The possible positions of chlorine atoms on the benzene rings are denoted by numbers assigned to the carbon atoms.
Polychlorinated biphenyls (PCBs) are a class of organic compounds
with 1 to 10 chlorine atoms attached to biphenyl, which is a molecule composed
of two benzene rings.
PCBs were widely used for many applications,
especially as dielectric fluids in transformers, capacitors, and coolants. Due to PCB's toxicity and classification as a persistent organic pollutant, PCB production was banned by the United States Congress in 1979 and by the Stockholm Convention on Persistent Organic Pollutants in 2001.
Concerns about the toxicity of PCBs are largely based on compounds within this group that share a structural similarity and toxic mode of action with dioxin.
Toxic effects such as endocrine disruption and neurotoxicity are also associated with other compounds within the group.
Physical and chemical properties
PCB congeners are odorless, tasteless, clear to pale-yellow, viscous liquids (highly chlorinated mixtures are more viscous and deeper yellow). They are formed by electrophilic chlorination of biphenyl with chlorine gas.
PCBs have low water solubilities — 0.0027-0.42 ng/L for Aroclors, and low vapor pressures at room temperature, but they have high solubilities in most organic solvents, oils, and fats. They have high dielectric constants, very high thermal conductivity, high flash points (from 170 to 380 °C) and are chemically fairly inert, being extremely resistant to oxidation, reduction, addition, elimination, and electrophilic substitution.
The density varies from 1.182 to 1.566 kg/L. Other physical and chemical properties vary widely across the class. As the degree of chlorination increases, melting point and lipophilicity increase, and vapour pressure and water solubility decrease.
PCBs readily penetrate skin, PVC (polyvinyl chloride), and latex (natural rubber). PCB-resistant materials include Viton, polyethylene, polyvinyl acetate (PVA), polytetrafluoroethylene (PTFE), butyl rubber, nitrile rubber, and Neoprene.
In terms of structural relationship to toxicity, PCBs fall into 2 distinct categories, referred to as coplanar or non-ortho-substituted arene substitution patterns and noncoplanar or ortho-substituted congeners. The coplanar group members have a fairly rigid structure, with the 2 phenyl rings in the same plane.
This gives the molecule a structure similar to polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans, and allows it to act in the same way as these molecules as an agonist of the aryl hydrocarbon receptor (AhR) in organisms. These type of PCBs are considered as contributors to overall dioxin toxicity, and the term dioxin is often used interchangeably when the environmental and toxic impact of these compounds is considered. Noncoplanar PCBs, with chlorine atoms at the ortho positions, have not been found to activate the AhR, and are not considered part of the dioxin group; however, studies have indicated some neurotoxic and immunotoxic effects, but at levels much higher than normally associated with dioxins, and thus of much less concern to regulatory bodies.
PCBs are very stable compounds and do not decompose readily. Their destruction by chemical, thermal, and biochemical processes is extremely difficult, and presents the risk of generating extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis (see Methods of destruction below).
Written by Carole Pointet Student in Health Safety & Environement
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