The Halogens

Abstract

Abstract The halogens are those elements in group XVII of the periodic table, and include fluorine, chlorine, bromine, iodine, and astatine, the latter of which is a radioactive element of no industrial importance. The physical properties of the halogens are described. They indicate an almost perfect doubling of atomic weights progressing from fluorine to bromine, paralleled by increases in specific gravity and melting and boiling points, and by decreases in water solubility. Chlorine and fluorine exist in the earth's crust in almost equal proportions (770 ppm for fluorine, 550 ppm for chlorine). The relative abundance of bromine and iodine are only about 2 and 0.04%, respectively, of that for chlorine. Seawater contains almost 19,000 ppm chlorine, compared to 65 ppm for bromine and less than 2 ppm for fluorine. Iodine exists only in trace quantities in seawater (about 0.05 ppm). Elements become progressively less electronegative and have less oxidizing potential as atomic weight increases. Each halogen forms an acid in water and combines with metals to form salts; the reactivity of these compounds shows the same relationship as the elemental halogens. Chemically, fluorine is the most powerful oxidizing agent known. It is the most reactive of all the elements. Free fluorine is rarely, if ever, found in nature. Elemental fluorine is produced on a commercial scale by electrolysis. Common operations where fluorine exposure occurs include the manufacture of fluorochemicals and plastics, rocket propellants, and fluorinated intermediates, metal production such as aluminum potroom work, the fluorination of pharmaceuticals and consumer products such as dentifrices, and the fluoridation of public drinking‐water supplies. Environmental contamination and air pollution with fluorine and fluorides may occur as a result of emissions from facilities for aluminum production, glass and ceramic manufacture, fertilizer manufacture, and the processing of fluorspar. Because of the reactivity of fluorine, exposures of humans and animals, and environmental contamination problems are almost always the result of fluorides, rather than fluorine gas. Most studies of fluorine itself are artificial and of academic rather than practical interest. Functionally, acute exposures to fluorine gas must be regarded as severe and potentially lethal corrosive exposures, with the added nuances of disturbances in calcium metabolism because of the reaction between the calcium ion and the fluoride ion. Chlorine is associated with the largest array of industrially useful compounds of all the halogens; it is the ninth highest volume chemical produced in the United States. Chemically, chlorine is more reactive than either bromine or iodine; it displaces bromine and iodine from their salts and enters into substitution and addition reactions with both inorganic and organic substances. When moist, but not when dry, chlorine unites directly with most elements. The physical properties of chlorine are given. Recognition of widespread environmental contamination problems associated with persistent chlorinated organic compounds has led to pressure to reduce the use of chlorinated compounds in industrial chemicals. The outcome of this concern is unclear at this time, but it is likely to lead to substitution of other compounds for those containing environmentally persistent and toxic chlorinated materials. Although most of the identified compounds that present environmental contamination problems are higher molecular weight organic compounds, the use of chlorinated solvents in cleaning and degreasing operations has already been reduced and will be further reduced because of their impact on the ozone layer. Higher molecular weight organic compounds containing chlorine have been identified as possible xenoestrogens. The concern of the scientific community is high, and much research is under way to clarify the role of halogenated xenoestrogens in biological systems. It is likely that if the halogenated organic compounds are confirmed to be biologically detrimental, there will be a further reduction in the production and use of these compounds. The introduction of chemically reactive inorganic halogens, principally chlorinated compounds, into the environment in groundwater and air may lead to active halogenation of natural products to xenoestrogens. If this pathway is confirmed, this, too, will lead to a reduction in the use of chlorinated compounds. Chlorine is noncombustible in air but will support the combustion of other materials. It reacts explosively or forms explosive mixtures with many common materials, including acetylene, turpentine, ammonia gas, fuel gas, hydrocarbons, hydrogen, and finely divided metals. Chlorine may also combine with water or steam to produce hydrogen chloride (HCl) fume. The reactivity of bromine lies between that of chlorine and iodine. Bromine will cause ignition of organic materials, including wood, cotton, and straw. It reacts violently on contact with natural rubber and reacts explosively with a number of common substances, including aldehydes, ketones, carboxylic acids, acetylene, acrylonitrile, ammonia, ethyl phosphine, hydrogen, nickel carbonyl, ozone, oxygen difluoride, phosphorus, potassium, sodium, and sodium carbide. Because of its explosive potential, facilities where bromine is manufactured or used should be designed to dispose rapidly of liquid bromine spills. Although it is estimated that 1015–1016 tons of bromine are contained in the earth's crust, it is widely distributed and found only in low concentrations in the form of bromide salts. The most readily recoverable form of bromine occurs as soluble salts in salt lakes, inland seas, brine wells such as those in Michigan and Arkansas, and seawater. Today, little bromine is extracted from seawater, which contains bromide salts in a concentration of only 65 ppm. The largest single current use of bromine is for the production of fire retardants. Agricultural chemical production consumed about 10% of bromine production, primarily as methyl bromide. However, recent restrictions on the use of brominated pesticides, such as ethylene dibromide and DBCP and proposed limitations on such fumigants as methyl bromide are expected to further reduce the use of these brominated chemicals in the future. Skin or eye contact with vapor or liquid bromine pentafluoride causes painful, deep‐seated, long‐lasting burns. The acute effect of this substance on the lung is similar to that of phosgene. Iodine is the 47th most abundant element in the earth's crust. The name iodine derives from the Greek word for violet‐colored, ioeides , which was used to describe the purple vapor generated by heating iodine. It is the heaviest of the halogens that are of industrial interest. Under ordinary conditions, iodine takes the form of gray‐black plates or granules that have a metallic, crystalline luster. It volatilizes at room temperature to yield a sublimed, violet vapor. Iodine's physical properties are shown. Although iodine resembles other members of the halogen group, it is the least electronegative; it is thus the least chemically reactive of the halogens and forms the weakest bonds with more electropositive elements. Iodine is used both in animal and human medicine, where its disinfectant and antiseptic properties are valued. The lack of iodine causes goiter (compensatory hypertrophy of the thyroid gland), and iodine is used both to treat iodine deficiency and hyperthyroidism. Principal iodine compounds and their industrial uses are shown. The inorganic iodine compounds of commercial interest, and their physical properties, are discussed. The iodides, an important class of inorganic iodine compounds, have less tendency to form complexes than the other halides. Chlorine and bromine freely displace iodine from the iodides. Iodine forms industrially useful and important compounds with hydrogen, metals, the other halogens, and oxygen. Those presented here are typical.