Computational chemistry study of the environmentally important acid‐catalyzed hydrolysis of atrazine and related 2‐chloro‐s‐triazines

Abstract

Many chlorine-containing pesticides, for example 2-chloro-s-triazines, are of great concern both environmentally and toxicologically. As a result, ascertaining or predicting the fate and transport of these compounds in soils and water is of current interest. Transformation pathways for 2-chloro-s-triazines in the environment include dealkylation, dechlorination (hydrolysis), and ring cleavage. This study explored the feasibility of using computational chemistry, specifically the hybrid density functional theory method, B3LYP, to predict hydrolysis trends of atrazine (2-chloro-N4-ethyl-N6-isopropyl-1,3,5-triazine-2,4-diamine) and related 2-chloro-s-triazines to the corresponding 2-hydroxy-s-triazines. Gas-phase energetics are described on the basis of calculations performed at the B3LYP/6-311++G(d,p)//B3LYP/6-31G* level of theory. Calculated free energies of hydrolysis (delta h G298) are nearly the same for simazine (2-chloro-N4,N6-diethyl-1,3,5-triazine-2,4-diamine), atrazine, and propazine (2-chloro-N4,N6-di-isopropyl-1,3,5-triazine-2,4-diamine), suggesting that hydrolysis is not significantly affected by the side-chain amine-nitrogen alkyl substituents. High-energy barriers also suggest that the reactions are not likely to be observed in the gas phase. Aqueous solvation effects were examined by means of self-consistent reaction field methods (SCRF). Molecular structures were optimized at the B3LYP/6-31G* level using the Onsager model, and solvation energies were calculated at the B3LYP/6-311++G(d,p) level using the isodensity surface polarizable continuum model (IPCM). Although the extent of solvent stabilization was greater for cationic species than neutral ones, the full extent of solvation is underestimated, especially for the transition state structures. As a consequence, the calculated hydrolysis barrier for protonated atrazine is exaggerated compared with the experimentally determined one. Overall, the hydrolysis reactions follow a concerted nucleophilic aromatic substitution (SNAr) pathway.

Theoretical predictions of isotope effects versus their experimental values for an example of uncatalyzed hydrolysis of atrazine

“…21,37 Sawunyama and Bailey in their computational studies on hydrolysis under acidic conditions have shown an important role of the solvation model. 38 However, the activation energy obtained theoretically was in poor agreement with the available experimental values.…”

“…Therefore, chlorine KIE determination seemed to us to be a complementary and necessary tool for the full description of underlying reaction mechanisms. 27,38 The previously determined value of chlorine KIE for the hydrolysis of atrazine at pH 12 was equal to 1.0069. 27 This value indicated significant carbon-chlorine bond elongation in the transition state and was also in agreement with the preliminary theoretically predicted value.…”

Determination of atrazine and propazine metabolites deemed endocrine disruptors in human seminal plasma by LC–ESI-MS/MS

“…Agriculturally used triazines can undergo biotic and non-biotic degradation through de-alkylation of amine groups at positions 4 and 6 on the triazine ring, dechlorination, de-amination or hydrolysis of the substituent at 2 [9], and to a lesser extent, even ring cleavage [10][11][12][13], to variety of derivatives. Atrazine, propazine and also their derivatives can accumulate in soil and contaminate ground and surface water as a result.…”

“…detected no N-bidealkylation or 2-hydroxylation metabolites in any of the previous systems. However, propazine 2-hydroxy (PP-2OH), which is similarly toxic to the parent triazine, has been identified among the degradation products formed in the natural environment [9]. Because it is hardly metabolized under physiological conditions, PP-2OH may be an effective marker for bioaccumulation.…”

Application of Molecular Modeling in the Optimization of Reactive Cotton Dyeing in Supercritical Carbon Dioxide

“…Molecular modeling has been extensively used for elucidating reactions and reaction mechanisms. − Therefore; it can be a very useful tool for understanding and optimizing the dyeing process of cotton. In this study, molecular modeling was used to elucidate the most plausible reaction mechanism of the dihalogenotriazine dyes (Figure ) with methanol and with cotton as well.…”