The Structure of PCB Molecules

Biphenyl molecules consist of two benzene rings linked by a single bond formed between two carbons that have each lost their hydrogen atom:

Like benzene, if biphenyl reacts with CI, in the presence of a ferric chloride (FeCl j) catalyst, some of its hydrogen atoms are replaced by chlorine atoms. The more chlorine initially present and the longer the reaction is allowed to proceed, the greater the extent (on average) of chlorination of the biphenyl molecule. The products are polychlorinated biphenyls, PCBs. The reaction of biphenyl with chlorine produces a mixture of many of the 209 congeners of the PCB family; the exact proportions depend upon the ratio of chlorine to biphenyl, the reaction time, and the reaction temperature. An example of a PCB molecule is shown below:


Although many individual PCB compounds are solids, the mixtures are liquids or are solids with low melting points. Commercially, individual PCB compounds were not isolated; rather they were sold as partially separated mixtures, with the average chlorine content in different products ranging from 21% to 68%.


The general formula for any PCB congener is C12H10_nCln, where n ranges from 1 to 10. Calculate the average number of chlorine atoms per PCB molecule in a mixture of congeners that is 60% chlorine by mass, a common value for commercial samples.

Continue reading here: The Numbering Systems for PCBs

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