complex of processes, operations, and organizations engaged in the manufacture of chemicals and their derivatives.

Although the chemical industry may be described simply as the industry that uses chemistry and manufactures chemicals, this definition is not altogether satisfactory because it leaves open the question of what is a chemical. Definitions adopted for statistical economic purposes vary from country to country. Also the Standard International Trade Classification, published by the United Nations, includes explosives and pyrotechnic products as part of its chemicals section. But the classification does not include the man-made fibres, although the preparation of the raw materials for such fibres is as chemical as any branch of manufacture could be.

The scope of the chemical industry is in part shaped by custom rather than by logic. The petroleum (petroleum refining) industry is usually thought of as separate from the chemical industry, for in the early days of the petroleum industry in the 19th century crude oil was merely subjected to a simple distillation treatment. Modern petroleum industrial processes, however, bring about chemical changes, and some of the products of a modern refinery complex are chemicals by any definition. The term petrochemical is used to describe these chemical operations, but, because they are often carried out at the same plant as the primary distillation, the distinction between petroleum industry and chemical industry is difficult to maintain.

Metals (metal) in a sense are chemicals because they are produced by chemical means, the ores sometimes requiring chemical methods of dressing before refining; the refining process also involves chemical reactions. Such metals as steel, lead, copper, and zinc are produced in reasonably pure form and are later fabricated into useful shapes. Yet the steel industry, for example, is not considered a part of the chemical industry. In modern metallurgy, such metals as titanium, tantalum, and tungsten are produced by processes involving great chemical skill, yet they are still classified as primary metals.

The boundaries of the chemical industry, then, are somewhat confused. Its main raw materials are the fossil fuels (coal, natural gas, and petroleum), air, water, salt, limestone, sulfur or an equivalent, and some specialized raw materials for special products, such as phosphates and the mineral fluorspar. The chemical industry converts these raw materials into primary, secondary, and tertiary products, a distinction based on the remoteness of the product from the consumer, the primary being remotest. The products are most often end products only as regards the chemical industry itself; a chief characteristic of the chemical industry is that its products nearly always require further processing before reaching the ultimate consumer.

Thus, paradoxically, the chemical industry is its own best customer. An average chemical product is passed from factory to factory several times before it emerges from the chemical industry into the market.

There are many routes to the same product and many uses for the same product. The largest use for ethylene glycol, for example, is as an automobile antifreeze, but it is also used as a hydraulic brake fluid. Further processing leads to many derivatives that are used as additives in the textile, pharmaceutical, and cosmetic industries; as emulsifiers in the application of insecticides and fungicides; and as demulsifiers for petroleum. The fundamental chemicals, such as chlorine or sulfuric acid, are used in so many ways as to defy a comprehensive listing.

Because of the competitiveness within the chemical industry and among the chemicals, the chemical industry spends large amounts on research (research and development), particularly in the highly industrialized countries. The percentage of revenue spent on research varies from one branch to another; companies specializing in large-volume products that have been widely used for many years spend less, whereas competition in the newer fields can be met only by intensive research efforts.

In most fields the United States is the largest producer of chemicals. Germany, the United Kingdom, France, Italy, and some other European countries are also large producers, and so is the Soviet Union. Japan in the 1960s came into prominence as a very large producer in certain areas. Investment in the chemical industry as a percentage of total investment in a given country may range from 5 to 15 percent for the less-developed countries; for the industrial countries it averages about 6 to 8 percent. For some developing countries this percentage can fluctuate widely; for example, the installation of one sizable fertilizer factory could change the percentage markedly in a specific country.

Early in the 20th century there was a marked distinction between economies that were based on coal as a fossil fuel and those based on petroleum. Coal was almost the unique source of the aromatic hydrocarbons. Two forces, however, have worked together to change this situation. First, aromatics can now also be obtained from petroleum, and indeed all hydrocarbon raw materials are now almost interchangeable; second, modern transportation technology makes possible very large-scale shipments by sea not only of petroleum, crude or in various stages of refinement, but also of natural gas, refrigerated and condensed to a liquid.

Statistics from the chemical industry as a whole can be misleading because of the practice of lumping together such products as inexpensive sulfuric acid and expensive dyes or fibres; included in some compilations are cosmetics and toiletries, the value of which per pound may be artificially high. Chemical industry statistics from different countries may have different bases of calculation; indeed the basis may change from time to time in the same country. An additional source of confusion is that in some cases the production is quoted not in tons of the product itself but in tons of the content of the important component.

For purposes of simplicity, various divisions of the chemical industry, such as heavy inorganic and organic chemicals and various families of end products, will be described in turn and separately, although it should be borne in mind that they interact constantly. The first division to be discussed is the heavy inorganic chemicals, starting at the historical beginning of the chemical industry with the Leblanc process. The terms heavy chemical industry and light chemical industry, however, are not precisely exclusive, because numerous operations fall somewhere between the two classes. The two classes do, however, at their extremes correlate with other differences. For example, the appearance of two kinds of plants is characteristically different. The large-scale chemical plant is characterized by large pieces of equipment of odd shapes and sizes standing immobile and independent of one another. Long rows of distilling columns are prominent, but, because the material being processed is normally confined in pipes or vessels, no very discernible activity takes place. Few personnel are in evidence.

The light chemical industry is entirely different. It involves many different pieces of equipment of moderate size, often of stainless steel or lined with glass or enamel. This equipment is housed in buildings like those for, say, assembling light machinery. Numerous personnel are present. Both types of plant require large amounts of capital.

In 1775 the French Academy of Sciences offered an award for a practical method for converting common salt, sodium chloride, into sodium carbonate, a chemical needed in substantial amounts for the manufacture of both soap and glass. Nicolas Leblanc (Leblanc, Nicolas), a surgeon with a bent for practical chemistry, invented such a process. His patron, the duc d'Orléans, set up a factory for the process in 1791, but work was interrupted by the French Revolution. The process was not finally put into industrial operation until 1823 in England, after which it continued to be used to prepare sodium carbonate for almost 100 years.

