The Numbering Systems for PCBs
The numbering scheme used for individual PCB congeners begins with the carbon that is joined to a carbon in the other ring; it is given the number 1, and the other carbons around the ring are numbered sequentially. As illustrated below, the positions in the second ring are also numbered 1 through 6, starting with the ring-joining carbon, but are distinguished by primes. By convention, the 2' position in the second ring lies on the same side of the C—C bond joining the rings as does the 2 position in the first ring, and so on.
In most instances, the two rings in a chlorinated biphenyl molecule are not equivalent since the patterns of substitution differ. The unprimed ring is chosen to be the one that will give a substituent with the lowest-numbered carbon. Using all these rules, we can deduce that the name of the PCB molecule shown on page 478 is 2,3',4',5''tetrachfarobiphenyl
Very rapid rotation occurs around carbon-carbon single bonds in most organic molecules, including the C—C link joining the two rings in biphenyl and in most PCBs. Thus it is not normally possible to isolate compounds corresponding to different relative orientations of the two rings ("rotamers") in a PCB. For example, 3,3'- and 3 ,5'-dichlorobiphenyl are not individually isolatable compounds, since one form is constantly being converted into the other and back again by rapid rotation about the C—C bond linking the rings:
3,3 '-dichlorobiphenyl 3,5' -dichlorobiphenyl
The name used for such a compound is that which has the lowest number for the second chlorine, so the system shown above is called the 3,3' isomer. Although the rings rotate rapidly with respect to each other, the energetically optimum orientation is the one having the rings coplanar or close to it, except, as we shall see later, when large atoms or groups occupy the 2 and 6 positions.
Using a systematic procedure, draw the structures of all unique dichloro-biphenyls, assuming first that free rotation about the bond joining the rings does not occur. Then deduce which pairs of structures become identical if free rotation does occur.
Continue reading here: Commercial Uses of PCBs
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