The name organic chemistry originated at the beginning of the 19th century, when scientists wished to differentiate between those substances derived from plant and animal (organic) sources and those derived from inanimate (inorganic) materials. Organic substances generally had more complicated compositions than did inorganic materials, and the scientists of the day were unable to synthesize any of these organic substances in the laboratory. It was believed that organic substances had special qualities and could be created only in the presence of the "vital force" found in living organisms. Even though the vital force theory was eventually disproved, the classification of chemical substances as organic or inorganic has continued to the present.

The modern usage of organic chemistry refers to the chemistry of compounds containing carbon, but this definition should be further clarified, because compounds such as carbon monoxide, carbon dioxide, and calcium carbonate, are considered to be inorganic. A better definition of organic substances is that they are generally characterized by chains of connected carbon atoms. More than two million such organic compounds are known.

Many of these are "natural products," or compounds found in nature. The study of the large organic molecules found in living systems and their reactions, which make up the life processes, has come to be called biochemistry. A large number of the known organic chemicals have been synthesized in the laboratory, and our society is dependent on such synthetic materials as plastics, synthetic fibers, dyes, detergents, and insecticides. The chemical and allied product industries contribute a large portion of the gross national product of the United States, and more than 150,000 people are employed as chemists. The vast majority of synthetic products are derived from petroleum, and as the world's supply of petroleum decreases, new sources of carbon-containing raw materials will have to be found. Also, it has become apparent that many synthesized compounds have deleterious effects both on the environment and on living organisms. Future developments in organic chemistry must take these effects into account.

HISTORY AND DEVELOPMENT

Organic chemistry began to evolve as a science in the early 19th century. Between 1769 and 1785, the Swedish chemist Karl Wilhelm \TScheele\t had isolated and characterized a large number of chemical substances derived from living systems. Little was known about the chemical composition of these substances, however, until the French scientist Antoine \TLavoisier\t developed his classic combustion experiments in the late 18th century. Lavoisier devised a method of burning these compounds in pure oxygen gas and found that the combustion products were carbon dioxide and water, indicating that the compounds contained carbon and hydrogen. Extensions of Lavoisier's work showed that all such compounds contained carbon and hydrogen and that many contained oxygen and nitrogen as well. It became clear that these four elements were combined in a variety of ways to give vastly different compounds. This behavior was unusual, because the number of compounds formed between the other chemical elements was relatively small. These substances isolated from living organisms were first called organic compounds by the Swedish chemist Jons Jakob \TBerzelius\t in 1807. So far it had proved to be impossible to prepare any of these compounds in the laboratory, and the belief was held that their synthesis involved a "vital force" present only in living organisms. Berzelius felt that the synthesis of organic substances was impossible. He was proved wrong, however, in 1828, when the German chemist Friedrich \TWohler\t unintentionally converted ammonium cyanate, a purely inorganic substance, into urea, which is an end product of animal metabolism. This discovery set the stage for the eventual overthrow of the vital force theory. Over the next 20 years other organic compounds were synthesized, and as evidence accumulated, the vital force theory was slowly abandoned.

Wohler's preparation of urea had another important consequence in that it furnished an example of two substances, urea and ammonium cyanate, which had the same chemical composition but very different properties. As experimental techniques improved, it became clear that there were other examples of more than one substance having the same composition. In the 1830s Berzelius used the term isomerism (composed of equal parts) to describe this phenomenon, but an understanding of isomerism was not possible until the structure of these compounds became understood later in the century.

The mid-19th century saw the steady development of systematic organic research. A key step in this development was the establishment of the idea of radicals as the organic equivalent of atoms. Radicals were groups of atoms that retained their chemical identity during a chemical reaction. In simpler compounds these radicals contained only hydrogen and carbon (hydrocarbon radicals). In 1832, Wohler and Justus von \TLiebig\t produced a series of compounds all containing the benzoyl radical, and their work encouraged others to discover new radicals by systematic reactions of organic compounds.