The first step in the Leblanc process was to treat sodium chloride with sulfuric acid. This treatment produced sodium sulfate and hydrogen chloride. The sodium sulfate was then heated with limestone and coal to produce black ash, which contained the desired sodium carbonate, mixed with calcium sulfide and some unreacted coal. Solution of the sodium carbonate in water removed it from the black ash, and the solution was then crystallized. From this operation derives the expression soda ash that is still used for sodium carbonate.

It was soon found that when hydrogen chloride was allowed to escape into the atmosphere, it caused severe damage to vegetation over a wide area. To eliminate the pollution problem, methods to convert the dissolved hydrogen chloride to elemental chlorine were developed. The chlorine, absorbed in lime, was used to make bleaching powder, for which there was a growing demand.

Because calcium sulfide contained in the black ash had a highly unpleasant odour, methods were developed to remove it by recovering the sulfur, thereby providing at least part of the raw material for the sulfuric acid required in the first part of the process. Thus the Leblanc process demonstrated, at the very beginning, the typical ability of the chemical industry to develop new processes and new products, and often in so doing to turn a liability into an asset.

The Leblanc process was eventually replaced by the ammonia-soda process (called the Solvay process), which was first practiced successfully in Belgium in the 1860s. In this process, sodium chloride as a strong brine is treated with ammonia and carbon dioxide to give sodium bicarbonate and ammonium chloride. The desired sodium carbonate is easily obtained from the bicarbonate by heating. Then, when the ammonium chloride is treated with lime, it gives calcium chloride and ammonia. Thus, the chlorine that was in the original sodium chloride appears as calcium chloride, which is largely discarded (among the few uses for this compound is to melt snow and ice from roads and sidewalks). The ammonia thus regenerated is fed back into the first part of the process. Efficient recovery of nearly all the ammonia is essential to the economic operation of the process, the loss of ammonia in a well-run operation being no more than 0.1 percent of the weight of the product.

Later in the 19th century the development of electrical power generation made possible the electrochemical (electrochemistry) industry. This not clearly identifiable branch of the chemical industry includes a number of applications in which electrolysis, the breaking down of a compound in solution into its elements by means of an electric current, is used to bring about a chemical change. Electrolysis of sodium chloride (salt) can lead to chlorine and either sodium hydroxide (if the NaCl was in solution) or metallic sodium (if the NaCl was fused). Sodium hydroxide, an alkali like sodium carbonate, in some cases competes with it for the same applications, and in any case the two are interconvertible by rather simple processes. Sodium chloride can be made into an alkali by either of the two processes, the difference between them being that the ammonia-soda process gives the chlorine in the form of calcium chloride, a compound of small economic value, while the electrolytic processes produce elemental chlorine, which has nearly innumerable uses in the chemical industry, including the manufacture of plastic polyvinyl chloride, the plastic material produced in the largest volume. For this reason the ammonia-soda process, having displaced the Leblanc process, has found itself being displaced, the older ammonia-soda plants continuing to operate very efficiently but no new ammonia-soda plants being built.

The need for sodium carbonate in the manufacture of soap and glass that led to the Leblanc process also led to the creation of the alkali industry and the chlor-alkali industry, another of the historic landmarks of the chemical industry (see Chlorine (chemical industry)).

Sulfuric acid is by far the largest single product of the chemical industry. The chamber process for its preparation on the scale required by the Leblanc process might be regarded as the most important long-term contribution of the latter.

When sulfur is burned in air, sulfur dioxide is formed, and this, when combined with water, gives sulfurous acid. To form sulfuric acid, the dioxide is combined with oxygen to form the trioxide, which is then combined with water. A technique to form the trioxide, called the chamber process, developed in the early days of the operation of the Leblanc process. In this technique the reaction between sulfur dioxide and oxygen takes place in the presence of water and of oxides of nitrogen. Because the reaction is rather slow, sufficient residence time must be provided for the mixed gases to react. This gaseous mixture is highly corrosive, and the reaction must be carried out in containers made of lead.

Lead is a material awkward to use in construction, and the process cannot deliver acid more concentrated than about 78 percent without special treatment. Therefore, the chamber process has been largely replaced by the contact process, in which the reaction takes place in a hot reactor, over a platinum or vanadium compound catalyst, a substance that increases the speed of the reaction without becoming chemically involved.

Of the large world production of sulfuric acid, almost half goes to the manufacture of superphosphate and related fertilizers (fertilizer). Other uses of the acid are so multifarious as almost to defy enumeration, notable ones being the manufacture of high-octane gasoline, of titanium dioxide (a white pigment, also a filler for some plastics, and for paper), explosives, rayon, the processing of uranium, and the pickling of steel.

Because sulfuric acid is indispensable to so many industries, its primary raw material is of the greatest importance. The needed sulfur is obtainable from a number of sources. Originally, sulfur came chiefly from certain volcanic deposits in Sicily. By the beginning of the 20th century this source was insufficient, but the supply was augmented by sulfur that occurs underground in the southern United States. This sulfur is not mined but is recovered by the so-called Frasch process, in which the sulfur is melted underground by hot water and the mixture brought to the surface in liquid form.

Other sources of sulfur include the ore iron pyrite, an iron-sulfur compound that can be burned to produce sulfur dioxide, and some natural gases, called sour gas, that contain appreciable quantities of hydrogen sulfide. Certain metal sulfides, such as those of zinc and copper, are contained in the ores of those metals. When these ores are roasted, sulfur dioxide is given off. Sulfur is usually shipped in its elemental form rather than in the form of sulfuric acid.

Under some circumstances, the sulfuric acid stage of manufacture can be avoided. Ammonium sulfate, a fertilizer, is normally made by causing ammonia to react with sulfuric acid. In many parts of the world, abundant supplies of calcium sulfate in any of several mineral forms can be used to make the ammonium sulfate by combining it with ammonia and water. This process brings the sulfur in the calcium sulfate deposits into use. Because deposits of calcium sulfate throughout the world are extensive, development of such a process would make the available resources of sulfur almost limitless.

