Glaser's "Chemistry is in the News"
To Accompany Wade Organic Chemistry 4/e.
Chapter 16. Aromatic Compounds.
Polychlorinated biphenyls are a group manufactured organic chemicals that were introduced into American industry in 1929 by Monsanto. PCBs are oily liquids or solids that range in color from clear to light yellow, and have no known smell or taste. The 209 possible isomers of PCBs result when two or more chlorine substituents replace a hydrogen atom at any position on a biphenyl ring (see the structure of PCBs). Chemical and physical properties such as low electrical and high thermal conductivity, high boiling point, chemical stability, and flame-retardant made PCBs valuable to manufacturers. These compounds were used widely in transformers, insulators, lubricants, carbonless copy paper, paint, adhesives, and pesticides. Production was halted in 1977 when PCBs were discovered to have serious adverse health effects. Still, between 1929 to 1977, pure PCB production exceeded 700,000 tons.
Adverse health affects of PCBs stem from their solubility in organic solvents, oils, and fats. This characteristic means that they do not degrade easily in the environment and bioaccumulate up the food chain. It takes several years for PCBs to break down in water; the sediments at the bottom of a body of water can act as a reservoir from which PCBs can be released in small amounts into the water. The trace amounts of PCBs in water are constantly absorped by the fish mainly because PCBs are more soluble in oils and fats than in water. Over a long period of time, the PCBs bioaccumulate in the fatty tissues of fish making the PCB concentration in fish hundreds of thousands of times higher than the PCB concentration in water. A Dutch study of seals in the Baltic Sea supports the evidence of bioaccumulation found in the dolphins from Matagorda Bay, Texas.
As the article pointed out, PCBs have significant health effects on animals and humans. Exposure to PCBs in food has been linked to delayed brain development and reduced IQ in children. Studies have been done on animals and found PCBs to be extremely carcinogenic. Bioaccumulated and sediment bound PCBs are the most carcinogenic. Epidemiological studies done on workers exposed to PCBs have shown increased rates of liver cancer and malignant melanomas. This research was supported by animal studies that found that PCB exposure caused liver cancer. Animal studies, particularly those done on primates, also showed non-carcinogenic toxic effects on various systems in the body. Significant decreases in the thymus gland were found in the immune system. Other significant problems were found in the neurological development of primates. Decreased sperm count was one of the more disastrous effects affecting the reproductive system.
For additional up-to-date information about PCBs, check out the Gascape Index for current articles on the subject.
Question 1: How do PCBs affect organisms?
A. Through bioaccumulation up the food chain and direct contact with PCBs, the immune, endocrine, neurological, reproductive systems of organisms are affected
Question 2: What were the properties that made PCBs useful to manufacturers?
A. Low electrical and high thermal conductivity, high boiling point, chemical stability, and flame-retardant made PCBs valuable to manufacturers.
Question 3: 3. What long-term dosage of PCBs (in mg/kg/day) has fatal affects in animals? What is the dosage over a short-term period (less than 14 days) that has fatal affects?
A. Over a long period of time, exposure of only 0.1 mg/kg/day can kill organisms. In a period less than 14 days, it takes 750 mg/kg/day to kill an organism.
Question 4: Are the affects of PCBs scientifically supported? Where?
A. One study performed by the Dutch National Institute of Public Health and Environmental Protection investigated the connection between hormone disrupting chemicals and the immune systems of seals. One test group was fed relatively clean North Atlantic fish while the second group was fed fish from the heavily polluted Baltic Sea. After being fed the Baltic fish for nearly two years, the seals in the second group showed signs of suppressed immune functions.
Question 5: The most toxic form of PCB is a non-ortho, coplanar isomer. Why must such a structure be non-ortho if it is coplanar? (Hint: refer to sections 5-9 and 16-12)
A. The structure cannot have chlorine substituents in the ortho positions, because it causing steric hindrance or ring strain. Refer to Figure 5-16 in book.
Question 6: Should geographic studies be completed to examine the exposure of PCBs on the human population? If these studies showed a problematic situation, what other action should be taken to counteract PCB exposure?