Prometryne Oxidation by Sodium Hypochlorite in Aqueous Solution: Kinetics and Mechanism

Abstract

The reaction of the herbicide prometryne (C 9 H 16 N 5 -S-CH 3 ) with sodium hypochlorite has been investigated from the kinetic and mechanistic stand point. Under the fixed experimental conditions : pH = 7, T = 25 °C, [NaClO]/[substrate] = (10 -3 M)/(10 -5 M), prometryne oxidation takes place according to the following pathway : R-S-CH 3 (P) → R-SO-CH 3 (a) → R-SO 2 -CH 3 (b) → R-O.SO 2 -CH 3 (x) → R-OH (c), where R stands for C 9 H 16 N 5 , i.e., the substituted triazine ring of prometryne and (x) is an unexpected intermediate never previously detected nor identified. After having synthesized the pure intermediates (a), (b), and (x), the values of the pseudo-first-order kinetic constants of the first three steps were experimentally obtained : [k 1 = (0.64 ± 0.03) s -1 , k 2 = (1.81 ± 0.05) x 10 -3 s -1 , k 3 = (1.50 ± 0.03) x 10 -4 s -1 ]. As for k 4 , its value [(2.5 ± 0.2) x 10 -5 s -1 ] has been calculated indirectly on the basis of the kinetic theory concerning consecutive reactions. All the steps were pseudo-first-order reactions with respect to their specific substrate. The effect of pH on the hydrolysis of (b) [(b)→(c)], in the absence of NaClO, has been also assessed to better elucidate the mechanism of the overall pathway.