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Nucleophilic substitution reaction of p‐nitrophenyl acetate (PNPA), p‐nitrophenyldiphenyl phosphinate, and pesticide parathion with different α‐nucleophiles [I] have been studied at 27 °C in different pH in the presence of a novel cationic surfactant. The kinetic study was performed spectrophotometrically under pseudo‐first order conditions with the α‐nucleophile in excess. The pKa of nucleophiles have also been determined by kinetic method. In the presence of surfactant, the rate constant increased with increasing surfactant concentration up to a limiting value. This behavior has been analyzed in quantitative terms on the basis of pseudo‐phase model of micellar catalysis. Finally the nucleophilic reactivity of hydroxamate ions has been compared with other α‐nucleophiles, like oxime, hydroxybenzotriazole, and 2‐iodosobenzoic acid (IBA). The order of cleavage of electrophilic centers, that is, CO, PO, and PS have also been discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Pseudo-first-order rate constants have been determined for the nucleophilic substitution reactions of p-nitrophenyl acetate with oxalo, malono, and succinodihydroxamate ions ( 2 ONHC(O)(CH 2 ) n C(O)NHO 2 ) in phosphate buffer (pH 5 7.9) at 278 8 8 8 8C. The rate data of the reaction revealed that the nucleophilic reactivity sequence of these hydroxamate ions is generally ODHA > MDHA > SDHA. The k obs value increases upon addition of cationic surfactants to the reaction medium which is typical behavior of micelle-assisted bimolecular reactions. The pseudo-phase ion exchange model has been successfully applied to determine binding constant.

Kinetic measurements were performed for the hydrolysis of p-nitrophenyl acetate as a model compound in micellar media. This reaction was studied both in the presence and absence of various α-effect nucleophiles, and in depth analyses were performed using acetohydroxamic acid (AHA). Rate surfactant profiles were obtained in the absence and presence of the AHA nucleophile for the alkyl triphenylphosphonium bromides (CnPPh3Br) surfactant series. All rate surfactant profiles were analyzed using the pseudophase model and modified pseudophase model (In the presence of nucleophile) in order to obtain the regression parameters, including binding constants and rate constants in the micellar pseudophase. Studies in the presence of acetohydroxamic acid and the alkyl triphenylphosphonium surfactants were also conducted as a function of pH to obtain the pKa value of the acetohydroxamic acid in the micellar media.

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