Autoxidation of p-toluic acid catalyzed by cobalt bromide

Abstract

The liquid-phase autoxidation of p-xylene proceeds with comparative ease in the presence of cobalt or manganese salts at 100–130°C to p-toluic acid. The oxidation of p-xylene to terephthalic acid in high yield may be achieved by using the cobalt or manganese salt in combination with an inorganic bromide. The present study, which was directed towards the elucidation of the latter reaction, was confined to the second part of the reaction, namely, the oxidation of p-toluic acid catalyzed by cobalt bromide. The reaction was observed to occur in glacial acetic acid at 90°C in two well-defined stages each of which was investigated. The first stage involved catalysis by both cobalt and bromide and during this phase of the oxidation bromide was present in the ionic form. The rate was proportional to the [cobalt][bromide] and independent of the substrate concentration above a limiting value and of the oxygen pressure in the range 2–32 cm Hg. At the onset of the second stage of the reaction the bromide ion was absent and the reaction was catalyzed by cobalt salt and p-bromomethyl benzoic acid. The principal intermediate in both stages was terephthaldehydic acid. Initiation occurred by the formation of atomic bromine either by direct electron transfer between cobaltic and bromide ions or by reaction of hydroperoxides with Br^– or HBr. The aldehydic intermediate and final acid were formed from peroxidic intermediates.