At about the time the radical theory was becoming firmly established, a new way of looking at organic reactions was introduced by two French scientists, Jean Baptiste \TDumas\t and Auguste \TLaurent\t. The basis of this new theory was substitution. Dumas recognized that an atom of one element may replace that of another element in an organic compound. Laurent generalized Dumas's ideas, saying that radicals replace each other by substitution. Although Laurent's idea was scoffed at by several noted chemists, he showed in 1837 that his theory was the basis for a classification of organic compounds. Laurent's associate, Charles \TGerhardt\t, was another scientist who found little recognition for his ideas. An important contribution made by Gerhardt was the theory of "residues." He applied this theory in 1839 to reactions in which two organic molecules combine, eliminating part of each to form a simple compound such as water or hydrochloric acid while the organic "residues or radicals" combine together. An example he used was the reaction of benzene and nitric acid to form nitrobenzene with the elimination of water. Gerhardt also introduced the term homologous series to describe a series of compounds in which members differ by multiples of the molecular fragment containing 1 atom of carbon and 2 atoms of hydrogen. An example of such a series is methane, ethane, propane, butane, and so on.

In 1846, Laurent postulated that water, alcohol, ether, potassium hydroxide, and potassium oxide were analogous compounds. The English scientist Alexander William Williamson (1824-1904) verified this idea when he showed (1850-52) that the ethyl radical was present in both alcohol and ether and that alcohol or ether could be viewed as an a water molecule in which one or two H's, respectively, have been replaced by an ethyl radical. In 1853, Gerhardt generalized this idea and formulated his theory of "types," in which he said there were four inorganic types from which all organic compounds were derivable. These "types" were water, hydrochloric acid, ammonia, and hydrogen. Organic compounds were formed when one or more hydrogen atoms in these compounds were replaced by organic radicals.

In spite of this progress in systematic organic research, organic chemistry was still in a state of confusion. This confusion existed primarily because a number of different atomic weight scales were in use, giving rise to different formulas for the same compound. In 1860, the First International Chemical Congress was held in Karlsruhe in an attempt to settle the confusion. Although not immediately successful, the meeting did prove to be a turning point, in that the Italian scientist Stanislao \TCannizzaro\t proposed that the hypothesis of Amedeo \TAvogadro\t be accepted. Avogadro had suggested in 1811 that equal volumes of different gases under the same conditions of temperature and pressure should contain equal numbers of particles. Thus by determining the masses of equal volumes of different gases, the ratios of their molecular weights could be determined. Cannizzaro argued for these ideas, but his arguments were not immediately accepted. When the scientists returned home and considered Cannizzaro's ideas further, however, they eventually did recognize their validity. Thus 50 years after Avogadro's work, Cannizzaro's use of gas densities to determine molecular weights was accepted as correct, and much of the earlier confusion associated with organic chemistry was removed.

Also contributing to the confusion was the fact that the structures of molecules were not understood. A milestone in the development of structural organic chemistry came in 1858 when Friedrich August \TKekule\t in Heidelberg and Archibald Scott Couper in Paris independently introduced the general rules of valence bonds and the pictorial representation of a molecule as a group of connected atoms. They postulated that a carbon atom always has a constant valence of four, hydrogen and chlorine one, oxygen two, and nitrogen three. Kekule introduced a graphical representation of valence in which a line between elements means that one valence of each atom has been used to establish a valence bond. The idea that carbon is tetravalent led Kekule to propose that in many organic compounds carbon atoms are linked together. To support his theory of constant valence, Kekule had to invent the multiple bond concept. The valence of an atom then became the number of bonds that the atom can form. For example, in ethylene, carbon is bonded to only three atoms, but it still has a valence of four because of the double bond between the carbon atoms. Kekule was able to solve a particularly puzzling structural problem through the application of his theory of constant valence. Benzene had been discovered in 1825 in pyrolyzed whale oil, and although its formula suggested to the chemists of the day that it should be very reactive, it was found to be relatively inert. Kekule proposed a cyclic structure for benzene in 1865, and when all the carbons were shown to be chemically equivalent, he further proposed that the structure actually corresponded to a rapid oscillation between two structures. This resultant representation of benzene is still called the Kekule structure. Eventually the theory of constant valence was shown not to hold for many atoms. It does hold for carbon, however, and the structural ideas and multiple-bond concept to which it gave rise are extremely important in modern chemistry.

This new structural theory did not appreciate that molecules are three-dimensional. In 1874, however, Jacobus Henricus \Tvan't Hoff\t and Joseph Achille LE BEL independently proposed that the four bonds of carbon were located at equal angles to each other in space. This meant the four bonds were directed at the four corners of a tetrahedron, with the carbon atom at its center. Their proposal was a correct description of the four carbon bonds, but at the time their ideas were not readily accepted.

