Permanganate oxidations of aromatic hydrocarbons in aqueous and organic solution

Abstract

The oxidations of a select group of arylalkanes by permanganate have been studied in aqueous solution and in toluene. In water at pH 7, the reactions of toluene, ethylbenzene, and cumene with KMnO$\sb4$ were studied. In toluene, $\rm\sp{n}Bu\sb4NMnO\sb4$ was used as the oxidant in the reactions of toluene, ethylbenzene, cumene, diphenylmethane, triphenylmethane, fluorene, 9,10-dihydroanthracene, and xanthene. The kinetics, which were monitored by UV/vis spectroscopy, show that the reactions, regardless of solvent, are first order with respect to both permanganate and substrate concentration, with no apparent induction periods. Despite these similarities, the reactions proceed by different mechanistic pathways, depending on the solvent.The oxidation of neat toluene by $\rm\sp{n}Bu\sb4NMnO\sb4$ exhibits a significant isotope effect and the presence of O$\sb2$ accelerates the disappearance of permanganate. Substitution at the para position of toluene has little effect on the rate of reaction. The products of toluene oxidation are benzoic acid with a trace of benzaldehyde. Permanganate is converted in all cases to colloidal MnO$\sb2$ which was confirmed by an iodometric determination of the oxidation state. In the reactions of dihydroanthracene and fluorene, the MnO$\sb2$ is consumed in a subsequent reaction that appears to form a charge transfer complex. All of the data are consistent with rate limiting hydrogen atom transfer from the substrate to an oxo group of permanganate. The enthalpies of activation for the various substrates are shown to be directly proportional to the $\rm\Delta H\sp\circ$ of the hydrogen atom transfer step, over a range of 14 kcal/mol in C-H bond strength. This type of correlation is typical of organic radical reactions. Permanganate's ability to react by hydrogen atom transfer is rationalized on the basis of its ability to form a strong bond to H$\cdot.$In neutral aqueous solution, toluene is oxidized by permanganate over three orders of magnitude faster than in neat toluene. The reaction rate shows no dependence on pH (over the range 6 to 11), buffer type, or the presence of O$\sb2.$ Substitution in the para position has only a slight effect on the rate constant. Above pH 11, the reaction accelerates dramatically. There is a large primary isotope effect. As in the neat toluene reaction, the products are predominantly benzoic acid and colloidal MnO$\sb2.$ Based on this evidence, a mechanism involving hydride transfer from the benzylic C-H to an oxo group of permanganate with stabilization of the incipient carbocation by the nucleophilic solvent is proposed. Studies of ethylbenzene and cumene under the same conditions, however, yield results which are difficult to rationalize with this mechanism. Specifically, these reactions show a dependence of the rate of permanganate disappearance on the presence of O$\sb2.$ This has raised doubts about the generality, of the proposed mechanism. Possible explanations for the observations are discussed.