Polychlorinated Biphenyls and Polybrominated Biphenyls - Organic Pollutants: An Ecotoxicological Perspective - page 134

PBBs have also been marketed as mixtures of congeners, produced in this case by
the bromination of biphenyl. Their main commercial use has been as fire retardants, for
which purpose they were introduced in the early 1970s. The most widely known com-
mercial PBB mixture was Firemaster, first produced in 1970 in the United States, with
production discontinued in 1974 following the recognition of pollution problems.
Many of the components of PCB and PBB mixtures are both lipophilic and stable,
chemically and biochemically. Similar to the persistent organochlorine insecticides
and their stable metabolites, they can undergo strong bioconcentration and bioaccu-
mulation to reach relatively high concentrations in predators.

6.2 PoLycHLorInated BIPHenyLS

6.2.1 c h e m i c a l p r o p e r T i e s
In theory, there are 209 possible congeners of PCB. In practice, only about 130 of
these are likely to be found in commercial products. The structures of some con-
geners are shown in Figure 6.1. The more highly chlorinated a PCB mixture is, the
greater the proportion of higher chlorinated congeners in it. Thus, in Aroclor 1242
(42% chlorine), some 60% of the mass is in the form of tri- or tetrachlorobiphenyls,
whereas in Aroclor 1260 (60% chlorine), some 80% of the mass is as hexa- and
heptabiphenyls. Small amounts of PCDFs are found in commercial products (see
Chapter 7).
Individual PCB congeners are often crystalline, but most commercial mixtures
exist as viscous liquids, turning into resins with cooling. Highly chlorinated mix-
tures, such as Aroclor 1260, are resins at room temperature. In general, PCBs are
very stable compounds of low chemical reactivity; they have rather high density, and
are fire resistant. They have low electrical resistance that, in combination with their
heat stability, makes them very suitable as cooling liquids in electrical equipment.
They have low water solubility and high lipophilicity, and low vapor pressures (see
Table 6.1). With increasing levels of chlorination, vapor pressure and water solubility
tend to decrease, and log K ow to increase. (Note: The values for vapor pressure and
water solubility in Table 6.1 are expressed as negative.)
Some PCB congeners have coplanar structures (see, e.g., 3,4,3′,4′-tetrachloro-
biphenyl in Figure 6.1). The coplanar conformation is taken up when there is no
chlorine substitution in ortho positions. If there is substitution of chlorine in only
one ortho position, the molecule may still be close to coplanarity, because of only
limited interaction between Cl and H on adjoining rings. Substitution of chlorines in

Cl Cl Cl

Cl Cl

Cl Cl Cl Cl Cl Cl

Cl Cl Cl Cl Cl

3, 3', 4, 4'-Te trachlorobiphenyl
(coplanar)

3, 3', 4, 4', 5, 5'-Hexachlorobiphenyl
(coplanar)

2, 2', 4, 4', 6, 6'-Hexachlorobiphenyl
(not coplanar)

fIgure 6.1 Some PCB congeners.