general reactivity sequence is: p–NO₂·C₆H₄Me < C₆H₁₂ < PhCH₂·CH₂Ph ∼ PhMe ∼ PhEt < Ph₂CH₂ < p–Bu^t·C₆H₄Me < Ph₂ < naphthalene < anthracene, phenanthrene, and an outer-sphere mechanism of oxidation is suggested. The fast oxidations of biphenyl and of the condensed polycycles are postulated to involve direct abstraction of a π-electron, but the initial attack on the aralkyl hydrocarbons may be on the side-chain, in view of the oxidisability of cyclohexane.

The products obtained from the oxidations of toluene, ethylbenzene, and bibenzyl vary markedly with solvent composition. In 50% methyl cyanide solution the oxidation of toluene is effected exclusively on the side-chain, but in 70% methyl cyanide other products are formed and the proportion of side-chain oxidation is decreased. Only the methyl substituent of p-t-butyltoluene is attacked, but no recognisable product could be obtained from the oxidation of biphenyl. The dependence of the rate of oxidation of toluene on methyl cyanide concentration, and on the acidity of the medium, are very similar to those found for the oxidation of benzyl ethers, and are explained similarly.