The sulfur present in low percentages in fossil fuels (fossil fuel) is a notorious source of air pollution in most industrial countries. Removal of sulfur from crude oil adds to the sulfur supply and reduces pollution. It is less easy to remove the sulfur directly from coal.

Carbon disulfide is made by the reaction of carbon and sulfur. Carbon comes from natural gas, and the sulfur may be supplied in the elemental form, as hydrogen sulfide, or as sulfur dioxide. The chief uses of carbon disulfide are for the manufacture of rayon and for regenerated cellulose film. These two products are made in such large quantity that carbon disulfide is a heavy chemical, by any standard.

Fertilizers represent one of the largest market commodities for the chemical industry. A very large industry in all industrialized countries, it is a very important one for introduction as early as possible into developing countries.

The crucial elements that have to be added to the soil in considerable quantities in the form of fertilizer are nitrogen, phosphorus, and potassium, in each case in the form of a suitable compound. These are the major fertilizer elements, or macronutrients. Calcium, magnesium, and sulfur are regarded as secondary nutrients; and it is sometimes necessary to add them. Numerous other elements are required only in trace quantities; certain soils may be deficient in boron, copper, zinc, or molybdenum, making it necessary to add very small quantities of these. As a great industry, however, fertilizers are based on the three elements mentioned above.

The simplest part of this diagram is the portion representing potassium. The element potassium is seventh in order of abundance in the Earth's crust, about the same order as sodium, which it resembles very closely in its properties. Although sodium is readily available in the sodium chloride in the ocean, most of the potassium is contained in small proportions in a large number of mineral formations, from which it cannot be economically extracted. When the use of potassium salts as fertilizers began in the second half of the 19th century, it was believed that Germany had a monopoly with the deposits at Stassfurt, but many other workable deposits of potassium salts were later found in other parts of the world. World reserves are adequate for thousands of years, with large deposits in the Soviet Union, Canada (Saskatchewan), and Germany (East and West).

Potassium chloride is the principal commercial form of potash, and some potassium nitrate is also produced. About 90 percent of the production of these goes to fertilizers. For other purposes, the similar sodium salts are cheaper, but for a few special uses potassium has the advantage. Some ceramic uses require potassium, and potassium bicarbonate is more effective than sodium bicarbonate in extinguishing fires.

Phosphorus presents a more complicated picture. It has many uses other than in fertilizers. By far the largest source is phosphate rock (phosphorite), although some use is made of phosphatic iron ore, from which the phosphorus is obtained as a by-product from the slag. As with potassium, there are extensive reserves. The largest deposits are in North Africa (Morocco, Algeria, Tunisia), the United States (largely Florida), and the Soviet Union, but there are also sizable deposits in numerous other countries.

Phosphate rock is found in deposits of sedimentary origin, laid down originally in beds on the ocean floor. The rock consists largely of the insoluble tricalcium phosphate, together with some other materials, including some fluorine. To be used as a fertilizer, phosphate must be converted to a form that is soluble in water, even if only slightly so.

The fertilizer industry is not only a matter of manufacturing the right chemical but also of distribution, getting the right material to the right place at the right time. Fertilizers are made centrally but must be distributed over a large agricultural area. A fertilizer factory is, typically, a large installation, characterized by enormous storage silos; the product is manufactured all the year round, but it requires considerable space to store it until the few weeks during which it is distributed on farmlands.

The weight of the superphosphate is greater than that of the original phosphate rock by the amount of the sulfuric acid added; the superphosphate also carries the dead weight of the calcium sulfate that is formed in the manufacturing process. This dead weight can be reduced by replacing sulfuric acid with phosphoric acid (itself obtained by the action of sulfuric acid on phosphate rock, followed by separating the products; or else by an electric furnace process). This process results in triple superphosphate, in which all the calcium originally in the phosphate rock appears as calcium monophosphate. The useful content of the fertilizer, expressed as the percent of phosphoric oxide, is increased from 20 percent in ordinary superphosphate to about 45 percent in the triple variety, resulting in a better than twofold reduction in the amount of material that must be distributed to provide a given amount of the useful oxide.

Instead of using either sulfuric or phosphoric acid to treat the phosphate rock, nitric acid can be employed. One of the resulting products, calcium nitrate, is itself a fertilizer, so what is obtained is one of the many varieties of mixed fertilizers. Instead of neutralizing phosphoric acid with calcium, which contributes nothing but dead weight, ammonia can be used, giving ammonium phosphate, in which both constituents contribute fertilizer elements. Such improvements in fertilizers are constantly being made.

Many other compounds of phosphorus are used. One group is composed of phosphoric acid and various phosphates derived from it. The acid itself is used in soft drinks for its pleasant taste when sweetened and its nutritive value. Other food applications include the use of disodium phosphate in processed cheese; and phosphates in baking powder, flameproofing, and the treatment of boiler water in steam plants. An important use of some of the phosphates is in detergents, discussed below.

Elemental phosphorus exists in many allotropic forms. White phosphorus is used in rodent poison and by the military for smoke generation. Red phosphorus, comparatively harmless, is used in matches. Ferrophosphorus, a combination of phosphorus with iron, is used as an ingredient in high-strength low-alloy steel. In addition, the many organic compounds of phosphorus have varied uses, including those as additives for gasoline and lubricating oil, as plasticizers for plastics that otherwise would be inconveniently rigid, and, in some cases, as powerful insecticides, related to nerve poisons.

The production of nitrogen not only is a major branch of the fertilizer industry, but it opens up a most important segment of the chemical industry as a whole.

Farm manure long supplied enough nitrogenous fertilizer for agriculture, but late in the 19th century it was realized that agriculture was outgrowing this source. A certain amount of ammonium sulfate was available as a by-product of the carbonization of coal, and the large deposits of sodium nitrate discovered in Chile helped for a time. The long-range problem of supply, however, was not solved until just before World War I, when the research of Fritz Haber (Haber, Fritz) in Germany brought into commercial operation the method of ammonia synthesis that is used, in principle, today. The immediate motivation for this great development was Germany's need for an indigenous source of nitrogen for military explosives. The close interrelation between the use of nitrogen for fertilizers and for explosives persists to this day.

