![]() Congeners can also be characterized by descriptors (CP0, CP1, 4Cl, PP, 2M) that give rapid access to geometry and substituent positions. ![]() The congener lipophilicity is given in the same table, and was expressed against capacity to partition in octanol and water (K ow) (see Section 1.1.2). The congener numbering presented in this table follows that in Table 1.1, with the revised numbering of congeners 107–109. The relationship between PCB congener number and the Chemical Abstracts Service (CAS) registry number is given in Table 1.3. ![]() Tridimensional chemical structures of selected PCBs The replacement of hydrogen atoms in the ortho positions with larger chlorine atoms forces the aromatic rings to rotate out of the planar configuration ( Fig. 1.2) such structures are referred to as “non-planar” or “non-coplanar” congeners. The probability of attaining a planar configuration is essentially determined by the number of substitutions in the ortho positions (2,2′,6,6′): the benzene rings of non -ortho substituted PCBs as well as mono -ortho substituted PCBs can assume a planar configuration and are referred to as “planar” or “coplanar” congeners ( Erickson, 1997). The two extreme theoretical configurations are “planar” or “coplanar,” in which the two benzene rings are in the same plane, and “non-planar” in which the benzene rings are at a 90° angle to each other ( Faroon et al., 2000). With PCBs, this conformation is dependent on the degree of chlorine substitution, since chlorine is larger than hydrogen and creates more steric hindrance to the rotation ( Erickson, 2001). As with all molecules, there is a low-energy preferred conformation. In the biphenyl molecule, the two aromatic rings can rotate about the connecting single 1,1′-bond ( Fig. 1.1). Physical and chemical properties of PCBs according to homologue group. (1992) where only the original BZ numbers 199–201 are changed. However, in the scientific literature, the revised numbering of congeners 107–109 has not been adopted systematically the numbering system commonly used has been that proposed by Ballschmiter et al. The nomenclature for PCB congeners based on this report is shown in Table 1.1 and will be preferred in this Monograph. As a result, they recommended that the congeners previously numbered 107, 108, 109, 199, 200, and 201 be renumbered 109, 107, 108, 200, 201, and 199, respectively (reviewed in Mills et al., 2007). (1993) used a computer program to systematically renumber the PCBs according to the strict IUPAC rules. Slight changes in the original BZ congener-numbering system were later recommended to correct some errors ( Schulte & Malisch, 1983 Ballschmiter et al., 1992), and this resulted in the renumbering of BZ numbers 199–201. This shorthand nomenclature has become quite popular and is convenient for many uses, although it is important to note that it obscures the chemical identity of the congener and does not strictly follow the IUPAC rules. This results in the congeners being numbered from PCB-1 to PCB-209. This number correlates the structural arrangement of the PCB congener and ascending order of number of chlorine substitutions within each sequential homologue ( Ballschmiter & Zell, 1980). In an additional strategy proposed by Ballschmiter & Zell (1980), a number (called “BZ number”) is attributed to each individual congener. A deviation in that system lists the unprimed and primed chlorinated ring positions separately, sometimes eliminating the prime symbols and the commas for clarity and ease of typing (e.g. For a given PCB congener, the name lists the numbers sequentially. Assemblies of unprimed and primed numbers are arranged in ascending numerical order. According to the International Union of Pure and Applied Chemistry (IUPAC) and in particular rule A-52.3 related to hydrocarbon systems, an unprimed number is considered lower (higher priority) than the same number when primed. Two different but correlated nomenclature systems are currently used. While positions 2,2′,6, and 6′ are called “ ortho,” positions 3,3′,5 and 5′ are named “ meta” and positions 4 and 4′ are called “ para.” The carbon positions are numbered 1 to 6 on one ring, and 1′ to 6′ on the other. Depending on the position and number of the chlorine atoms, there are theoretically 209 individual PCB compounds (congeners). The general chemical formula is C 12H (10-m-n)Cl (m+n), where (m + n) is the number of chlorine atoms on the two rings. Chemical structure of PCBs and the IUPAC numbering system
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