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World to Discuss Persistent
Wednesday, June 24, 1998
ENN News Staff
Article review, editorial and questions
by The Straight-Chain Alkanes
Presently there is considerable debate over the use of persistent organic pollutants (POP's). In July of 1998 over 100 governments met in Montreal, Canada to discuss this issue. In Montreal talks basically focused on 12 POP's, including aldrin, DDT, chlordane, dieldrin, and heptachlor. Most of these POP's are pesticides and herbicides. The environmental fate of many of these molecules has been extensively researched because of their wide use.
These pollutants persist in the environment due to properties such as polychlorinated ring structures, low solubility, and semi-volatility. For better understanding of these ring structures, 3D models are available for such molecules as chlordane, heptachlor/heptachlor epoxide and aldrin/dieldrin. The breakdown of these pollutants also depends on these properties and their interaction in the environment. Soil properties, temperature, and the presence of microorganisms help determine the rate at which these molecules break down. The environmental interactions of endrin, dieldrin, and heptachlor help to show trends of POP breakdown.
Epoxidation in several of these molecules occurs quickly and results in molecules with a greater degree of stability and, ultimately, persistence. All of these molecules and their derivatives are readily absorbed in lipids, and thus are said to be very lipophillic. This lipophillicity causes bioaccumulation in animals and some plants which can lead to severe health problems even at small doses. Biomagnification, the result of eating polluted organisms leads to even greater problems in predatory animals, especially to those high in the food chain.
What are the hazards to humans of exposure of even very low doses of POP's?Answer:
Cancer, damage to central and peripheral nervous systems, diseases of the immune system, reproductive disorders and interference with normal infant and child development.
Look at the heptachlor using ChemFinder. Now draw the epoxidized version of heptachlor and try to include the transition state. Do you think this epoxidized molecule is stable? Propose a mechanism to go from heptachlor to a glycol in one step (ignore any possible enantiomerism).Answer:
Using ChemFinder, look at heptachlor epoxide. The epoxidized molecule is very stable unless in a very acidic environment. Formation of the glycol can occur with a strong acid in carbon tetrachloride solution.
Why are animals higher in the food chain more likely to be affected by POP's and does this category include humans?Answer:
Due to their lipophillicity, POP's accumulate in fatty tissue. As you go higher in the food chains, animals have a greater magnification of POP's because they have eaten lower animals contaminated by POP's.
Look at the links to chlordane, aldrin, and dieldrin. What is the trend in solubility? Why would this characteristic make them persist in the environment?Answer:
They are not very soluble in water and are hydrophobic. This lack of interaction with aqueous solution disallows aqueous chemical reactions.
Based on your knowledge of POP's, do you think that the Montreal talks should focus on minimizing emissions and releases of POP's? Would it be more feasible to ban POP's altogether and find nontoxic alternatives?Answer: