Chlorinated hydrocarbons are a very large and diverse group of organic molecules. Hydrocarbons are molecules composed entirely of hydrogen (H) and carbon (C) atoms, often derived from carbon-based fossil fuels like petroleum oils and coal. Chlorinated hydrocarbons are specific hydrocarbon molecules that also have atoms of the element chlorine (Cl) chemically bonded to them. The number of chlorine atoms bonded to a specific chlorinated hydrocarbon determines, in part, the properties of the molecule. The number of carbon atoms and how they are arranged in three-dimensions also determines the chemical and physical properties of chlorinated hydrocarbons. Because there is such an immense number of possible forms of chlorinated hydrocarbons, this class of useful compounds has a wide set of applications that are of great economic and practical importance.
For example, chlorinated hydrocarbons produced from the refinement of crude oil comprise such things as synthetic rubbers used in automobile tires and tennis shoes. They also create plastics used in packaging, and products like fluid pipes, furniture, home siding, credit cards, fences, and toys, to name just a few. Chlorinated hydrocarbons can also be used as anesthetics, industrial solvents, and as precursors in the production of non-stick coatings like Teflon®. Chlorinated hydrocarbons are some of the most potent and environmentally persistent insecticides, and, when combined with the element fluorine, they function as refrigerants called chlorofluorocarbons, or CFCs. (Since CFCs have been found harmful to Earth’s atmosphere, they are being phased out in many countries of the world.) Because of their wide array of uses, chlorinated hydrocarbons are among the most important industrial organic compounds. Since they are derived from distillates of petroleum fossil fuels, however, the depletion of global oil and coal reserves looms as a concern for the future.
Chemistry, the study of matter and its interactions, can be divided broadly into two groups: inorganic chemistry and organic chemistry. Inorganic chemistry is concerned with atoms and molecules that, largely, do not contain the element carbon. For example, table salt, or sodium chloride (NaCl), is an inorganic compound. The production of table salt and water from the reaction of sodium hydroxide (NaOH) and hydrochloric acid (HCl) is an example of a reaction in inorganic chemistry, since none of the elements within the compounds are carbon. Exceptions to the no-carbon rule are oxides like carbon dioxide, which animals exhale when they breathe, and carbonates, like calcium carbonate (blackboard chalk). Although these substances contain carbon, they are considered to be inorganic in nature.
Organic chemistry, then, is the branch of chemistry dealing with most carbon-containing compounds. Carbon, the sixth element listed in the periodic table of elements, is a very versatile element. Atoms of carbon have the capacity to form chemical bonds with other carbon atoms in many configurations. This great variety makes carbon containing, or organic, molecules very important. Most biological molecules involved in the very chemical processes of life and in most of the cellular structures of living things are organic molecules. Approximately 98% of all living things are composed of organic molecules containing the three elements carbon, hydrogen, and oxygen (O).
Organic molecules vary both in the number of carbon atoms they contain and in the spatial arrangement of the member carbon atoms. Examples of organic molecules containing only one carbon atom are methane (natural gas), chloroform (a general anesthetic), and carbon tetrachloride (an industrial solvent). However, most organic molecules contain more than one carbon atom. Like people holding hands, carbon atoms can form molecules that look like chains. The carbon atoms are chemically linked with each other, like people linked together hand-in-hand in a chain.
Some organic molecules are very short chains of three or four carbon atoms. Other organic molecules are very long chains, containing many carbon atoms linked together. In addition, just as people in a chain can form a ring when the first person and the last person in the chain join hands, the carbon atoms in an organic molecule can form ring structures, called aromatic rings. The most common rings are five and six-member rings, containing five or six atoms of carbon, respectively.
Hydrocarbons, then, are specific organic molecules that contain only carbon and hydrogen chemically bound together in chains or in rings. Many hydrocarbons are actually combinations of chains and rings, or multiple rings linked together. In addition, some hydrocarbons can be branched chains. These carbon chains have portions branching from a main chain, like limbs from the trunk of a tree. The number of carbon atoms involved, and the pattern of chain formation or aromatic ring formation, determines the unique chemical and physical properties of particular organic hydrocarbons (like rubber, or plastic, or volatile liquid). Chlorinated hydrocarbons are organic hydrocarbon chains and/or aromatic rings that also contain chlorine atoms chemically linked within the molecule.
