Oxidations of organic compounds by cobaltic salts. Part VIII. The oxidation of benzyl ethers

Abstract

The oxidations of three benzyl ethers and diphenyl ether in aqueous methyl cyanide solution are essentially first-order with respect to both substrate and oxidant. The overall reactivities are in the sequence PhCH₂OMe < PhCH₂OCH₂Ph < PhCH₂OC₆H₁₁ < PhOPh. The oxidations are faster than those of similar hydrocarbons, but very much slower than that of benzyl alcohol. An outer sphere mechanism of oxidation is suggested. Quantitative analyses show that the oxidation products from the benzyl ethers are benzaldehyde, benzoic acid, and sometimes the corresponding benzoate ester. Attack occurs on the benzylic CH₂ group, and there is negligible ring fission.

The increase in the rate of oxidation with increase in methyl cyanide concentration is interpreted as being due to a highly reactive cobaltic–methyl cyanide complex. This enhanced activity results from a lower activation energy for the oxidation, which outweighs the effect of a less favourable activation entropy. The oxidation rates also depend inversely on the acidity of the medium.