Aromatic hydrocarbons congeners

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

Baker, J.E., S.J. Eisenreich, and B.J. Eadie. 1991. Sediment trap fluxes and benthic recycling of organic carbon, polycyclic aromatic hydrocarbons and polychlorobiphenyl congeners in Lake Superior. 

Dioxins (PCDDs) and furans (PCDFs) are polyhalogenated aromatic hydrocarbons of high toxicity. There are a total of 210 different congeners 75 dioxin congeners and 135 furan congeners, of which 17 are potentially toxic. Dioxins and furans are now found prevalent in air, water, and soil in almost all natural environments. PCDD/Fs are strongly bound to organic matter, where half-life in soil has been estimated at 10-20 years (Ryan et al., 1987). PCDD/Fs enter the environment primarily as unintentional byproducts of combustion and chemical processes. 

Doucette, W., Andren, A.W. (1988a) Aqueous solubility of biphenyl, furan, and dioxin congeners. Chemosphere 17, 243-252. Doucette, W.J., Andren, A.W. (1988b) Estimation of octanol/water partition coefficients Evaluation of six methods for highly hydrophobic aromatic hydrocarbons. Chemosphere 17, 345-359. 

Figure 13.3 An example of a substitution reaction of an aromatic hydrocarbon compound (biphenyl) to produce an organochlorine product (2,3,5,2, 3 -pentachlorobiphenyl, a PCB compound). The product is 1 of 210 possible congeners of PCBs, widespread and persistent pollutants found in the fat tissue of most humans and of considerable environmental and toxicological concern.

Dioxin and dioxin-like compounds. Dioxin and dioxin-like compounds are structurally related groups of chemicals from the family of halogenated aromatic hydrocarbons. Depending on the number of chlorine-substituted positions, there are several congeners in each group. The most studied congener is TCDD. 

PCDDs, PCDFs, and PCBs are chemically classified as halogenated aromatic hydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are classified as tricyclic aromatic compounds with similar physical and chemical properties, and both classes are similar structurally. Certain PCBs (the so-called coplanar or mono-ortho coplanar congeners) are also structurally and conforma-tionally similar. The most widely studied of these compounds is TCDD this compound, often called simply dioxin, represents the reference compound for this class. 

Figure 32.5. Molecular structures of chlorinated aromatic hydrocarbons. Chlorine substitutions can exist in any or all of 8 or 10 positions, resulting in over 400 possible congeners.

Krauss and Wilcke examined the TiC -photocatalyzed oxidation of 12 PCB congeners [and 20 polycyclic aromatic hydrocarbons (PAHs)] on various soil samples (four mineral topsoil horizons, six organic horizons, and four particle-site fractions in three different soils) [107]. When the Ti02/soil mixture was irradiated in the absence of H2O, no photooxidation of the chlorobiphenyls occurred. When slurried with water, however, chlorobiphenyl concentrations decreased by 40-50% after 48 hours of irradiation, while the PAH concentrations were unchanged. By way of contrast, PAHs and PCBs doped onto quartz sand diminished by 95-100% after 8 hours of photolysis. The pollutants are clearly more accessible to hydroxyl radicals on sand than on soil. It is also clear that the photooxidation occurred in the soil and not in solution. Thus, OH is generated on one surface (Ti02), diffuses in the water to the other surface (soil), where the oxidation occurs. 

Chemical/Pharmaceutical/Other Class Halogen-ated aromatic hydrocarbon Chemical Structure There are 74 chlorinated dibenzo-p-dioxin congeners. The basic structure for unsubstituted dibenzo-p-dioxin (showing the carbon numbering scheme that is used to name specific congeners) and the structure of 2,3,7,8-tetrachlorodibenzo-p-dioxin (one of 22 tetrachlo-rinated dibenzo-p-dioxins) are shown below... 

Fig. 3. Representative congeners of aromatic hydrocarbons (AHs). AHs have similar chemical structure, are highly persistent in the environment, produce similar patterns of toxic response that vary in potency, and act via a common mechanism. AHs are, or were, manufactured as commercial products, or result as a by-product from several different manufacturing processes. TCDD is most studied, and often serves as the prototype of the AHs.

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