Many organic molecules, and many chlorinated hydrocarbons, are actually polymers. Organic polymers are large molecules made of many smaller repeating units joined together. The smaller subunits of polymers are called monomers. Just as a locomotive train is made of many train cars linked together, polymers are many smaller monomers linked together in a line. For example, DNA (deoxyribonucleic acid) in the chromosomes of cells is a polymer of nucleotide monomers. Many repeating nucleotide subunit molecules are joined together to form a large molecule of DNA. Similarly, polystyrene plastic that is used to make foam cups, toys, and insulation, is a hydrocarbon polymer consisting of carbon chains and aromatic rings. To illustrate their importance, of all the organic petrochemicals (hydrocarbons and their derivatives) produced industrially, over three-fourths are involved in the production of polymers. Some of the most important polymers are chlorinated hydrocarbons.
Chloroform is the name given to the chlorinated hydrocarbon compound trichloromethane. Trichloromethane, as its name implies (tri-, meaning three) contains three chlorine atoms. Carbon tetrachloride (tetra-, four), in contrast, has four atoms of chlorine bonded to a single atom of carbon. Both molecules are essentially a methane molecule with chlorine atoms substituted in place of hydrogen atoms. Since both contain only one atom of carbon, they are among the simplest chlorinated hydrocarbon molecules.
Chloroform is a colorless liquid at room temperature and is very volatile. Characterized as having a heavy sweet odor somewhat like ether, chloroform is actually sweeter than cane sugar. Chloroform cannot mix well with water. Like salad oil in water, chloroform separates into a layer. However, it does mix well with other hydrocarbons, so one of its uses is as a solvent or cleaner to dissolve other organic substances like gums, waxes, resins, and fats. In addition, chloroform is used in the industrial synthesis of the non-stick coating called Teflon® (polytetrafluoroethylene), which is an organic polymer. However, in the past, the primary use for chloroform was as a general anesthetic.
General anesthetics are drugs that cause the loss of consciousness in order to avoid sensations of extreme pain, such as those encountered during surgery. First synthesized in the laboratory in 1831, chloroform was used as a general anesthetic for the first time in 1847 by British physician Sir James Young Simpson (1811–1870), a professor of midwifery at the University of Edinburgh, during an experimental surgical procedure. Before the discovery of chloroform’s utility as a general anesthetic, drugs such as opium, alcohol, and marijuana were used to dull the pain of medical procedures. However, none were effective enough to allow pain-free surgery within the body. Other substances, like ether and nitrous oxide, were also used as general anesthetics around that same time. Because it was used for Queen Victoria of England’s labor pain during childbirth in 1853, chloroform became very popular. It was soon discovered that chloroform can cause fatal cardiac paralysis in about one out of every 3, 000 cases; it therefore is seldom used as an anesthetic today.
Carbon tetrachloride, like chloroform, is a clear, organic, heavy liquid. Consisting of one carbon atom and four chlorine atoms, carbon tetrachloride has a sweet odor and evaporates very easily, and so it is most often encountered as a gas. The compound does not occur naturally. Rather, it is manufactured industrially in large amounts for use as a solvent to dissolve other organic materials or as a raw material in the production of chlorofluorocarbons (CFCs) used as aerosol propellants and refrigeration fluids. For many years, carbon tetrachloride was used as a cleaning agent to remove greasy stains from carpeting, draperies, furniture upholstery, and clothing. In addition, prior to 1960, carbon tetrachloride was used in fire extinguishers, since it is inflammable. Because it is an effective and inexpensive pesticide, before 1986, carbon tetrachloride was used to fumigate grain.
These applications, however, have been discontinued since the discovery that the compound is probably carcinogenic, or cancer causing. Given its potential to cause cancer in humans, carbon tetrachloride is especially dangerous since it does not break down in the environment very easily. It can take up to 200 years for carbon tetrachloride to degrade fully in contaminated soil. Fortunately, the carcinogenic effects seen in laboratory experiments were due to very high levels of exposure that are not characteristic of the levels encountered by most people. Currently, it is not known what long-term low levels of exposure might have on human health.
