Micellar catalysis of the nucleophilic substitution reactions of organophosphorus compounds: Computational and kinetic studies of dicapthon degradation by o-iodosobenzoic acid.

摘要

Micellar solutions of cetyltrimethylammonium bromide (CTAB) were employed to enhance dicapthon degradation by o-iodosobenzoic acid (IBA) in an attempt to develop systems capable of rapid detoxification of organophosphate pesticides. Both phosphorylation and alkylation were observed by NMR spectroscopy. Although regeneration of IBA was reported by other investigator when reacting with several phosphate esters, nucleophile turnover was insignificant in this system. Consequently, when dicapthon was added in excess of the nucleophile, “burst” kinetics with rapid appearance of a degradation product, 2-chloro-4-nitrophenol (CNP), were seen. This was followed by zero order kinetics which corresponded solely to the spontaneous dicapthon hydrolysis.;The phosphorylation reaction between the two reactants was further investigated with the aid of a computational chemistry study. It was found that the reactive anionic form of IBA appeared to have two nucleophilic sites, the endocyclic (carboxylate) and exocyclic (iodoso) oxygens of the iodoxolone ring. The nucleophilic substitution reactions might occur via either apical or equatorial addition. However, for dicapthon, these two pathways lead to the same reaction end products. Therefore, it was predicted that two phosphorylation reaction end products would be generated. This postulate was consistent with the observations from 1H-NMR spectroscopy, wherein two phosphorylation end products were observed. By analogy, the formation of a methyl ester of the carboxylate group of IBA could also be explained.;Rate equations appropriate for these phosphorylation reactions were derived. The phosphorylation reaction rate constant for dicapthon degradation was investigated as a function of pH; the kinetic pKa of IBA was estimated to be approximately 7.3. This value was consistent with the value determined by spectroscopic and titrimetric methods (7.4 and 7.2, respectively). The initial rates of dicapthon degradation by IBA were obtained as a function of the concentrations of surfactant present as micelles. A pseudophase model was employed to obtain estimates of the substrate-micelle binding constants. A separate binding experiment was employed which utilized the change in the apparent pKa of the nucleophile as a function of surfactant concentration. IBA binding constants obtained from the two methods agreed well (KIBA = 120 and 124 from the kinetic method and the titrimetric method, respectively).