THE ROLE OF PROTONATION, HYDRATION, ELIMINATION, AND RING-OPENING IN THE ELECTROREDUCTION OF HEXAZINONE

The herbicide hexazinone ohexyl-6-(dimethylamino)-1-methyl,3,5-triazin e-2,4 (1H,3H)-dione) is a 1,3,5-triazine derivative which exists in a single cyclic tautomeric form. In the presence of water its azomethine bond is covalently hydrated (K-d = 0.79, in acetonitrile as a predomi nant co-solvent) and protonated (pK(a) = 1.2). Following dehydration, the protonated form of the azomethine bond undergoes a two-electron re duction in wave I-1. The decrease in current at pH > 2.5 results from the decrease in the rate of protonation (k(r) = 6.3 x 10(6) mol(-1) s( -1)). The current at pH 2 to 4 is smaller than theoretical due to a co mpetitive hydration. The two-electron reduction product formed in wave s undergoes elimination of dimethylamine. The resulting azomethine bon d in the product of elimination is reduced in wave I-2. The oxidized f orm of hexazinone undergoes an acid-catalyzed ring opening at pH < 1.3 . The acid catalysis is attributed to a protonation of the hydrated fo rm of the heterocycle. The formyl group of the open-chain form is redu ced in a more negative wave I-1a. Ac polarography indicated adsorption of the oxidized form of hexazinone, dc polarography and voltammetry s how also an adsorption of its reduced form. (C) 1998 Elsevier Science S.A. All rights reserved.