Polychlorinated possible congeners

Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) [1,2] are environmental pollutants particularly relevant for their toxicity. Their structure formulas are shown in Figure 21.1. Each of the carbon atoms at the 1,23/4-, and 6,73,9-positions can bond either with chlorine or with hydrogen atoms, so yielding 210 possible congeners (75 PCDDs and 135 PCDFs), different in the number and/or position of chlorine atoms. The PCDDs and PCDFs are chemically and thermally stable and very persistent in the... 

Polybrominated diphenyl ethers (PBDEs, 12-133) are compounds structurally similar to polychlorinated biphenyls (PCBs). The numbering of209 individual PBDE congeners is analogous to the lUPAC nomenclature used for numbering PCBs (Figure 12.46). The basic skeleton is diphenyl ether with a different number and location of bromine atoms. 209 possible congeners can be derived, which... 

Nakata, H., S. Tanabe, R. Tatsukawa, M. Amano, N. Miyazaki, and E.A. Petrov. 1997. Bioaccumulation profiles of polychlorinated biphenyls including coplanar congeners and possible toxicological implications in Baikal seal (Phoca sibirica). Environ. Pollut. 95 57-65. 

Biphenyl was once used extensively for the production of polychlorinated biphenyls (PCBs) before their production was banned in the United States in 1979. PCBs are formed by direct substitution of hydrogen atoms in biphenyl with chlorine using chlorine gas under pressure with a ferric chloride (FeCl3) catalyst. There are 209 possible PCB compounds referred to as congeners. PCBs were discovered in 1865 as a by-product of coal tar and first synthesized in 1881. Commercial production of PCBs, originally called chlorinated diphenyls, began in 1929 by the Swann Chemical Company located in Anniston, Alabama. Swann was taken over by Monsanto in 1935. 

There are 75 possible isomers of polychlorinated-p-dibenzodioxin (PCDD) and 135 such isomers of polychlorinated dibenzofuran (PCDF). These related compounds are known as congeners. In addition, PCBs form a similar group of 209 related congeners that differ only in the number and pattern of chlorine atoms attached to the parent biphenyl molecule. 

Like dioxin and DDT, these chemicals are associated in the public mind with pollution and nasty chemicals. What are they PCB is an abbreviation for polychlorinated biphenyls this means the chemical biphenyl has various numbers of chlorine atoms attached to it. There are 200 possible substances, known as congeners, with the number of chlorine atoms ranging from one to ten in the molecule. The PCBs that are usually manufactured are a mixture of products with various numbers of chlorine atoms in the molecules. PBBs are similar chemicals called polybromi-nated biphenyls, in which various numbers of bromine atoms are attached to biphenyl. 

In 1993, the number of theoretically possible polychlorinated bornanes was calculated as 32,768 congeners (16,128 pairs of enantiomers and 512 achiral structures) [71]. This surprisingly high number is obtained by the product of the possibility to chlorinate nine positions on secondary or tertiary carbons (29) and three primary carbon positions (C8, C9, and CIO) with a variety of 4 giving... 

The occurrence of polychlorinated aromatic sulfur compounds in the environment has been reported during the last decade. PCDDs and PCDFs are formed by different chemical,photochemical and enzymatic reactions [10], It is possible that PCDTs and PCTAs are formed by chemical processes similar to those of the formation of PCDFs and PCDDs. Until now, PCDTs have been analyzed in stack gas and fly ash samples, in sediments, pulp mill effluents, and in some aquatic organisms. Some PCTA congeners have been observed in pulp mill effluents, stack gas, and soil and compost samples. The compound 2468-TeCDT has been found to accumulate from environmentally contaminated sediments to sandworms, clams, and grass shrimp. Accumulation factors have been calculated [11]. 

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