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

Abstract

The oxidation of 11 alkanes, 4 cycloalkanes, and 10 arenes by potassium permanganate in aqueous trifluoroacetic acid has been studied. The reaction rates are approximately first order in hR, the antilogarithm of the acidity function −HR, and the active oxidant is believed to be MnO3+, either free or complexed. The rate of abstraction of primary, secondary, and tertiary hydrogen atoms in the C5H12 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 cyclo-C6D12 is 4.3 at 25 °C. The mechanism of oxidation of the aliphatic hydrocarbons is believed to be hydrogen-atom abstraction by MnO3+. The oxidation of arenes C6H5Z has a ρ+ value of −5.29 when appropriate σ+ values of Z are used, and there is no isotope effect when the reaction rates of C6H5CH3, C6H5CD3, and C6D5CD3 are compared. The mechanism of oxidation of the aromatic compounds is believed to be electrophilic attack of MnO3+ at the activated positions of the ring followed by rapid degradation of the ring system.