Other Chloroorganic Pesticides 891 Lindane 8911 Synthesis

Lindane, the 7-isomer of 1,2,3,4,5,6-hexachlorocyclohexane, is prepared by the photochemical initiated addition of chlorine to benzene, as shown in reaction (8-40).

a Cl

In this reaction four of the eight possible isomers of 1,2,3,4,5,6-hexachlorocy-clohexane, are formed in appreciable amounts. These isomers differ in the relative orientations of the chloro substituents. Axial (a) groups are perpendicular to the cyclohexane ring and are labeled in the a-isomer in reaction (840). Equatorial groups are in the equatorial plane of the ring as shown in the p-isomer. Only one of these isomers, the ^-isomer, exhibits significant insecti-cidal action. Initially, the total reaction mixture was used as the insecticide. However, that practice was discontinued because it resulted in the distribution into the environment of large amounts of potentially toxic chlorocarbon compounds, which do not have insecticidal activity.

Initially the use of lindane was banned in the United States, but that ban was lifted, and it is now imported for some specific uses. For example, lindane is used as a soil insecticide and on cotton and forest crops. Lindane is excreted rapidly, and its acute mammalian toxicity is low.

The p-isomer tends to accumulate in mammalian tissues and exhibits chronic toxicity. The volatility of the 7-isomer is high, which makes it useful as a soil insecticide, but this volatility results in its distribution far from the point of use. 1,2,3,4,5,6-Hexachlorocyclohexane accumulates in plankton and fish (presumably the p-isomer), along with DDT residues and PCBs, with the highest concentrations being found in bottlenose dolphins and other animals at the end of the food chain. Environmental Degradation

Lindane is an inert chloroorganic that undergoes a very slow elimination of HCl in aqueous solution in path (a) of reaction (8-41). Presumably this is an El reaction [see reactions (8-14) and (8-17)]. Lindane is also degraded by microorganisms by a combination of reductive and elimination pathways to 1,2,4-trichorobenzene [see path (b) of reaction (8-41)].


Cl Cl

Cl microorganisms (b)

Cl Cl

(and other isomers)

(and other isomers)

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