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 h R , the antilogarithm of the acidity function −H R , and the active oxidant is believed to be MnO 3 + , either free or complexed. The rate of abstraction of primary, secondary, and tertiary hydrogen atoms in the C 5 H 12 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-C 6 D 12 is 4.3 at 25 °C. The mechanism of oxidation of the aliphatic hydrocarbons is believed to be hydrogen-atom abstraction by MnO 3 + . The oxidation of arenes C 6 H 5 Z 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 C 6 H 5 CH 3 , C 6 H 5 CD 3 , and C 6 D 5 CD 3 are compared. The mechanism of oxidation of the aromatic compounds is believed to be electrophilic attack of MnO 3 + at the activated positions of the ring followed by rapid degradation of the ring system.