Environmental Degradation

The first step in the degradation of DDT is the relatively rapid elimination of HCl to form DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene) (reaction 8-37) when the DDT is heated in water. However, the subsequent hydrolysis of DDE to DDA is extremely slow, as indicated in reaction (8-37), because there are only unreactive vinyl and aryl chlorides in DDE.

Reduction

CHClo.

CHClo.

Fast

Slow H2O '

DDD (also known as TDE), a major microbial metabolite of DDT formed by reductive dechlorination, has also been used as an insecticide (Rothane). Because of reactions (8-36) and (8-37), both DDD and DDE accompany DDT and DDA in the environment, so when the mixture of DDT and its degradation products is reported in environmental samples, it is often referred to as DDT.

Methoxychlor and other DDT analogues that do not contain the p,p'-chloro group are much less persistent than DDT in the environment. This is due to their metabolism by soil microorganisms to phenols that are readily degraded to acetate in accordance with reactions (8-38) and (8-39). o,p-DDT (8-38) is known to have estrogenic activity (Section 8.5.1).

CH3O-

Methoxychlor

Further degradation

-OCH3

CCl3

CCl3

Further degradation

The enzymes involved in these oxidations are probably of the cytochrome P-450 type mentioned earlier (Section 8.5). Cytochrome P-450 enzymes are also present in insects. The lower toxicity of methoxychlor (vs DDT) may reflect the effective oxidative detoxification of methoxychlor by some insects.

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