Because air is 78 percent nitrogen, there is a little more than 11 pounds of nitrogen over every square inch of the earth's surface. Nitrogen, however, is a rather inert element; it is difficult to get it to combine with any other element. Haber succeeded in getting nitrogen to combine with hydrogen by the use of high pressure, moderately high temperatures, and a catalyst.

The hydrogen for ammonia (NH₃) is usually obtained by decomposing water (H₂O). This process requires energy, in some cases supplied by electricity, but more often from fossil fuels. In some cases the hydrogen is obtained directly from the fossil fuel, without decomposing water.

Haber used coke as a fuel. Carbon can burn either to carbon dioxide or, if the supply of air is kept short, to carbon monoxide, by a process known as the producer gas reaction. The gaseous product is a mixture of carbon monoxide with the nitrogen that was originally in the air.

The red-hot coke can also be heated with steam to yield carbon monoxide and hydrogen, a mixture known as water gas. It is also possible to carry out a water-gas shift reaction by passing the water gas with more steam over a catalyst, yielding more hydrogen, and carbon dioxide. The carbon dioxide is removed by dissolving it in water at a pressure of about ten atmospheres; it can also be utilized directly, as noted below. Starting then from water gas, and converting a certain proportion of the carbon monoxide to carbon dioxide and hydrogen, it is possible to arrive at a mixture of carbon monoxide and hydrogen in any proportion.

The above description is a simplified account of how synthesis gas, either for ammonia or for methanol, is obtained from fossil fuel as a source of energy, but it gives an idea of the versatility of the operations. There are many possible variations in detail, depending largely on the particular fuel that is used. The nitrogen industry, which has grown steadily since shortly after World War I, was originally based largely on coke, either from coal or lignite (brown coal). There has been a gradual change to petroleum products as the fossil fuel. As is true with many other branches of the chemical industry, the latest trend is to move to natural gas.

The carbon dioxide removed during the preparation of the synthesis gas can be caused to react with ammonia, often at the same plant, to form urea, CO(NH₂)₂. This is an excellent fertilizer, highly concentrated in nitrogen (46.6 percent), and also useful as an additive in animal feed to provide the nitrogen for formation of meat protein. Urea is also used for an important series of resins and plastics by reaction with formaldehyde, derived from methanol.

By far the most important use of ammonia within the chemical industry is to produce nitric acid (HNO₃). Nitrogen and oxygen can be made to combine directly with one another only with considerable difficulty. A process based on such a direct combination, but employing large quantities of electrical power, was in use in the 1920s and 1930s in Norway, where hydroelectric power is readily available. It has not proved economical in modern conditions.

Ammonia burns in air, or in oxygen, causing the hydrogen atoms to burn off, forming water and leaving free nitrogen. With the aid of a catalyst, platinum with a small percentage of the related metal rhodium, ammonia is oxidized to oxides of nitrogen that can be made to react with water to form nitric acid.

Nitric acid treated with ammonia gives ammonium nitrate, a most important fertilizer. Ammonium nitrate, moreover, is also an important constituent of many explosives (explosive). Three fundamental explosive materials are obtained by nitrating (treating with nitric acid, often in a mixture with sulfuric acid): cellulose, obtained from wood, gives cellulose nitrate (formerly called nitrocellulose); glycerol gives glyceryl trinitrate (formerly called nitroglycerin); and toluene gives trinitrotoluene, or TNT. Another explosive ingredient is ammonium picrate, derived from picric acid, the relationship of which appears more clearly in its systematic name, 2,4,6-trinitrophenol.

A minor but still important segment of the explosives industry is the production of detonating agents, or such priming compositions as lead azide 【Pb(N₃)₂】, silver azide (AgN₃), and mercury fulminate 【Hg(ONC)₂】. These are not nitrates or nitro compounds, although some other detonators are, but they all contain nitrogen, and nitric acid is involved in their manufacture.

Related to the explosives are the rocket propellants. A rocket-propelled missile or spacecraft launch vehicle must carry both reactive components (fuel and oxidizer) with it, either in different molecules or in the same molecule. Essentially, rocket propellants consist of an oxidant and a reductant. The oxidant is not necessarily a derivative of nitric acid but may also be liquid oxygen, ozone (O₃), liquid fluorine, or chlorine trifluoride.

Other uses for nitric acid not related to explosives or propellants include the production of cellulose nitrate (nitrocellulose) for use in coatings. Without a pigment it forms a clear varnish much used in furniture finishing. Pigmented, it forms brilliant shiny coatings referred to as lacquers. At one time a fibre similar to rayon was made from cellulose nitrate.

The above account gives an idea of the importance, not only for fertilizers but for many other products, of the process known as fixation of atmospheric nitrogen (nitrogen fixation)—that is, taking nitrogen from the air and converting it into some form in which it is usable. The tremendous increase in the production of fertilizers has led to the erection of huge ammonia plants. The plants must have available a source of fossil fuel, but petroleum and natural gas are easily transported, so that there is a tendency to locate the plants near the ultimate destination of the product.

A typical plant comprises all the equipment for the preparation of the synthesis gas on the requisite scale, along with equipment for purifying the gas. In the synthesis of ammonia (but not of methanol) any compound of oxygen is a poison for (reduces the effectiveness of) the catalyst, and so traces of carbon dioxide and carbon monoxide must be carefully removed. Compressing the gas to the desired pressure requires extensive engineering (chemical engineering) equipment. The higher the pressure the greater the yield but the higher the actual cost of compression. The higher the temperature the lower the yield, but the temperature cannot be lowered indefinitely to obtain better yields because lower temperatures slow down the reaction. The temperature used is of the order of 500° C (930° F). The choice of temperature and of pressure is a carefully worked out compromise to give optimal results. The yield equals the amount of nitrogen and hydrogen that combine to form ammonia in any one pass through the converters. Only a fraction is converted each time, but after each pass the ammonia is removed and the remaining gas is recycled. Atmospheric nitrogen contains about 1 percent argon, a totally inert gas, which must be removed from time to time so that it does not build up in the system indefinitely. There is also usually a nearby nitric acid factory and equipment for producing ammonium nitrate in the exact grain size for convenient application as fertilizer.

