Polychlorinated biphenyls commonly known as PCBs are man made chemicals that never existed in nature until the 1900's when they started to be released into the environment by manufacturing companies and consumers. First manufactured by Monsanto (the only American company to manufacture PCBs) in 1929, PCB's were quickly acclaimed as an industrial breakthrough. These chlorinated oils have a low degree of reactivity. They are not flammable, have high electrical resistance, good insulating properties and are very stable even when exposed to heat and pressure. All in all, they seemed to be the perfect oil for use in dielectric fluids, and as insulators for transformers and capacitors. Not only were PCBs hailed for their role in preventing fires and explosions, they were actually required by fire code. Uses for PCBs quickly expanded to include hydraulic fluids, casting wax, carbonless carbon paper, compressors, heat transfer systems, plasticizers, pigments, adhesives, liquid cooled electric motors, fluorescent light ballasts, and the list goes on.
Polychlorinated Biphenyls make up a group of 209 individual chlorinated biphenyl rings known as congeners. They were typically manufactured as mixtures of 60 to 90 different congeners. In the concentrated form, PCBs are either oily liquids or solids with no discernable taste or odor. As the number of chlorines in a PCB mixture increases the flash point rises and the substance becomes less combustible. Also, PCBs with large numbers of chlorines are more stable and thus resistant to biodegradation. The most highly favored PCBs tended to be the ones with large numbers of chlorines. These congeners are also proving to be the ones that present the greatest environmental and health risks.
PCBs were used for many different industrial purposes, including the afore mentioned electrical industry and:
Hydraulic fluids; casting wax; carbonless carbon paper; compressors; heat transfer systems; plasticizers; pigments; adhesives; liquid cooled electric motors; fluorescent light ballasts
The list seems to be endless. There is little wonder that this group of chemicals was so readily accepted.
One of the qualities that make PCBs so desirable is one of the characteristics that make PCBs so hazardous to the environment. The high thermal and chemical resistance of PCBs means that they do not readily break down when exposed to heat or chemical treatment. This is a very desirable trait for lubricants. However, since PCBs do not break down they remain in the environment and continue to build up as more are introduced into the environment. Worms consume organic matter contaminated with PCBs, and small fishes eat the worms. Small fish are then eaten by larger fish, and perhaps the large fish are eaten by an eagle. Worms may only live for a short time, but eagles live for a long time. They continue to eat lots of large fish that ate lots of small fish that ate lots of contaminated worms. Over a lifetime, the PCB concentration in the fatty tissue of the eagle would continue to increase as it ate more and more contaminated fish. This accumulation of a chemical in animals at the top of the food chain is known as bioaccumulation. If the chemical happens to be toxic, the consequences become obvious.
The structural formula for cyclic carbon compounds can be abbreviated by eliminating the carbons and hydrogens and using an inner ring to indicate that the double bonds continuously flip between the carbons. For simplicity sake, the abbreviated form of the aromatic rings will be used in this article.
PCBs consist of a biphenyl (two benzene rings with a carbon to carbon bond between carbon 1 on one ring and carbon 1' on the second ring) with a varying number of chlorines.
Chlorines can be attached to any of the carbons by removing the hydrogen from that carbon and substituting the chorine in its place. The common nomenclature used for identifying the location of chlorine atoms on the biphenyl rings is as follows:
When the position of the chlorine is expressed in terms of its relationship to the carbon-to-carbon bond between the two aromatic rings, the ortho, meta and para designations make sense. Students of organic chemistry will remember that two side chains or substitutions in the following positions are referred to as ortho, meta an para.
If there are only 2 chlorines, one in the ortho position (carbon #2) and one on the ortho position (carbon 2'). The fact that there are only two chlorines, and that both of the chlorines is in an ortho position would tend to make this congener of PCB less toxic than the case in which the congener contains 6 chlorines, but chlorines are lacking in all of the four ortho positions (carbons 2, 6, 2' and 6'). The large number of chlorine present coupled with the fact that there are no ortho carbons makes this congener a particularly toxic and bioaccumulative species of PCB.
The toxicity of a PCB is dependent not only upon the number of chlorines present on the biphenyl structures, but the positions of the chlorines. For instance congeners with chlorines in both para positions (4 and 4') and at least 2 chlorines at the meta positions (3, 5, 3', 5') are considered to be "dioxin like" and are particularly toxic. When there is just 1 or no substitution in the ortho position, the atoms of the congener are able to line up in a single plane (sometimes referred to as coplanar). The planar or "flat configuration is particularly toxic.
Biodegradability is related to the amount of chlorination of a specific PCB. The higher the chlorine content of a PCB, the less the biodegradability. The lack of degradability of PCB compounds results in bioaccumulation of PCBs in the environment.
Specific PCB congeners are sometimes given specific names. Arochlor was the brand name for Monsanto PCBs, and Phenochlor and Pyralene were brand names for PCBs manufactured by the French company Prodelec. Most often, however, they are given numbers.
PCB's have been given four digit code designations. Since PCBs typically have 12 carbons (6 in each phenyl ring), the first two numbers of the code are typically 12, indicating the number of carbons in the structures. The second 2 digits in the code indicate the average percent weight of the chlorine. Codes for the PCB Arochlor for instance include 1242, 1254, 1260 and 1260.
There is still much to be learned about the chemistry of industrial PCBs. Current research focuses on finding ways to break the molecules down into harmless compounds. The biodegradation of PCBs utilizing microorganisms presently appears to be our best hope for removing PCBs from the environment.
If you need to cite this page, you can copy this text:
Roberta C. Barbalace. The Chemistry of Polychlorinated Biphenyls. EnvironmentalChemistry.com. Sept 2003. Accessed on-line: 10/24/2014