Find out how halogens are used in the manufacture of herbicides and polymers and environmental problems associated
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Find out how halogens are used in the manufacture of herbicides and polymers and environmental problems associated. Organohalogen compounds are organic compounds that contain at least one halogen (fluorine [F], chlorine [Cl], bromine [Br], or iodine [I]) bonded to carbon. They are subdivided into alkyl, vinylic, aryl, and acyl halides. In alkyl halides all four bonds to the carbon that bears the halogen are single bonds; in vinylic halides the carbon that bears the halogen is doubly bonded to another carbon; in aryl halides the halogen-bearing carbon is part of an aromatic ring; and in acyl halides (also called acid halides) the halogen-bearing carbon is doubly bonded to oxygen. Organohalogen compounds differ widely in chemical reactivity, depending on the halogen and the class to which they belong, and they may even differ within a class. A halogen substituent is considered a functional group, and the transformations of organohalogen compounds rank among the most important in organic chemistry. Many organohalogen compounds, especially organochlorine compounds, are important industrial chemicals; they are used as solvents and pesticides and as intermediates in the preparation of dyes, drugs, and synthetic polymers.
are sometimes applied by the jar method, whereby the tops of weeds are bent over and immersed in jars of poisonous solution. The herbicide is drawn into the rest of the plant and into connecting plants, gradually killing the entire system. Wild morning glory, poison oak, and camel thorn are sometimes treated in this manner. Chlorinated benzene and certain aromatic solvents are used to control aquatic weeds by adding them directly to the watery medium. Herbicides are chemicals used to kill plants. Their potential to produce toxicity in humans is rather low. High doses of 2,4-D, however, can produce muscular and neurological symptoms (Table 1). The systemic toxicity of 2,4,5-T is lower than that of 2,4-D, but 2,4,5-T is more irritating.During the Vietnam War, Agent Orange, a mixture of 2,4-D and 2,4,5-T, was used as a defoliant. The 2,4,5-T used in the Agent Orange was contaminated with tetrachlorodibenzodioxin (TCDD), or dioxin. Although TCDD is extremely toxic to some animals, it is less so to others, but it does cause birth defects and cancer in laboratory animals.
High DDT levels in birds were associated with fragile eggshells and reproductive abnormalities. Potential harm to wildlife and humans, along with the fact that many insects had become resistant to DDT, prompted the United States Environmental Protection Agency (EPA) to impose an almost complete ban on its use in 1972.A second chlorine-containing aromatic compound that is widespread in the environment is 2,3,7,8-tetrachlorodibenzo-p-dioxin (known simply as dioxin).Dioxin is formed in small amounts as a by-product in the synthesis of the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), in chlorine-based bleaching processes during paper production, and whenever organic material burns in the presence of a source of chloride ion Cl-, as in forest fires, for example. It is a very stable compound and gradually accumulates in the environment. Because dioxin is exceedingly toxic, carcinogenic, and teratogenic to test animals, regulations designed to limit further environmental contamination have been implemented.A group of aryl halides called polychlorinated biphenyls (PCBs) were formerly prepared on a large scale for use as heat-transfer mediums and insulating materials in transformers and other electrical equipment. Many of the problems associated with DDT and dioxin as environmental pollutants apply to PCBs as well, and PCB production was banned by the EPA in 1979.
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