Lindane, structure

For lindane, structural considerations suggest that stepwise 1,2-HCl elimination should occur with relative ease, and that the final product should be 1,3,5-trichlorobenzene. Roberts et al. (1993) included lindane in a QSAR and found close agreement between the predicted and experimentally reported rate constant, with an environmental half-life of about 6 years. The QSAR is useful for compounds that can be analyzed by that approach, especially because the range of reactivity is so vast for the halogenated hydrocarbons. However, the limitations of QSARs are also evident, in that various mechanisms can operate, and a QSAR is developed only for one mechanistic pathway. 

Figure 7. Conformations of the BHC (HCH) isomers and recent analogues of the aaaeee (lindane) structure, some of which have additional biodegradable groups (54)...

Figure 15.7 Structure of hexachlorocyclohexane (lindane) showing the axial and equatorial arrangement of the chlorine atoms in the active isomer.

The most fundamental properties of a chemical substance are those of the substance in pure form, in most cases as a solid or liquid. Molecular mass can be deduced readily from the chemical formula or structure, although a range of values may exist for commercial mixtures. In some cases, the substance may adopt different structural (e.g., cis-trans) or enantiomeric forms, usually with relatively small physical property differences but with potentially substantial differences in ability to induce toxicity or other biological responses. The hexachlorocyclohexane isomers and enantiomers are examples, the insecticide lindane or y HCH being the most active form. 

Lindane is a chlorinated pesticide with the following structure ... 

First, draw out the hydrogen suppressed structure for lindane and assign delta values to each of its atoms, as shown below. For 1%, the delta value for each atom is equal to the number of atoms is bonded to. 

The method is based on the observations that gas-phase OH radical reactions with organic compounds proceed by four reaction pathways, assumed to be additive H-atom abstraction from C-H and O-H bonds, OH radical addition to >C=C< and -C=C-bonds, OH radical addition to aromatic rings, and OH radical "interaction" with N-, S-, and P-atoms and with more complex structural units such as ->P=S, >NC(0)S- and >NC(0)0- groups. The total rate constant is assumed to be the sum of the rate constants for these four reaction pathways (Atkinson, 1986). The OH radical reactions with many organic compounds proceed by more than one of these pathways estimation of rate constants for the four pathways follow. Section 14.3.5 gives examples of calculations of the OH radical reaction rate constants for the "standard" compounds lindane (y-hexachlorocyclohexane), trichloroethene, anthracene, 2,6-di-ferf-butylphenol, and chloropyrofos. 

As examples of the calculation of OH radical reaction rate constants using the method discussed above (Kwok and Atkinson, 1995), the OH radical reaction rate constants for lindane [y-hexachlorocyclohexane cyclo-(-CHCl-)6], trichloroethene (CHC1=CC12), 2,6-di-tert-butylphenol, and chloropyrofos appear below. As the section dealing with OH radical addition to aromatic rings mentions, at present the rate constant for the reaction of the OH radical with anthracene (and other PAH) cannot be estimated with the method of Kwok and Atkinson (1995). In carrying out these calculations, one first must draw the structure of the chemical (the structures are shown in the appendix to Chapter 1). Then one carries out the calculations for each of the OH radical reaction pathways which can occur for that chemical. 

ICI Scientists working on lindane received the news of the new Swiss insecticide, but not its structure, around Christmas time in 19 3 So similar were the reported insecticidal properties of DDT to those of lindane that there was speculation as to whether the two compounds were variants of the same chemical. 

The resemblance between lindane and the cyclodiene structure is particularly striking if one compares models of lindane and the photoisomer of the molecule SD (Figure 8), in which the S-chlorine is replaced by hydrogen and the usual double bond is absent ... 

The crystals composing the deposits of lindane and dieldrin were upright and easily dislodged by the insect, though markedly different in structure (Figure 1). Lindane crystals were platelike, usually 20-200/x wide and 50-300/x long. Dieldrin crystals were extremely fine and needlelike, usually 20-500/ long and less than 5/x wide. 

Many other chlorinated insecticides have been developed. Some of them also accumulate in the environment, gradually producing toxic effects in wildlife. Others can be used with little adverse impact if they are applied properly. Because of their persistent toxic effects, chlorinated insecticides are rarely used in agriculture. They are generally used when a potent insecticide is needed to protect life or property. For example, lindane is used in shampoos to kill lice, and chlordane is used to protect wooden buildings from termites. The structures of some chlorinated insecticides are shown next. 

These are usually abbreviated as HCHs for obvious reasons. The most well known of these compounds is lindane, which is also known as the gamma isomer of HCH or y-HCH. Its structure is given below. 

Internal perfluoroalkenes react with trimethyl(perfluorophenyl).silanc under more forcing conditions than terminal alkenes. The reactions can also give biphenyl derivatives. An interesting dependence of the reaction results on the structure in the case of perfluorophenylation of isomeric perfluoro(methylpentenes) has been identified. Perfluoro(2-methylpent-2-ene) is transformed into pcrfluoro(2-methyl-3-phenylpent-2-cnc) (4) perfluoro(4-methylpent-2-cnc) forms pcrfluoro( 1.1,3-triinethy lindan) (6) as well as perfluoro(4-inethyl-2-phciiylpen t-2-ene) (5). 

Organochlorine insecticides may be divided into three broad groups dichlorodiphenylethanes, such as DDT and methoxychlor cyclodienes, such as chlor-dane and dieldrin and hexachlorocyclohexanes, such as lindane. Mirex and chlordecone, however, are organochlorine insecticides whose caged structures do not fit well into the previous groups. 

The biggest exception to this, of course, is the GABA system which is associated with a variety of active drugs. Besides the older picrotoxinin (I) and bicuculline (II), there are barbiturates (III), nipecotic acid (IV), muscimol (V), benzodiazepines (VI), baclofen (VIII), and avermectins to guide the search for active leads. The recent suggestion of similarities in structure and mode of action of lindane, cyclodienes and picrotoxinin analogs (J ) throws open even more possibilities. 

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