The chlorinated hydrocarbons discussed above are considered to be simple because they contain only one carbon atom in their molecules. Many chlorinated hydrocarbon substances, however, are much larger than this. Having molecules consisting of numerous carbon atoms, some of the most important examples of complex chlorinated hydrocarbons are polymers and biologically active compounds that act as poisons.
Organic polymer materials are prevalent in modern society. The common term plastic really refers to synthetic organic polymer materials consisting of long carbon chains. One of the best known polymers, home plastic wraps used to cover food, is polyethylene. For use as plastic food wrap, polyethylene is prepared as a thin sheet. A greater thickness of polyethylene is used to mold plastic house ware products, like plastic buckets, carbonated drink bottles, or brush handles. Another common and closely related polymer is polypropylene. In addition to also being used for similar products, polypropylene is used to make clothing, the fibers of which are woven synthetic polymer strands. Both polypropylene and polyethylene are used extensively. As chemicals, though, they are classified as hydrocarbons. The chemical addition of chlorine atoms into the molecular structure of hydrocarbon polymers gives the polymers different useful properties. Organic polymers containing chlorine are called chlorinated hydrocarbon polymers.
Perhaps the best known chlorinated hydrocarbon polymer is polyvinyl chloride, or PVC. Because PVC also contains atoms of chlorine incorporated into the polymer molecule structure, it has different uses than polyethylene or polypropylene. PVC polymer molecules are created by chemically linking monomers of vinyl chloride molecules into very long chains. Vinyl chloride is a two-carbon molecule unit, also containing chlorine. The polymer structure is very similar to polyethylene. Vinyl chloride is made from the addition of chlorine to ethylene with hydrochloric acid as a byproduct. In the United States, about 15% of all ethylene is used in PVC production.
PVC was first discovered as a polymer in 1872 when sealed tubes containing vinyl chloride were exposed to sunlight. The solution inside the tubes polymerized into PVC. In the United States, the first patents for the industrial production of PVC were submitted in 1912, making PVC one of the earliest plastics in use. The prevalence of PVC and its importance to everyday lives is immense. To name just a few products made, PVC is found in pipes for household plumbing and waste management, phonograph records, soles and heels of shoes, electrical wire insulation, coated fabrics like Naugahyde®, plastic films like Saran Wrap®, patio furniture, vinyl floor tiles, novelty toys, yard fences, home siding, and credit cards. Its properties make it very useful in making many of the products that we take for granted each day. PVC is inexpensive to synthesize relative to other polymers, making it an attractive material to use.
Because the polymer molecules of PVC are able to fit closely together, they prevent the seepage of fluids through the plastic. Therefore, PVC has important advantages over other organic polymers in clean water transport, preventing food contamination, and securing sterile products. For instance, PVC blood bags allow blood products to be stored longer than do glass containers while allowing for flexibility. PVC packaging protects fresh food from deterioration, and PVC pipes and liners provide safe drinking water supplies from reservoirs, preventing contamination during transport.
PVC is also fire retardant, making it a very safe chlorinated hydrocarbon polymer. Because they are derived from petroleum products, organic polymers are often very flammable. PVC, however, is difficult to ignite. When PVC is burned, it releases less heat than other materials. Because of its heat resistant property, PVC is used to insulate cables that can build up heat. Additional safety characteristics of PVC include its durability and shatterproof qualities. Therefore, PVC is used to make protective eyewear, shatterproof bottles, life jackets, and inflatable personal flotation devices. The durability and corrosion resistance of PVC makes it useful in auto underbody sealing, gutters, window frames, shutters, and cladding of homes.
In addition to the previously listed uses, PVC is an important polymer because it requires less energy to manufacture than other plastics and can be recycled into new products after first use. Other closely related chlorinated hydrocarbon polymers include polychloroethylene and trichloroethylene.
A very important example of a chlorinated hydro-carbon polymer that is a synthetic rubber is polychloroprene. Polychloroprene is an example of an elastomer, or polymer that has the elastic properties of rubber. Along with butadiene, isoprene, and styrene, polychloroprene accounts for 90% of all worldwide synthetic rubbers produced. Closely related in chemical structure to natural rubber extracted from rubber-tree plants, polychloroprene is used to make hoses, belts, shoe heels, and fabrics, because it is resistant to corrosive chemicals.