The growing plants of agricultural crops do not receive all of their nitrogen from synthetic fertilizer. A certain proportion is supplied by natural means. Some plants, notably beans, have a symbiotic relationship with nitrifying bacteria that are able to “fix” the nitrogen in the air and to combine it into a form available for plant life. This natural synthesis takes place without the necessity for pressures of several hundred atmospheres or of high temperatures. In many laboratory syntheses of natural products, great success has been obtained by quiet reactions, without extreme conditions, by a process of following nature gently, rather than by brute force. Ammonia may some day be synthesized by some process that more nearly resembles the natural way; research is being carried out along these lines, and it may be that a far easier approach to fixation of atmospheric nitrogen will be part of the chemical industry of the future.

The first large-scale use of chlorine was in the manufacture of bleaching powder for use in making paper and cotton textiles. Bleaching powder was later replaced by liquid chlorine, which also came into widespread use as a germicide for public water supplies. Presently the principal use of chlorine is in making chemical compounds. Important inorganic chemicals made by direct action of chlorine on other substances include sulfur chloride, thionyl chloride, phosgene, aluminum chloride, iron(III) chloride, titanium(IV) chloride, tin(IV) chloride, and potassium chlorate.

Organic chemicals made directly from chlorine include derivatives of methane (methyl chloride, methylene chloride, chloroform, and carbon tetrachloride); chlorobenzene and ortho- and para-dichlorobenzenes; ethyl chloride; and ethylene chloride.

Of several processes that have been used for the manufacture of chlorine, the oldest employed the reaction of hydrochloric acid with manganese dioxide. The procedure was inefficient, and its commercial application was short-lived.

A process introduced about 1868 by the English chemist Henry Deacon was based on the reaction of atmospheric oxygen with hydrochloric acid, which was available as a by-product of the Leblanc process for making soda ash; when the Leblanc process became obsolete, the Deacon process fell into disuse.

The chlor-alkali industry—in which chlorine and caustic soda (sodium hydroxide) are produced simultaneously by electrolytic (electrolysis) decomposition of salt (sodium chloride)—has become the principal source of chlorine during the 20th century. As noted earlier, in the two important versions of the electrolytic process, brine is the electrolyte (in which the passage of electric current occurs by the movement of charged particles called ions), and graphite rods are the anodes (positive terminals). The difference between the two processes derives from the distinct behaviour of iron and of mercury when those metals are used as cathodes (negative terminals).

In brine, the two substances susceptible to chemical reduction are positively charged sodium ions and neutral water molecules. At a reversible cathode, reduction of sodium ions requires a higher voltage than does the reduction (oxidation–reduction reaction) of water molecules, and application of a voltage high enough to reduce sodium ions would effect reduction of a considerable amount of water but of a very small number of sodium ions. The reaction occurring at the surface of an iron cathode is represented by the following equation:

At a mercury cathode, on the other hand, appreciable reduction of water requires a much higher voltage than that needed at an iron cathode. This so-called overpotential is so great, in fact, that the electrode voltage can be raised to that needed for the reduction of sodium ions without affecting the water molecules.

Passage of a direct electric current through brine is attended by chemical changes at the surfaces where the electrodes come in contact with the electrolyte. At the graphite anode, chloride ions present in the dissolved salt are converted by oxidation to elemental chlorine, which is led away through a vent. At the iron cathode, reduction of water takes place, according to the equation shown above. The hydrogen gas is removed, while the hydroxide ions remain in the solution. The net result is that chloride ions and water are consumed and chlorine gas, hydrogen gas, and hydroxide ions are produced. Complete conversion of chloride to hydroxide is not practical, but as brine is continuously introduced at the top of the cell, a solution containing nearly equal amounts of salt and caustic soda is withdrawn at the bottom. Purification of the effluent liquor yields solid sodium hydroxide containing only a small amount of salt.

Successful production of chlorine and caustic soda in these cells requires that the two products be separated, because upon mixing they would react with one another. The chlorine is kept away from the caustic by interposing a diaphragm between the electrodes: such cells are commonly called diaphragm cells.

In the other main variant of the chlor-alkali process, the so-called mercury cell is employed. The cathode in such a cell is a shallow layer of mercury flowing across the bottom of the vessel; graphite anodes extend down into the brine electrolyte. A powerful direct current is caused to pass between the graphite rods and the mercury surface. At the anodes, chloride ions are converted to chlorine gas, as in the diaphragm cell; the reaction occurring at the mercury cathode, however, differs from that at an iron cathode. Positively charged sodium ions in the brine migrate to the mercury surface, where the voltage is high enough to reduce them to sodium metal without reducing the water because of the above-noted overpotential of mercury. The metallic sodium formed at the cathode dissolves in the mercury, and the solution (called an amalgam) flows out of the cell into another vessel, where it is brought into contact with water, which reacts with the sodium to form sodium hydroxide and hydrogen.

The overall result of operating a mercury cell is the same as that of operating a diaphragm cell: sodium chloride and water are changed into sodium hydroxide, chlorine, and hydrogen. Use of the mercury cell, however, makes it possible to generate the sodium hydroxide in the absence of salt, so that evaporation of the caustic liquor produces solid sodium hydroxide completely free of sodium chloride. The higher purity of the product makes it more desirable for certain applications, notably in the manufacture of rayon.

