Aqueous trifluoroacetic acid as a medium for organic reactions. II. The oxidation of hydrocarbons by manganese(VII)

Abstract

Ross STEWART and UDO A. SPITZER. Can. J. Chem. 56, 1273 (1978). The oxidation of 11 alkanes, 4 cycloalkanes, and 10 arenes by potassium perrnanganate in aqueous trifluoroacetic acid has been studied. The reaction rates are approximately first order in /in, the antilogarithm of the acidity function -HR, and the active oxidant is believed to be MnO,+, either free or complexed. The rate of abstraction of primary, secondary, and tertiary hydrogen atoms in the C5Hl, isomers is in the ratio 1 :60:2100 and the rate per methylene in normal alkanes increases with increasing chain length. Cyclohexane is the least reactive of the cycloalkanes and the isotope effect in the oxidation of c~c~o -C~D~~ is 4.3 at 2S°C. The mechanism of oxidation of the aliphatic hydrocarbons is believed to be hydrogen-atom abstraction by MnO,+. The oxidation of arenes C6H5Z has a p+ value of -5.29 when appropriate o f values of Z are used, and there is no isotope effect when the reaction rates of C6H5CH3, C6H5CD,, and C6D5CD3 are compared. The mechanism of oxidation of the aromatic compounds is believed to be electrophilic attack of MnO,+ at the activated positions of the ring followed by rapid degradation of the ring system.

Manganese Dioxide–Methanesulfonic Acid Promoted Direct Dehydrogenative Alkylation of sp3C–H Bonds Adjacent to a Heteroatom

“…Two mechanisms have been postulated for the generation of stabilized carbocation 9 through MnO 2 –CH 3 SO 3 H mediated C–H oxidation (Scheme ). The first pathway proceeds through an initial benzylic hydrogen atom abstraction from 1a to 6a or 6b to form free radical 7 ; the radical diverges either via hydroxyl rebound to form 8 or one-electron oxidation to directly generate cationic intermediate 9 . Indeed, employing synthesized 8 as a substrate under the standard oxidation conditions provided the alkylation product 3a in 93% yield (Scheme ), indicating that 8 could be a potential intermediate.…”

Hydrogen Atom Abstraction by Permanganate: Oxidations of Arylalkanes in Organic Solvents

“…Rate-limiting hydrogen atom transfer from the substrate (eq 9) to a permanganate oxo group is consistent with all of the experimental evidence. This is probably the most commonly proposed mechanism for permanganate oxidations of C−H bonds. − , The lack of a solvent effect is characteristic of radical reactions and is not consistent with polar transition states or products. The observed influence of added O 2 is an indicator of radical intermediates (see above).…”

“…Permanganate has been widely used as a strong, easily handled, readily available, and versatile oxidant, reacting with alcohols, alkenes, aldehydes, saturated C−H bonds, and other functionalities . The lack of selectivity of permanganate is due, at least in part, to its ability to react readily by either one-electron or two-electron pathways, and its conversion to even stronger oxidants such as “MnO 3 + ” . The reaction pathway is influenced by solvent, pH, substrate, and other variables, complicating mechanistic understanding.…”

Permanganate Treatment of DNAPLs in Reactive Barriers and Source Zone Flooding Schemes - Final Report

“…However, this idea cannot explain why TECA utilizes MnO 4 -more rapidly than TCA. Reportedly, the permanganate oxidation of alkanes may proceed via abstraction of a hydrogen atom from a tertiary position to yield alkyl hypomanganate ester (Stewart and Spitzer, 1978;Wiberg and Fox, 1963). The presence of a tertiary hydrogen atom in alkanes would determine the rates of MnO 4 -consumption through the initial bond-breaking step.…”