Acidic permanganate oxidation of sulfamethoxazole by stepwise electron-proton transfer

Permanganate is a versatile chemical oxidant, and has undergone a dramatic evolution toward deep insight into its reaction mechanism. However, the hydrogen abstraction of the N-H bond by permanganate remains unclear. We studied the permanganate oxidation of the emerging micropollutant sulfamethoxazole in acidic aqueous solution. The reaction followed autocatalytic kinetics and demonstrated first-order with respect to each reactant. The presence of HMnO4 accelerated the reaction rate, which was four orders of magnitude higher than that of MnO4-. Based on the identified products, the rate-limiting step was determined to be simple N-H bond oxidation by metal-oxo species permanganate. The mechanism was then studied computationally by density functional theory (DFT) using ammonia as the simplest model. Results showed that the N-H bond oxidation by MnO4- (32.86 kcal/mol) was a concerted mechanism similar to that of C-H bond oxidation, whereas HMnO4 oxidation of the N-H bond (10.44 kcal/mol) was a stepwise electron-proton transfer. This reminds us that coordination of Bronsted acid could not only produce the stronger electrophile but also change the reaction mode by avoiding the bond cleavage in electron transfer process. (C) 2019 Elsevier Ltd. All rights reserved.