Hexachlorocyclohexane structure

Rings with more than two differently substituted carbons can be dealt with on similar principles. In some cases, it is not easy to tell the number of isomers by inspection. The best method for the student is to count the number n of differently substituted carbons (these will usually be asymmetric, but not always, e.g., in 68) and then to draw 2" structures, crossing out those that can be superimposed on others (usually the easiest method is to look for a plane of symmetry). By this means, it can be determined that for 1,2,3-cyclohexanetriol there are two meso compounds and a dl pair and for 1,2,3,4,5,6-hexachlorocyclohexane there are seven meso compounds and a dl pair. The drawing of these structures is left as an exercise for the student. 

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

The six axial bonds are directed upward or downward from the plane of the ring, while the other six equatorial bonds are more within the plane. Conversion of one chair form into another converts all axial bonds into equatorial bonds and vice versa. In monosubstituted cyclohexanes, for electronic reasons, the more stable form is usually the one with the substituent in the equatorial position. If there is more than one substituent, the situation is more complicated since we have to consider more combinations of substituents which may interact. Often the more stable form is the one with more substituents in the equatorial positions. For example, in ct-1,2,3,4,5,6-hexachlorocyclohexane (see above) four chlorines are equatorial (aaeeee), and in the /Tisomer all substituents are equatorial. The structural arrangement of the /3-isomer also greatly inhibits degradation reactions [the steric arrangement of the chlorine atoms is unfavorable for dehydrochlorination (see Chapter 13) or reductive dechlorination see Bachmann et al. 1988]. 

The issue of bioavailability is further clouded by the physical characteristics of soil and the role of a possible mass transfer limitation. Soil constituents are not simply flat surfaces with free and equal access to all bacterial species. The formation of aggregates from sand-, silt-, and clay-sized particles results in stable structures which control microbial contact with the substrate (Figure 2.7). Discussion of sorption mechanisms and binding affinities must include the possible impact of intra-aggregate transport of the substrate. If the substrate is physically inaccessible to the microorganism then both desorption from soil constituents and diffusion to an accessible site are necessary. The impact of intra-aggregate diffusion on degradation kinetics has been modeled for y-hexachlorocyclohexane (Rijnaarts et al., 1990) and naphthalene (Mihelcic Luthy, 1991). 

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. 

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. 

Therapeutic Function Pediculicide, Scabicide Chemical Name la,2a,3p,4a,5a,6p-Hexachlorocyclohexane Common Name y-BHC Structural Formula ... 

An early chlorinated phenoxy acid herbicide (2,4-D) was first discovered in 1932. Although this compound rapidly breaks down in the environment, the seed fungicide hexachlorobenzene (HCB), introduced in 1933, was found to be far more persistent.2 The structurally similar insecticide hexachlorocyclohexane or... 

The 7 isomer of 1,2,3,4,5,6-hexachlorocyclohexane (Fig. 4.1-9A), an insecticide, gives rise to a large number of sharp bands which have the same IR and Raman frequencies. The 0 isomer (Fig. 4.1-9B), on the other hand, exhibits fewer bands and shows no bands which coincide in the IR and the Raman spectrum. This confirms its structure, which possesses a center of symmetry (rule of mutual exclusion). The insecticide endrine, too, shows a complicated pattern of bands, but due to the low symmetry the IR and Raman spectrum coincide to a large extent (Fig. 4.1-9 C). 

Although there are nine stereoisomers of 1,2,3,4,5,6-hexachlorocyclohexane, one stereoisomer reacts 7000 times more slowly than any of the others in an E2 elimination. Draw the structure of this isomer and explain why this is so. 

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. 

Fig. 7.25 Chemical structures of the contrasting persistent organic pollutants (POPs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs), p,p -dichlorodiphenyl trichloroethane (DDT) and hexachlorocyclohexane (HCH). Symbols x and y indicate the possible number of chlorines attached to the ring structures.

Therapeutic Function Pediculicide scabicide Chemical Name 10, 2a,3j3,4Ci ,5Ci ,6 3-hexachlorocyclohexane Common Name gamma-BHC Structural Formula ... 

Organochlorine pesticides = A unique class of pesticides because of their cyclic structure, number of chlorine atoms, and low volatility. They can be classified into four categories dichlorophenyl-ethanes (e.g., DDT), cyclodienes, chlorinated benzenes (e.g., hexachlorobenzene (HCB)), and cyclohexanes (e.g., hexachlorocyclohexane (HCH)). 

Figure 5.48 shows the eight isomeric structures of hexachlorocyclohexane. 

Investigation of the hexachlorocyclohexane configurational isomers has continued. The structure of the y-isomer (of the seven known configurational isomers this is the only one which is a useful insecticide, known as Lindane or Gammexane) is confirmed as 1,2,3,4,5,6-hexachlorocyclohexane. The jS-isomer, also re-... 

Modern DFT (density functional theory) functional and the continuum solvation model have been applied in order to predict theoretically carbon, chlorine, and hydrogen kinetic isotope effects during aerobic degradation of four hexachlorocyclohexane (HCH) isomers (a, p, 5, and y) Dehydrochlorination of all the HCH isomers occurs by the E2 mechanism. However, distinctive features of -HCH versus the other three isomers have been identified. It has been shown that the transition state (TS) structure for the -HCH is different from the TS of the other three isomers. Furthermore, the TSs for the elimination reactions for a-, S-, and y-HCH are T-like in water and TcB-like in an enzymatic environment -HCH reacts by a syn- rather than an anti-E2 mechanism. 

Crystals of carbon dioxide, COg, /3-hexachlorocyclohexane, CgHgClg, and /S-hexabromocyclohexane, CgHgBre, have face-centered cubic structure of the class T. The molecular axes are... 

Chlorinated pesticides are a small but diverse group of artificially produced chemicals characterized by a cyclic structure and a variable number of chlorine atoms. Most members of the group are resistant to environmental degradation and relatively inert toward acids, bases, oxidation, reduction, and heat. The parent compounds often have a number of related analogs and isomers, which show significant variation in toxicity and persistence. In some instances, these isomers have been used to develop highly specific insecticides, such as y-hexachlorocyclohexane, which shows low toxicity to plants and mammals. 

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