Structure and Properties

Polychlorinated biphenyls (PCBs) are a group of industrial chemicals that have not been found to occur naturally. The generic structure of PCBs is shown in Figure 1, and consists of biphenyl chlorinated in one or more positions. The positions which can be chlorinated are numbered separately on each phenyl ring, using a prime (') to differentiate between the rings. For the purposes of assessing reactive sites the relative positions on the rings are of most importance and are most

Figure 1 Chemical structure of polychlorinated biphenyls, showing numbered and named ring positions, wherex is 1-5 and y is 0-5. Named ring positions: o = ortho; m = meta; p = para.

Figure 1 Chemical structure of polychlorinated biphenyls, showing numbered and named ring positions, wherex is 1-5 and y is 0-5. Named ring positions: o = ortho; m = meta; p = para.

easily represented as ortho, meta, and para to the bond between the phenyl rings. There are 209 different PCBs (called congeners), due to the structural isomers possible at each chlorination level (homolog). There are also a number of PCBs which exist as enantiomers, the chiral center being the bond between the phenyl rings, for which free rotation is precluded by steric hindrance when there are three or four chlorines present at ortho positions. PCBs were produced industrially by the chlori-nation of biphenyl, leading to complex (but racemic) mixtures of congeners. Individual PCB congeners are generally named in two ways, a IUPAC structural name (e.g., 2,2',5-trichlorobiphenyl) and a numbering system originally devised by Ballschmitter and Zell, and adopted by IUPAC (e.g., PCB 18, or #18), in which PCB 1 is 2-monochlorobiphenyl and PCB 209 is decachlorobiphenyl.

PCBs are chemically stable (e.g., they are stable to both strong acids and strong bases, are not easily oxidized or reduced, are nonflammable and thermally stable), are soluble in a wide range of organic solvents, and have low electrical conductivity. Pure PCBs are either liquids or noncrystalline solids at room temperature, and most commercial mixtures are viscous liquids (although the highest chlorine content mixtures are solids). The vapor pressures of PCBs range from 1.2 Pa for monochlorinated congeners to 7 x 10~6Pa for decachlorobiphenyl at 25 °C, and the log octanol-water partition coefficients (log K0W) of PCBs are in the range 4.2 (monochlorobiphenyl) to 8.3 (decachlorobiphenyl) at 25 °C. PCBs are poorly soluble in water, with a solubility of 2 x 10~6molm~3 for decachlorobiphenyl to 29molm-3 for monochlorobiphenyl, at 25 °C.

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