In addition to making very useful polymers, rubbers, plastics, solvents, and cleaners, chlorinated hydrocarbons also are potent pesticide substances. Perhaps the best known chlorinated hydrocarbon insecticide is Dichlorodiphenyltrichloroethane, or DDT. First synthesized in the 1800s, the insecticidal properties of DDT were not discovered until 1939. Paul Muller, while working for the Swiss company Geigy, first uncovered the effectiveness of DDT against insects. After demonstrations of its effectiveness and relative safety to humans was established, the use of DDT exploded around the globe in the war against disease-carrying and agricultural insect pests.
The first major use of DDT was during World War II to control lice infestation in Allied troops. Its success led to the large-scale use of DDT to control the blood-sucking insects spreading yellow fever, malaria, typhus, and plague. Its initial use met with exceptional results. For example, by the 1960s, malaria cases in India fell from tens of millions of infections to fewer than 200, 000. Fatal cases of malaria in India dropped from near one million to just two thousand per year.
However, the success of DDT application in the fight against insect-transmitted diseases led to massive misuse of the chemical. Widespread overuse quickly led to the development of resistant insects, upon which the poison had no effect. At the same time, evidence was accumulating that toxic levels of DDT were accumulating in the fatty tissues of animals higher on the food chain, including fish, mammals, birds, and humans. Like other chlorinated hydrocarbons, the persistence of DDT in the environment allows for biological magnification in nature, a process where minute quantities in run-off water are concentrated into toxic levels and travel upward in the food chain. Because of its harmful effects on vertebrates as well as insects, its creation of resistant insect species, its environmental persistence, and its biological magnification, the use of DDT has been banned in many countries, despite its general effectiveness.
Apart from DDT, there are other chlorinated hydrocarbon pesticides that have been developed. These include Chlordane, Aldrin, Mirex, and Toxaphene. Because other, less persistent, insecticide alternatives have been developed, the use of chlorinated hydrocarbon insecticides in general has fallen by the wayside in most places.
Close cousins to chlorinated hydrocarbons like chloroform or carbon tetrachloride are chlorofluoro-carbons, or CFCs. Chlorofluorocarbons are single carbon atoms with both the elements chlorine and fluorine chemically bonded to them. The compounds trichlorofluoromethane (Freon-11) and dichlorodifluoromethane (Freon-12) are widely used CFCs. They are odorless, nonflammable, very stable compounds used as refrigerants in commercial refrigerators and air conditioners. CFCs are also used as aerosol propellants, which launch products like hairspray and spray paint outward from cans. Very useful compounds, over 1, 500 million lb (700 million kg) of CFCs were made worldwide in 1985. However, because they destroy the ozone layer, they are being phased-out as propellants and refrigerants. By 2010 all CFCs are expected to be eliminated from all developed countries, per the Montreal Protocol (more formally, the Montreal Protocol on Substances That Deplete the Ozone Layer, which is an international treaty to protect Earth’s ozone layer).
Chlorinated hydrocarbons are incredibly useful in an astounding variety of products, making them an important part of modern life. Because they are environmentally persistent, new ways of cleaning up areas contaminated with chlorinated hydrocarbons are being developed. The term bioremediation refers to the use of living organisms to clean up chemically contaminated habitats. Currently, scientists are using genetic engineering to develop microorganisms that can degrade chlorinated hydrocarbons and plants that can absorb them from contaminated soil. In this way, pollution with chlorinated hydrocarbons, and hydrocarbons in general, can be efficiently remedied.
The limited supply of fossil fuels is a threat to future industry because all of the uses of chlorinated hydrocarbons, including the wide array of polymers, depends on building-block monomer molecules extracted from crude oil and other carbon-based fossil fuels. Global oil and gas reserves are dwindling at the same time demand for their use is skyrocketing. One possibility to meet demand for chlorinated
Chlorinated hydrocarbon —An important class of organic molecules composed of carbon, hydrogen, and chlorine atoms chemically bonded to one another.
Chloroform —A simple chlorinated hydrocarbon compound consisting of one carbon atom, one hydrogen atom, and three chlorine atoms. An important chemical, chloroform was widely used as a general anesthetic in the past.
DDT —The chlorinated hydrocarbon, dichlorodiphenyltrichloroethane. Once widely used as a potent insecticide, DDT use has been reduced due to its persistence in the environment over long periods of time.