The fluorine industry is intimately related to the production of aluminum. Alumina (aluminum oxide, Al₂O₃) can be reduced to metallic aluminum by electrolysis when fused with a flux consisting of sodium fluoroaluminate (Na₃AlF₆), usually called cryolite. After starting the process, the cryolite is not used up in massive quantities, but a small supply is needed to make up for inevitable losses. Cryolite is a rare mineral, however, found in commercial quantities only in Greenland. The supply is limited, and it has other uses in glass, in enamels, and as a filler for resin-bonded grinding wheels.

The supply problem was solved by the development of synthetic cryolite. For this synthetic, however, a source of fluorine was needed. Fluorine is actually somewhat more abundant in the Earth's crust than chlorine, but most of it is distributed in various rocks in very small quantities. In a form available to the industrial chemist, it is much scarcer than chlorine. Until the 1960s almost the only source was fluorspar (fluorite) (CaF₂), a mineral long known and used as a flux in various metallurgical operations. It is still so used, in quantities larger than before, because the processes that are coming into greatest use for making steel, the basic oxygen process and the electric furnace, use two to three times as much flux as the earlier open-hearth furnaces did. The mineral fluorspar is widely distributed, but the supplies of good quality ore are not large; it has been found necessary to utilize lower grade ores, making the processing more expensive. A very large reserve that can be tapped for fluorine is the 3 percent or so that is present in some phosphate rock. In the past this fluorine content was seldom recovered; the future will undoubtedly see a major reversal.

These inorganic uses, as a flux and in the manufacture of aluminum, formerly constituted almost the whole of the fluorine industry. The organic fluorine industry, a separate branch, began in the late 1920s with the discovery by Thomas Midgley, Jr. (Midgley, Thomas, Jr.), of the United States, of the fluorine-containing refrigerants. A new refrigerant was needed for the domestic refrigerators that were just beginning to be produced on a large scale. Ammonia was unsuitable because even a minute leak would give an unpleasant smell, and breakdown would release poisonous quantities of the gas. Although many fluorine compounds were known to be poisonous, Midgley found some that were remarkably nontoxic. They also had the physical properties required for a refrigerant and were totally odourless.

The most used of these is Freon 12 (CCl₂F₂), dichlorodifluoromethane; also used is Freon 22 (CHClF₂), chlorodifluoromethane. Several analogous compounds containing carbon, fluorine, chlorine, and sometimes hydrogen are available.

The next advance in the fluorine industry was connected with the development of the atomic bomb during World War II. It was necessary to separate the small proportion of the fissionable isotope uranium-235 from other, nonfissionable uranium isotopes. This separation could be done by diffusion, working with uranium hexafluoride, a gas. Fluorine at that time was made only occasionally on a small laboratory scale, and it had a reputation for intense chemical reactivity and for being difficult to handle. The solution to the problem of large-scale preparation of elemental fluorine, which required the development and introduction of novel, fluorine-resistant materials of construction, made this important element generally available. Fluorine manufacture is now routine. Other uses have been developed: as a component in some rocket propellants, for the preparation of the extremely reactive interhalogen compounds such as chlorine trifluoride (ClF₃), used for cutting steel, and for the preparation of sulfur hexafluoride, an extremely stable gas that has been employed as an insulator in electrical applications.

Nonstick frying pans have been coated with a fluorocarbon resin, the best known of which is polytetrafluoroethylene. There are several other fluorocarbon and fluorinated hydrocarbon resins; some have highly specialized applications in the aerospace industry.

Fluorinated compounds are also used in textile treatments; some are soil-release agents that make fabric easy to wash. The salt sodium fluoroacetate is an extremely powerful rodenticide; it has been reported to give good control of rats, but it must be used with great care. Sodium bifluoride is used as a laundry sour; it also removes iron stains without weakening the fabric.

A minor but important use of fluorine in some countries is in the fluoridation of drinking water in the interest of dental health.

The properties of bromine are significantly different from those of fluorine and chlorine, and it is far less abundant. Discovered in the early 19th century, in the form of its salts (bromides) in the bitterns remaining after evaporating seawater and extracting the sodium chloride, it was obtained later from Stassfurt, Germany, as a by-product in the production of potassium salts and from other salt deposits and salt lakes. Its main use was originally for bromides in medicine, still a minor use. Bromine first became of industrial importance with the development of the modern photographic (photography, history of) process, in which the light-sensitive material is an emulsion of minute particles of silver bromide (together with silver chloride, or iodide, or both) in gelatin.

Tetraethyllead (tetraethyl lead) was another of Thomas Midgley's discoveries in the 1920s. Long the only effective agent in preventing “knock” in gasoline engines, tetraethyllead is now supplemented by tetramethyl lead, a similar compound. Although the knock problem was solved, a method was needed to get all traces of lead out of the engine cylinder. This removal was achieved by the addition of small quantities of a scavenger, ethylene dibromide, often in a mixture with ethylene dichloride.

For a time the expanding world automobile industry threatened a scarcity of bromine, obtained from brines from the Great Lakes region and Searles Lake in the United States, and from the Dead Sea, which contains about 0.5 percent bromine. To meet the demand it was necessary to turn to seawater, which contains about 70 parts per million bromine.

To produce bromine from seawater, very large volumes of water must be processed. A preferable site for the operation is a neck of land projecting into the ocean so that water can be taken from one side and discharged to the other, avoiding the problem of processing the same water. The water is made acid with a little sulfuric acid and then treated with chlorine, which releases bromine from the bromides.

A current of air removes the bromine as a very dilute mixture of bromine with air. The bromine is absorbed in sodium carbonate, after which treatment with sulfuric acid releases the bromine again in a much more concentrated form.

By far the greater part of the bromine produced is converted to ethylene dibromide (ethylene bromide) by treatment with ethylene. Most of the ethylene dibromide is used in gasoline as a scavenger for lead; but it is also used as a fumigant, as a solvent for certain gums, and for further syntheses. The next most important bromine compound is methyl bromide, which is used as a fumigant, sometimes as a fire extinguisher, and for further syntheses.

