Degradation of the Pesticide Fenitrothion as Mediated by Cationic Surfactants and α-Nucleophilic Reagents
journal contributionposted on 10.10.2006, 00:00 by Xiumei Han, Vimal K. Balakrishnan, Gary W. vanLoon, Erwin Buncel
The reaction of fenitrothion with a series of α-nucleophile oximates having pKa values in the range of 7.7−11.8 was studied both in the absence and presence of cetyltrimethylammonium (CTA+) surfactants. Reaction with CTA−oximates was found to proceed through two pathways: SN2(P) and SN2(C); an SNAr pathway was not observed. Accordingly, the observed rate constants were dissected into the two corresponding SN2(P) and SN2(C) pathways. Use of the pseudophase ion exchange (PPIE) model for micellar catalysis in the CTA+ system allowed evaluation of micellar second-order rate constant (k2m) parameters and binding constants, (KS). KS values for CTA−oximates were found to vary with the counterion, and the rate enhancement depended on a combination of KS and k2m values. k2m/k2w values ranged from 0.0025 to 0.64, suggesting that a concentration effect is mainly responsible for the rate enhancement. In the absence of surfactant, an α-effect (i.e., kα/knormal) varying from 8 to 450 was observed for the oximate reaction, decreasing with increasing pKa. It is proposed that differential solvation (transition-state imbalance) is a cause of the α-effect in this system.