On the limiting stage of oxidation of alkylarenes by potassium permanganate

Abstract

The first stage of hydrocarbon benzyl C–H bond oxidation with permanganate is considered. Using a thermochemical approach, three possible mechanisms of this stage are analyzed: electron transfer from hydrocarbon to oxidant, homolytic abstraction of H-atom from benzyl C–H bond, insertion of O-atom into benzyl C–H bond. The reaction enthalpies were used as a characteristic of the thermodynamic favorability of these reactions. The thermochemical characteristics of the species involved in oxidation reactions and the reaction enthalpies were calculated with the quantum chemical method PM7. It is the most accurate among the semi-empirical methods. The calculated reaction enthalpies of various oxidant species formed during the oxidation of the cumene with KMnO 4 showed that their reactivity increases in the series MnO 4 – < HMnO 4 < H 2 MnO 4 + < MnO 3 + . It is established that the protonation of the oxidant species increases their reactivity both in homolytic reactions, where after the neutral H-atom abstraction free radical is formed, and in heterolytic ones, when electron or hydride ion transfer occurs. The obtained fact of increasing the reactivity of the species in oxidation reactions due to protonation explains the experimentally established significant effect of medium acidity on the rate of permanganate reactions with C–H bonds in hydrocarbon functionalization processes, and also provides understanding of the significant role of acidity in the oxidation of organic matter by permanganate. It should be noted that dehydration (removal of water molecule) of the oxidant species increases its reactivity in oxidation reactions. The results of the calculations of the enthalpies of the reactions show greater thermochemical favorability of hydride ion transfer reactions in comparison with reactions that proceed through homolysis of the C–H bond is shown. Keywords: potassium permanganate, oxidative functionalization, alkylarenes, PM7 method.