Iodine enters the chemical industry on a smaller scale. The largest producer is Japan, where iodine is obtained from seaweed. Seawater contains only about 0.05 part per million iodine, but some species of seaweed are able to concentrate this iodine manyfold, so that commercial extraction of the iodine is possible.

The most important industrial use of iodine compounds is the small amount of silver iodide used with silver bromide in photography. Iodine is important also in medicine (although this is not a large-scale use) in the treatment of certain thyroid conditions, and it is added to common table salt to prevent such conditions. It is also used directly as a disinfectant. Iodine is a component of a few useful dyes. The laboratory chemist frequently makes use of iodine or iodine compounds in synthesis and also in analysis. Crystalline silver iodide is useful in cloud seeding.

The heavy chemical industry, in its classical form, was based on inorganic chemistry, concerned with all the elements except carbon and their compounds, but including, as has been seen, the carbonates. Similarly the light chemical industry uses organic chemistry, concerned with certain compounds of carbon such as the hydrocarbons, combinations of hydrogen and carbon. In the late 1960s the phrase heavy organic chemicals came into use for such compounds as benzene, phenol, ethylene, and vinyl chloride. Benzene and phenol are related chemically, and they are also related to toluene and the xylenes, which can be considered together as part of the aromatic group of organic chemicals, the aromatic compounds being most easily defined as those with chemical properties like those of benzene.

Chemically, the hydrocarbon benzene, which forms the basis of the aromatics, is a closed, six-sided ring structure of carbon atoms with a hydrogen atom at each corner of the hexagonal structure. Thus a benzene atom is made up of six carbon (C) atoms and six hydrogen (H) atoms and has the chemical formula C₆H₆. Benzene has long been an industrial chemical. Initially it was obtained from the carbonization (heating) of coal, which produces coke, combustible gas, and a number of by-products, including benzene. Carbonization of coal (coal gas) to produce illuminating gas dates back in England to the very early years of the 19th century. The process is still employed in some countries, but more use is being made of natural gas. The carbonizing process is also used (with slight modifications) to produce metallurgical coke, indispensable for the manufacture of iron and hence steel. The supply of benzene from the carbonizing process, however, is not sufficient to meet the demand. For every ton of coal carbonized only about two to three pounds (0.9 to 1.35 kilograms) of benzene are obtained.

The shortage of aromatics first became evident during World War I, when toluene was in great demand for the manufacture of trinitrotoluene, or TNT, the principal explosive used then. Methods were worked out to obtain toluene from petroleum. Much later, after World War II, benzene and all the other aromatics derived from it were needed in far greater quantities than metallurgical coke could supply, and by far the greater part of these aromatics now comes from petroleum.

Toluene differs from benzene in that one of the hydrogen atoms is replaced by a special combination of carbon and hydrogen called a methyl group (−CH₃). The xylenes have two methyl groups in different positions in the benzene ring, and thus all aromatics are to some extent interchangeable. In fact, one of the uses for toluene is to produce benzene by removing the methyl group.

All of these hydrocarbons are useful as gasoline additives because of their antiknock properties.

Toluene is also used as a solvent. The expression “as a solvent,” which occurs frequently in describing the uses for chemicals, covers a multitude of applications. The substance dissolved is usually also organic, and the process is used in coatings, adhesives, textiles, pharmaceuticals, inks, photographic film, and metal degreasing. An application that reaches the ultimate consumer is dry cleaning (although the solvent used here is not toluene, but other hydrocarbons or chlorohydrocarbons). Toluene has a multitude of other uses, such as in the polyurethane plastics and elastomers discussed below.

The three isomeric xylenes (isomeric means that they have exactly the same number and kind of atoms but are arranged differently) occur together, and with them is another isomer, ethylbenzene, which has one ethyl group (−C₂H₅) replacing one of the hydrogen atoms of benzene. These isomers can be separated only with difficulty, but numerous separation methods have been worked out. The small letters o-, m-, and p- (standing for ortho-, meta-, and para-) preceding the name xylene are used to identify the three different isomers that vary in the ways the two methyl groups displace the hydrogen atoms of benzene. Ortho-xylene is used mostly to produce phthalic anhydride, an important intermediate that leads principally to various coatings and plastics. The least valued of the isomers is meta-xylene, but it has uses in the manufacture of coatings and plastics. Para-xylene leads to polyesters, which reach the ultimate consumer as polyester fibres under various trademarked names.

Because of the interlocking network of the chemical industry, it will be helpful to return briefly to the original raw materials. Earlier the aromatic group of organic chemicals was described; contrasted with these are the aliphatics, of which a number of quite simple chemicals are of industrial importance.

Propylene is not produced in as large volume as ethylene and is mostly used chemically. It is an important raw material for certain detergents. It leads to derivatives that are used in antiknock gasoline additives. It can also be polymerized to a product with uses generally similar to those of polyethylene. When made into a fibre, polypropylene is especially useful for carpets.

Interest in a synthetic material that could be used in automobile tires began in Germany as early as World War I, when supplies from the tropical, rubber-producing countries were cut off. A synthetic rubber of a sort was produced that could be used for tires, although the vehicle had to be jacked up when not in motion to prevent developing a flat spot on the tires. Much research in Germany and the United States led to the development, shortly before World War II, of several elastomers. The most important of these, and by far the best for tires, was made of a copolymer of 75 parts of butadiene and 25 parts of styrene. This synthetic was first known as GR-S (Government Rubber–Styrene) but later came to be called SBR—styrene-butadiene rubber. It is produced in far greater quantity than any of the other synthetics. It is better than natural rubber in some respects, but poorer in others. It is often used in blends with other rubbers.

Acrylonitrile contains nitrogen, and therefore is decidedly different in chemical constitution from natural rubber, which contains only carbon and hydrogen. Natural rubber has a repeating unit of five carbon atoms. By starting with the unsaturated hydrocarbon isoprene (C₅H₈), a polymer can be made with the spatial arrangement of the atoms the same as in natural rubber and with very similar properties. This polymer is sometimes referred to as synthetic natural rubber. Another hydrocarbon elastomer starts with isobutylene (C₄H₈) and gives butyl, a rubber characterized by resistance to oxygen and impermeability to gases, which is used widely in cable insulation and as a coating for fabrics.

