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Mechanism of permanganate oxidation of alkanes, arenes and related compounds Spitzer, Udo Anthony

Abstract

The permanganate oxidation of alkanes and arenes has been studied in trifluoroacetic acid (TFA)-water. This solvent system has the unique advantage of being virtually inert to oxidative degradation and yet providing adequate solubilities for the reactants. The mechanistic investigation involved kinetic studies to determine the orders with respect to oxidant, substrate,and acid. Complementary techniques such as product studies, substituent effects, activation parameters, and isotope effects completed the mechanistic investigation. The oxidation of several alcohols, aldehydes, and ketones was also investigated to aid in the interpretation of the results. Because of the poor fit of the kinetic data with the previously determined H[sub O] function for TFA-water the H[sub R] function was determined using the Hammett approach of overlapping indicators. The identity of the oxidants in the acidic medium was established by cryoscopic and spectrophotometric means. It was found that the most vigorous oxidant was permanganyl ion (MnO₃⁺), with some contributing oxidation by both permanganic acid (HMnO₄) and permanganate ion (MnO₄⁻) in the case of easily oxidized compounds such as alcohols, aldehydes, or enols. The mechanism of the acidic permanganate oxidation of alkanes (ethane to n-tridecane) was found to proceed via rate-determining homolytic carbon-hydrogen bond scission as depicted below. [See Thesis for Diagrams] The -mechanism of arene oxidation was shown to proceed via rate-determining electrophilic attack by permanganyl ion on the aromatic ring to yield ring degradation products. Phenols are believed to be intermediates in this process as depicted below. [See Thesis for Diagrams] The mechanisms of the oxidation of alcohols, ketones, aldehydes, and formic acid were determined and shown to be consistent with mechanisms previously established under other conditions. The behaviour of two electrophiles, nitrosonium ion (NO⁺) and nitronium ion (NO₂⁺), generated respectively from sodium nitrite and sodium nitrate, was examined in the TFA-water medium. It was found that the nitronium ions thus generated could be successfully used to carry out electrophilic aromatic nitrations in excellent yields but that the nitrosonium ions were inert. It was also determined that the tetra(n-hexyl)ammonium permanganate salt could be prepared in good yield and used as an oxidant for a variety of substrates in benzene solution.

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