Hydrolysis Methoxychlor

Methoxychlor. Methoxychlor is strongly adsorbed to the soil and does not leach, and volatilization is slow. There is no evidence for oxidation, and although photolysis is rapid in aquatic systems, it is assumed not to occur in the soil environment. The hydrolysis half-life is a year in aquatic systems (33) and probably longer in soil systems because of adsorption. Biodegradation does occur in soil systems, however, with a half-life of from 1 to 3 weeks (34). Methoxychlor would not persist in the soil environment. 

The apparent rate of hydrolysis and the relative abundance of reaction products is often a function of pH because alternative reaction pathways are preferred at different pH. In the case of halogenated hydrocarbons, base-catalyzed hydrolysis will result in elimination reactions while neutral hydrolysis will take place via nucleophilic displacement reactions. An example of the pH dependence of hydrolysis is illustrated by the base-catalyzed hydrolysis of the structurally similar insecticides DDT and methoxy-chlor. Under a common range of natural pH (5 to 8) the hydrolysis rate of methoxychlor is invariant while the hydrolysis of DDT is about 15-fold faster at pH 8 compared to pH 5. Only at higher pH (>8) does the hydrolysis rate of methoxychlor increase. In addition the major product of DDT hydrolysis throughout this pH range is the same (DDE), while the methoxychlor hydrolysis product shifts from the alcohol at pH 5-8 (nucleophilic substitution) to the dehydrochlorinated DMDE at pH > 8 (elimination). This illustrates the necessity to conduct detailed mechanistic experiments as a function of pH for hydrolytic reactions. 

Many xenobiotics contain alkyl groups, such as the methyl (-CH3) group, attached to atoms of O, N, and S. An important step in the metabolism of many of these compounds is replacement of alkyl groups by H, as shown in Figure 7.6. These reactions are carried out by mixed-function oxidase enzyme systems. Examples of these kinds of reactions with xenobiotics include O-dealky-lation of methoxychlor insecticides, N-dealkylation of carbary 1 insecticide, and S-dealkylation of dimethyl mercaptan. Organophosphate esters (see Chapter 18) also undergo hydrolysis, as shown in Reaction 7.3.12 for the plant systemic insecticide demeton ... 

Furthermore as a result of industrial emissions the Teltow Canal sediments are highly polluted by the pesticides DDT and methoxychlor, accompanied by several metabolites (Schwarzbauer et al. 2001). Accordingly, numerous DDT-related compounds were identified in the hydrolysis extracts including DDE 6, DDMU 7, DDNU 8 and DDM 9. Highest concentrations were observed for DDA 10 and DBP IT, the more polar degradation products of DDT. 

Further investigations revealed not only the presence of DDT and its metabolites in Teltow Canal sediments but also the occurrence of 2,4 and 4,4 -methoxychlor (MDT) at elevated concentrations (up to 1100 pg/kg) (22). As methoxychlor is structurally related to DDT, MDT-related compounds were also included in the quantitative analyses (see Tab. 6). We detected MDT, MDD, MDE, MDB and MDA in the extracts and partly in the hydrolysis product mixtures of all four sediment samples. The total amounts ranged between 600 and 8000 pg/kg in the extracts and between 1200 and 6000 pg/kg after application of the hydrolysis procedure. In degradation products after BBr3-treatment and RuC -oxidation no MDT-related substances were analysed likely due to the lower concentration level as compared to the DDT-related compounds. 

Furthermore, numerous methoxychlor-related compounds were detected not only in the extracts but also in parts of the hydrolysis products. 

Figure 2.3. Proposed reaction pathway to account for methoxychlor hydrolysis products (Adapted from Wolfe et al., 1977).

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