Less Persistent Analogs of DDT
A number of compounds having the same general molecular structure as DDT are found to display similar insecticidal properties. This similarity arises from the mechanism of DDT action, which is due more to its molecular shape than to its chemical interactions. The shape of a DDT molecule is determined by the two tetrahedral carbons in the ethane unit and by the two flat benzene rings. In insects, DDT and other molecules with the same general size and 3-D shape become wedged in the nerve channel that leads out from the nerve cell. Normally, this channel transmits impulses only as needed via sodium ions. But a continuous series of Na+-initiated nerve impulses is produced when the DDT molecule holds open the channel. As a consequence, the muscles of the insect twitch constantly, eventually exhausting it with convulsions that lead to death. The same process does not occur in humans and other warm-blooded animals since DDT molecules do not exhibit such binding action in nerve channels.
Examples of other molecules with DDT-like action include DDD (sometimes called TDE), fjara-dichlorodiphenyldichloroethane, which is an environmental degradation product of DDT: It differs only in that one chlorine from the —CCI3 group is replaced by a hydrogen. Since the overall shapes and sizes of DDT and DDD are similar, their toxicity to insects is as well. In the past, DDD was itself sold as an insecticide, but it has also been discontinued because it bioaccumulates. Notice that DDE, unlike DDT and DDD, is based upon a planar C=C unit rather than a C—C linkage which has tetrahedral groups at each end. Thus, whereas DDD is a DDT-like insecticide, DDE is not, since its three-dimensional shape is very different: DDE is flat rather than propeller-shaped, so it does not become wedged in the insect's nerve channels.
Scientists have devised analogs to DDT that have its same general size and shape; consequently, they possess the same insecticidal properties but are more biodegradeable and thus do not present the bioaccumulation problem associated with DDT. The most important of these analogs is methoxychlor:
The para-chlorines of DDT are replaced in methoxychlor by methoxy groups, —OCH3, which are approximately the same size as chlorine but which react much more readily. In particular, under reducing conditions, the O—CH3 bonds in methoxychlor are subject to attack by hydrogen ions in water, converting the O—CH3 units to Q—H bonds and free methane molecules, CH4. The hydroxy-lated products ate water-soluble products that not only degrade in the environment but are excreted rather than accumulated by organisms. Methoxychlor is still used both domestically and agriculturally to control flies and mosquitoes.
Draw the molecular structure of DDD.
Methyl groups are approximately the same size as chlorine atoms, but hydrogen atoms are significantly smaller. Would you expect insecticidal properties for DDT molecules in which (a) the —CC13 group is replaced by —(CH3)3, and (b) the para-chlorines are replaced by hydrogens?
Continue reading here: Toxaphene
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