Elastomer —An organic polymer that has rubber-like, elastic qualities.
Monomer —The repeating chemical unit of a polymer. Like cars in a train, monomers are chemically linked together to form long chains within a polymer.
Organic chemistry —Chemistry dealing with carbon-containing molecules. Most molecules containing carbon are considered to be organic.
Polymer —A large organic molecule consisting of a chain of smaller chemical units. Many plastic and rubber compounds are polymers.
PVC —PVC, or polyvinylchloride, is an important chlorinated hydrocarbon polymer plastic used in thousands of everyday household and industrial products. PVC is, perhaps, best known for its use in water pipes.
hydrocarbon products in the future might be the synthesis of organic molecules from coal, which is much more abundant than oil.
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"Chlorinated Hydrocarbons." The Gale Encyclopedia of Science. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/chlorinated-hydrocarbons
"Chlorinated Hydrocarbons." The Gale Encyclopedia of Science. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/chlorinated-hydrocarbons
Chlorinated hydrocarbons are compounds made of carbon , hydrogen , and chlorine atoms. These compounds can be aliphatic, meaning they do not contain benzene , or aromatic, meaning they do. The chlorine functional group gives these compounds a certain character; for instance, the aromatic organochlorine compounds are resistant to microbial degradation; the aliphatic chlorinated solvents have certain anesthetic properties (e.g., chloroform); some are known for their antiseptic properties (e.g., hexachloraphene). The presence of chlorine imparts toxicity to many organochlorine compounds (e.g., chlorinated pesticides).
Chlorinated hydrocarbons have many uses, including chlorinated solvents, organochlorine pesticides, and industrial compounds. Common chlorinated solvents are dichloromethane (methylene chloride), chloroform, carbon tetrachloride, trichloroethane, trichloroethylene, tetrachloroethane, tetrachloroethylene . These compounds are used in drycleaning solvents, degreasing agents for machinery and vehicles, paint thinners and removers, laboratory solvents, and in manufacturing processes, such as coffee decaffeination. These solvents are hazardous to human health and exposures are regulated in the workplace. Some are being phased out for their toxicity to humans and the environment , as molecules have the potential to react with and destroy stratospheric ozone .
The organochlorine pesticides include several subgroups, including the cyclodiene insecticides (e.g., chlordane , heptachlor, dieldrin), the DDT family of compounds and its analogs, and the hexachlorocyclohexanes (often incorrectly referred to as BHCs, or benzene hexachlorides). These insecticides were developed and marketed extensively after World War II, but due to their toxicity, persistence, widespread environmental contamination, and adverse ecological impacts, most were banned or restricted for use in the United States in the 1970s and 80s. These insecticides generally have low water solubilities, a high affinity for organic matter, readily bioaccumulate in plants and animals, particularly aquatic organisms, and have long environmental half-lives compared to the currently-used insecticides.
There are many chlorinated industrial products and reagent materials. Examples include vinyl chloride , which is used to make PVC (polyvinyl chloride ) plastics ; chlorinated benzenes, including hexachlorobenzene; PCB (polychlorinated biphenyl), used extensively in electrical transformers and capacitors; chlorinated phenols, including pentachlorophenol (PCP); chlorinated naphthalenes; and chlorinated diphenylethers. They represent a diversity of applications, and are valued for their low reactivity and high insulating properties.
There are also chlorinated byproducts of environmental concern, particularly the polychlorinated dibenzo-p-dioxins (PCDDs) and the polychlorinated dibenzofurans (PCDFs). These families of compounds are products of incomplete combustion of organochlorine-containing materials, and are primarily found in the fly ash of municipal solid waste incinerators. The most toxic component of PCDDs, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), was a trace contaminant in the production of the herbicide 2,4,5-T and is found in trace amounts in 2,4,5-trichlorophenol and technical grade pentachlorophenol. PCDDs and PCDFs can also be formed in the chlorine bleaching process of pulp and paper mills , and have been found in their effluent and in trace amounts in some paper products.
[Deborah L. Swackhammer ]
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"Chlorinated Hydrocarbons." Environmental Encyclopedia. . Encyclopedia.com. (August 20, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/chlorinated-hydrocarbons
"Chlorinated Hydrocarbons." Environmental Encyclopedia. . Retrieved August 20, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/chlorinated-hydrocarbons