Most of the above-mentioned groups of chemicals that can be used either as plastics or elastomers can also be made into the form of coherent films. In the more highly industrialized countries there is a very high demand for films for wrapping (packaging) purposes, largely for food, and also in the building construction industry. The requirements for a film vary greatly. For many food products the wrapping film must have the ability to “breathe”; that is, it must have some permeability to water vapour and also to oxygen. Films can be developed with high permeability or with none at all. In some applications the film should be self-sealing. Films can be made of any thickness, and for some purposes extreme toughness is required. Paper, or treated paper, has of course been used for many of these purposes for many years, but it has such disadvantages as low strength, particularly when wet, and it is difficult to make it transparent. cellophane was produced commercially starting in the 1920s; its transparency attracted attention at once, beginning a revolution in wrapping materials.

Cellophane is regenerated cellulose. It is like viscose rayon, except that it is extruded flat, instead of in the form of a fibre. It is still very popular but is highly sensitive to water and to changing humidity. Many other polymers now supplement it and compete with it. Polyethylene makes fine, tough films; there is no sharp distinction between a thin extrusion, useful for a wrapping film, and thicker products used for nonbreakable bottles. Many vinyl products are used in films, as are polystyrene, polyesters, and nylon. A chemical derivative from natural rubber, chlorinated rubber, gives films of extraordinary stretchability.

From coherent films that can stand by themselves, it is a short step to one of the components of a paint. In the days before chemical technology, commercial paints were based on linseed oil as a film-former. Linseed oil and the pigment made a mixture that was too thick, so that it was normally thinned with turpentine.

The thinner in paint is the component that has undergone least change. Turpentine, obtained from pine trees, and sometimes as a by-product in the manufacture of paper, is still used. A petroleum distillate, however, is equally effective. The thinner completely evaporates very shortly after the paint is applied. In latex paints, the paint itself is in the form of minute droplets in water, and water is the thinner.

The outstanding black pigment is the versatile product known as carbon black. Carbon black is one of the most important industrial chemical products. Carbon black is considered a petrochemical because it is made from natural gas or petroleum residues. There are several processes involving either incomplete combustion (burning off the hydrogen of a hydrocarbon, such as methane, and leaving the carbon) or by externally applied heat in a furnace, splitting the hydrocarbon into hydrogen and carbon.

The most important of all the uses of carbon black is in compounding rubber to be used in tires. An average tire of a passenger automobile contains about four pounds of carbon black. Carbon black is not only used as a pigment but also is employed in printing ink, an ink being little different from an applied coating. Carbon black creates the principal difficulty in recycling newsprint because no practical way has been found to destroy the black ink. A specialized use of carbon black is as an additive to phonograph records. A special form of carbon black, derived from acetylene, has its principal use in electrochemical dry cells.

Methanol is a large-volume chemical; about half of the production goes to making formaldehyde (CH₂O), a very reactive chemical with a large number of uses. A small amount of formaldehyde comes from non-methanol sources, via the direct oxidation of hydrocarbons. Methanol also enters into the production of various plastics; leads to such useful derivatives as methyl chloride, a solvent for inks and dyes; and is used in the purification of steroidal and hormonal medicines.

The greatest uses of formaldehyde are in the formation of important groups of plastics, the urea-formaldehyde resins and the phenol-formaldehyde resins. In addition, it is used as a fungicide and as a preservative, in paper and textile treatments, and in the synthesis of further products.

Methanol (CH₃OH) is the simplest of the alcohols. The next number of the series, called either ethanol or ethyl alcohol (CH₃CH₂OH), has two carbon atoms. It is most familiar as the active constituent of fermented beverages, but it is also widely used in industry. When intended for human consumption ethyl alcohol is always produced by fermentation of some suitable material to form beer, wine, or distilled spirits of various kinds. For industrial use it is sometimes produced by fermentation from some cheap material, such as molasses, but more often it is made from ethylene by causing it to combine with water under the influence of a catalyst, which may be sulfuric acid or phosphoric acid.

Acetaldehyde made from ethanol is generally used in the next step by the same company, most often in the same plant, so that the ethanol is really an intermediate that is used at once. For other uses ethanol is often shipped from one plant to another. Alcohol intended for human consumption, however, is in all countries subject to a tax, which would make the cost of ethanol prohibitive for any industrial use. Industrial alcohol, therefore, is denatured (denaturation) by the addition of small amounts of substances that are carefully chosen to be highly unpleasant in taste and hard to remove but that do not interfere with the intended industrial use.

In the alcohols with three carbon atoms, there are two possible structures, or isomers. One is called n-propyl alcohol (or 1-propanol), the other isopropyl alcohol (or 2-propanol).

The alcohol 1-propanol, not manufactured in very large quantities, finds major use in printing inks. The alcohol 2-propanol, on the other hand, is manufactured on the million-ton scale. It is made from propylene by a process similar to that used to convert ethylene to ethanol, and manufacture of 2-propanol by this process initiated the petrochemical industry in the 1920s.

The principal use of 2-propanol is in the manufacture of acetone, which is used extensively as a solvent and as a starting material in the manufacture of numerous other organic compounds. Smaller amounts of 2-propanol are converted to other chemical products or used as a solvent, as rubbing alcohol, or as a denaturing agent for ethyl alcohol.

Higher alcohols—that is, with more than three carbon atoms—too numerous to detail here, are also manufactured. Mention should also be made of the dihydric alcohol, ethylene glycol. This chemical is produced in large volume and is made from ethylene by an indirect route. Its principal use is in antifreeze mixtures for automobile radiators. It is also used in brake fluids and has numerous derivatives used in resins, paints, and explosives and in the manufacture of polyester fibres. Similar reactions for propylene give propylene glycol, the principal use of which is as a moistening agent in foods and tobacco.