Encyclopedia of Environmental Science and Engineering, Volume I and II

PCBs AND ASSOCIATED AROMATICS 869

do not occur as major constituents in commercial PCB

mixtures. Substitution patterns such as the (2,6), (3,5),

(2,4,6) and (3,4,5) are not commonly found in commercial

mixtures. The number of chlorines on the two rings seldom

differs by more than one for an odd number of chlorines in

the molecule, and is usually equal for an even number of

chlorines. Because of this, the number of congeners found

in technical PCBs is far less than the theoretical number.

The introduction of a single ortho-chloro substituent into

the biphenyl ring results in decreased coplanarity between the

two phenyl rings due to steric interactions. Nevertheless, the

isomers bind to the Ah receptor and cause dioxin-like responses

although their potency is less than the coplanar PCBs. The

mono-ortho- PCBs are shown in Figure 12.

Di-ortho- substitution of the four coplanar PCBs results

in 13 possible di-ortho-coplanar PCBs. This substitution

pattern would result in more steric hindrance to coplanarity

than the mono-ortho- congeners and it would therefore be

expected that they would exhibit weaker dioxin-like effects.

The di-ortho- compounds are shown in Figure 14.

In more general terms, PCBs can be separated into six dif-

ferent structural classes. Class I and Class II compounds are

the coplanar and mono-ortho-coplanar PCBs, respectively.

Class III are mono-ortho-coplanars, which lack a chlorine

in a paraposition. Class IV PCBs are the di-ortho-coplanar

compounds. Classes I to IV are dioxin-like in decreasing

order of potency. Class V and Class VI are the tri- and tetra-

ortho-PCBs which are not coplanar and do not elicit sig-

nificant dioxin-like activity. The six classes are shown with

representative examples in Figure 15.

In summary, the coplanar PCB congeners are toxic

and bind with high affinity to the Ah receptor and are highly

dioxin-like. The mono-ortho-coplanar PCBs exhibit less

toxicity than the coplanar isomers and interact with the

FIGURE 10 PCB congeners are isomers contained in homologous series.

C016_003_r03.indd 869C016_003_r03.indd 869 11/18/2005 1:12:27 PM11/18/2005 1:12:27 PM