Hydrolysis of Amides and Phenylamide Pesticides at the Mineral-Water Interface

Abstract

In this study, batch experiments on the hydrolysis of pesticides or pesticide-like amides were performed at the mineral-water interface. The main goal is to elucidate the influence of minerals on surface mediated hydrolytic reactions of amides. First, the results show that hydrolysis of 4’- and 2’-nitrophthalanilic acid is intramolecular catalyzed reaction in acidic solutions (pH < 4.3). The neutral species are the active forms for the intramolecular catalysis. The mass loss was observed during the hydrolysis, and a by-product N-phenylphthalimide may be yielded. For all selected mineral suspensions of SiO2, Al2O3, goethite, kaolinite, hectorite or Na-rich montmorillonite, the observed rate constants of 4’- and 2’-nitrophthalanilic acid at certain pH are of the same order of magnitude as those in the corresponding aqueous solution. Therefore, the investigated minerals have an insignificant effect on the intramolecular catalyzed hydrolysis of 4’- and 2’-nitrophthalanilic acid. In addition, no measurable adsorption was observed on mineral surfaces for 4’- and 2’-nitrophthalanilic acid as well as the hydrolysis products, 4- and 2-nitroaniline. Second, hydrolysis of N-methyl-N-(4-nitrophenyl)-2-pyridinecarboxamide (N-Me-PCA) in mineral suspensions as well as aqueous solutions is general base-catalyzed reaction throughout pH 9.5-11. Mineral surfaces have no strong effects on the hydrolysis of N-Me-PCA, the observed rate constants are only slightly lower in hectorite and Na-montmorillonite suspensions compared with aqueous solution and the other mineral suspensions. N-Me-PCA can adsorb on hectorite and Na-montmorillonite to different extent depending on suspension pH. Third, the hydrolysis of racemic metalaxyl and enantiopure metalaxyl-M in mineral suspensions was also investigated. The reaction kinetics and enantiomeric ratios of R- and S-enantiomer of metalaxyl were measured by enantioselective high performance liquid chromatography (HPLC). For racemic metalaxyl, the hydrolysis of R- and S-metalaxyl follows pseudo-first-order kinetics and is completely base-catalyzed reaction over pH 9.5-12 in aqueous solutions and mineral suspensions. Furthermore, hydrolysis of R- and S-metalaxyl occurs with retention of configuration. Although hydrolysis of R-metalaxyl is slightly faster than that of S-metalaxyl, the chemically mediated hydrolysis is basically nonenantioselective. Hydrolysis rate constants in mineral suspensions are similar to those in the corresponding aqueous solutions, suggesting an insignificant effect of minerals on metalaxyl hydrolysis. In the case of enantiopure metalaxyl-M, a computer model was developed using MATLAB® to describe different processes observed in the hydrolysis experiments. The results show that in mineral suspensions and aqueous solutions over pH 9.5-11 the enantiopure R-metalaxyl is configurationally unstable and readily enantiomerizes to its antipode,S-metalaxyl, while the hydrolysis products, R- and S-MX-acid, are configurationally stable and do not interconvert to each other. The enantiomerization of R- and S-metalaxyl also follows first-order kinetics and is base-catalyzed reaction over pH 9.5-11. The rate constants for enantiomerization are in the range of 22-32% of those for hydrolysis. In addition, there is not much difference in rate constants of enantiomerization between aqueous solutions and mineral suspensions at the same pH, indicating unimportant influence of minerals on chiral conversion of metalaxyl enantiomers.