Chlorinated aromatic compounds, benzene derivatives

Table 2. Production and uses of chlorinated aromatic compound (Benzene derivatives) [105]...

Allylchlorosilanes undergo Friedel-Crafts alkylation with aromatic compounds such as benzene derivatives and ferrocene to give [p-(chlorosilyl)alkyl]arene compounds in the presence of Lewis acid catalyst. Allylsilanes containing two or more chlorine atoms on silicon react smoothly with benzene under mild conditions to give alkylation products in good yields [Eq. (15)]. In alkylations of benzene, the reactivity of the allylsilanes increases as the number of chlorine atoms on the silicon increases, but decreases as the number of methyl groups increases. Because the reactivity of allylsilanes is sensitive to the electronic nature of the substituents on the silicon atom, allylsilane selection is an important factor for alkylation reactions. 

Such xenobiotics as aliphatic hydrocarbons and derivatives, chlorinated ahphatic compounds (methyl, ethyl, methylene, and ethylene chlorides), aromatic hydrocarbons and derivatives (benzene, toluene, phthalate, ethylbenzene, xylenes, and phenol), polycyclic aromatic hydrocarbons, halogenated aromatic compounds (chlorophenols, polychlorinated biphenyls, dioxins and relatives, DDT and relatives), AZO dyes, compounds with nitrogroups (explosive-contaminated waste and herbicides), and organophosphate wastes can be treated effectively by aerobic microorganisms. 

Benzene and its derivatives are used widely throughout the chemical industry as solvents and raw materials. Mono-, di-, and trichlorobenzenes are used directly as pesticides for their insecticidal and fungicidal properties. Benzene, toluene, and chlorobenzene are used as raw materials in the synthesis of at least 15 pesticides, although their main use is as a carrier solvent in 76 processes. Additional priority pollutant aromatics and chlorinated aromatics exist as impurities or as reaction byproducts because of the reactions of the basic raw materials and solvent compounds. 

As seen from the structural formulas in Figure 16.4, the organochlorine insecticides are of intermediate molecular mass and contain at least one aromatic or nonaromatic ring. They can be placed in four major chemical classes. The first of these consists of the chloroethylene derivatives, of which DDT and methoxychlor are the prime examples. The second major class is composed of chlorinated cyclodiene compounds, including aldrin, dieldrin, and heptachlor. The most highly chlorinated members of this class, such as chloredecone, are manufactured from hexachlorocyclo-pentadiene (see Section 16.3). The benzene hexachloride stereoisomers make up a third class of organochlorine insecticides, and the third group, known collectively as toxaphene, constitutes a fourth. 

As noted above, the first definition of "aromaticity" was in terms of substitution rather than addition. This is certainly true for many benzene derivatives. However, it must be used with some care since thiophene is by most criteria about as "aromatic" as benzene, but when treated with chlorine or bromine it gives an addition product. The latter is, however, the kinetically controlled product, for when heated or treated with base it loses hydrogen halide and gives the 2-halothiophene.20 Compounds such as anthracene and phenanthrene, which are recognized as having considerable resonance stabilization, also undergo addition reactions. 

FIGURE 6.5 The infrared spectrum of a query compound compressed by Hadamard transform for the prediction of benzene derivatives by a CPG neural networks. The spectrum exhibits some typical bands for aromatic systems and chlorine atoms. 

The questions raised by Kekule s interpretation of the mechanism of this reaction led to a number of investigations which involved a further study of the oxidation of aromatic phenols and quinones. Thus, for example, Zincke in collaboration with Kuster48 and Rabinowitch 4 undertook a systematic investigation of the action of chlorine (potassium chlorate plus hydrochloric acid) on various derivatives of the three dihydroxybenzenes. The results of these experiments showed that these classes of benzene derivatives split up under conditions to give aliphatic compounds. The intermediate compounds which were formed from tetrahydrobenzene during these transfonnations, were identified as the hexachloro-o-diketone derivative (I) and the pentachloro-m-diketone derivative (II). 

Examples of toxic compounds, including some important intermediates and starting materials in the chemical industry, are shown in Figure 3.3. Many alkali fluorides, such as alkali hexafluorosilicate, alkali hydrogen difluoride, or alkali sulfuryl fluoride, are well known toxic substances. Sulfur dioxide and ammonia (ubiquitous gases) are toxic, as are chlorine, metallic mercury vapors, many organic phenol compounds, amino aromatic compounds such as aniline, and many substituted amino-benzene derivatives. Additionally, many diisocyanates are toxic, e.g., 2,4- and 2,6-toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), chloro-, bromo-, and iodoacetic acid, methyl bromide, tribromomethane (bromoform), carbon tetrachloride, and formaldehyde. Also, many natural compounds present in many plants have toxic properties, and a selection of these are listed in Table 3.4. 

Whereas the nitro derivatives of benzene are produced by electrophilic aromatic substitution, further important derivatives of toluene are predominantly obtained through reactions of the methyl group they include the production of oxidation products such as benzoic acid and the side-chain chlorinated toluene compounds. 

The Friedel-Crafts process is frequently the most useful method for the introduction of an alkyl group. The reaction is capable of many practical applications, and a large number of patents have appeared on the preparation of alkyl derivatives of various aromatic compounds such as xylene, naphthalene, and phenols. Patents have covered the utilization of such alkylating agents as the olefins derived from cracking, the mixtures prepared by chlorination of petroleum fractions, and various naturally occurring waxy esters. The most important application is the synthesis of ethylbenzene from ethylene and benzene. 

The majw work to date on synthetic applications of remote functionalization has involved free radical chlorination. The earliest studies involved the direct attachment of aryliodine dichloride units to the steroid substrates, then intramolecular free radical chain chlorination in benzene or chlorobenzene solution (Scheme 14). Yields were only in the 50% region, but fairly good selecdvities were observed compound (6) afforded chiefly the 9-chloro derivative, while compound (7) produced the 14-chloro steroid. The yields and selectivities were considerably improved when it was realized that aromatic solvents promote intermolecular random processes by forming complexes with C1-, and when the radical relay method was developed. 

For exposure to water, food, sediment or soil, some general relationships exist which enable us to predict the concentration in many organisms. However, in particular for the latter three types of exposure, little data are available. In addition, the present knowledge for derivation and application of the relationships is based on only a few classes of organic compounds, such as polycyclic aromatic hydrocarbons and chlorinated benzenes and biphenyls. 

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