Find out how halogens are used in the manufacture of herbicides and polymers and environmental problems associated
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. More than 1,500 organohalogen compounds have been identified as naturally occurring materials and are produced by various plants, fungi, bacteria, and marine organisms. A variety of synthetic methods to introduce halogen substituents are available, and organic halogen compounds may be converted to other functional-group classes by reliable methods. Examples illustrating the preparation, applications, and natural occurrence of individual organohalogen compounds are described in the separate subsections dealing with alkyl, vinylic, and aryl halides.
Herbicides an agent, usually chemical, for killing or inhibiting the growth of unwanted plants--i.e., weeds. (See weed.) In the past, sea salt, by-products of chemical industries, and various oils were used as weed killers. Late in the 19th century the selective control of broad-leaved weeds among cereal crops came into practice. The central development in modern weed control was the introduction in 1945 of so-called organic herbicides; they were revolutionary in that their high toxicity allowed for effective weed control at very low dosages--about 1-2 percent of the dosage rates of such earlier weed killers as carbon disulfide, ...
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Herbicides an agent, usually chemical, for killing or inhibiting the growth of unwanted plants--i.e., weeds. (See weed.) In the past, sea salt, by-products of chemical industries, and various oils were used as weed killers. Late in the 19th century the selective control of broad-leaved weeds among cereal crops came into practice. The central development in modern weed control was the introduction in 1945 of so-called organic herbicides; they were revolutionary in that their high toxicity allowed for effective weed control at very low dosages--about 1-2 percent of the dosage rates of such earlier weed killers as carbon disulfide, borax, and arsenic trioxide.Modern weed killers are put in two categories: selective (affecting specific plant species) and nonselective (affecting plants generally). These, in turn, are classified as foliage-applied and soil herbicides. Contact herbicides (e.g., sulfuric acid, diquat, paraquat) kill only the plant organs with which they are in contact. Translocated herbicides (e.g., amitrole; picloram; 2,4-D) are effective against roots or other organs to which they are transported from aboveground treated surfaces (i.e., soil). With respect to planting time, herbicides are also classified as preplant, preemergence, or postemergence weed killers. Preplant herbicides may be applied to the soil or to weeds before crop planting.Certain herbicides (e.g., sodium arsenite) 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. The major toxicity of TCDD in humans is in the production of chloracne, a condition characterized by acne that appears between the eyes and the ears. In more severe form, acne may be found on the face, trunk, and buttocks. (Significant adverse health effects in the soldiers exposed to low amounts of TCDD in Vietnam have not been clearly established.) Polychlorinated biphenyls (PCBs) also produce chloracne by damaging the sebaceous glands in skin.
Aryl halides Many organohalogen compounds in which the halogen is directly attached to a benzenoid ring occur naturally. Unlike alkyl and vinylic halides, for which marine origins are the most common, aryl halides are found in a variety of sources. Thyroxine, for example, is an iodine-containing amino acid secreted by the thyroid gland that acts as a regulator of metabolism.The sex pheromone of the lone star tick Amblyomma americanum is 2,6-dichlorophenol, and 2,6-dibromophenol has been isolated from the acorn worm Balanoglossus biminiensis.
Several halogen-containing aromatic compounds, while not natural products in the customary sense of the word, have become widely dispersed in the environment.
The most familiar example is 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT).DDT was introduced in the early 1940s and soon became both the agricultural insecticide of choice and the principal means of combating disease-bearing insects. One of the advantages of DDT is that it is a persistent insecticide, meaning that it is only slowly degraded by natural processes and survives for a long time after its initial application. DDT proved so effective in increasing crop yields and controlling insect-borne diseases such as malaria that Paul Müller, the Swiss chemist who developed the insecticide, was awarded the Nobel Prize for Physiology or Medicine in 1948. Studies in the 1960s, however, revealed that DDT accumulated in the fatty tissue of fish, birds, and animals and that the DDT levels increased in moving up the food chain. 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.