The elucidation of the structures of organic molecules during the 19th century was an important step in the development of organic chemistry. In all this time, however, no clear understanding of the nature of the chemical bond had developed. It was not until 1917, when G. N. \TLewis\t described chemical bonds as electron pairs, that the modern period of organic chemistry can be said to have begun. Because chemical reactions involve the breaking and forming of chemical bonds, it was not possible to understand the mechanisms by which reactions occur until chemical bonds were understood. Only when quantum theory became available in the 1920s was it possible to understand more complicated problems.

PROPERTIES AND CLASSES OF ORGANIC COMPOUNDS

Why are there so many organic compounds and why is only carbon capable of forming such a multitude of compounds? The answer lies in the electronic structure of the carbon atom. Carbon has four valence electrons and can share all four electrons to form a maximum of four chemical bonds. More importantly, carbon atoms are able to bond to one another, forming chains of carbon atoms. Why is it that only carbon forms such compounds? It might be expected that silicon, which also has four valence electrons, would behave in the same way. In fact, silicon does form hydrides and halides that are analogous to the hydrocarbons and halocarbons. These silicon compounds react readily with water (hydrolyze), however, while the carbon compounds are stable in the presence of water. Hydrolysis of silicon compounds occurs readily because silicon has empty 3d orbitals through which it can coordinate to a water molecule. Carbon, on the other hand, has no such available d orbitals, and hydrolysis would have to occur through a more roundabout mechanism. Since the carbon atom is so small, however, the surrounding atoms to which it is bonded impede attack by other atoms. Therefore, both electronic structure and geometry combine to make hydrolysis of carbon compounds extremely slow, and most of the two million known organic molecules are stable to attack by both air and water.

Some generalizations can be made about the properties and reactions of these diverse molecules. Organic compounds are combustible, often charring to form elemental carbon. They are usually gases, liquids, or low-melting solids. The boiling points of compounds with similar structures increase with the number of carbon atoms; ethane boils at -88 degC, propane at -42 degC, and n-butane at -0.5 degC. The highest melting points are found in symmetrical molecules. Since the electronegativities of carbon and hydrogen are similar, bonds between carbon and hydrogen are not very polar. Consequently, many organic molecules are not polar and are insoluble in the polar solvent water. Water-soluble organic compounds usually contain a polar group such as the hydroxyl group--OH. The nonpolar nature of many organic molecules also influences their chemical reactivity. Reactions between organic molecules are usually slow, because interactions between nonpolar molecules are weak. Heat is often required to speed up organic reactions. Also, organic reactions are generally not quantitative. The yield of the product seldom approaches the amount theoretically possible. Often the heat necessary to drive the reaction destroys some of the reactants and products, thus producing impurities along with the desired product. The occurrence of side reactions that produce undesired products also reduces the yield. In most reactions, the greater part of the carbon skeleton of a particular organic molecule remains unchanged, and much of the chemistry of organic compounds is really the chemistry of functional groups. These functional groups, which are attached to the hydrocarbon skeleton, generally contain atoms other than C or H.

Organic molecules can be classified into a number of groups. The simplest group includes compounds known as hydrocarbons, which contain only C and H. Other organic compounds also contain elements such as oxygen, O, nitrogen, N, sulfur, S, or the halogens fluorine, F, chlorine, CI, or bromine, Br, but all these compounds are based on the simpler hydrocarbons and are known as hydrocarbon derivatives.

The simplest group of hydrocarbons consists of the alkanes, in which each carbon atom is bonded to four other carbon or hydrogen atoms. Because the maximum number of atoms to which a carbon atom can bond is four, the valence of carbon in these compounds is said to be saturated, and the alkanes are also known as saturated hydrocarbons. The alkanes make up a homologous series, of which the simplest member of the group is methane, and each successive member of the series is related by the addition of a unit having one carbon atom and 2 hydrogen atoms. The next members of the series are ethane, and propane. For methane, ethane, and propane there is only one structural form, but for butane there are two distinct forms leading to two different molecules. Molecules having the same formulas but different structures are known as structural isomers. As the number of atoms in a molecule increases, so does the number of structural isomers. Decane, for example, has 75 structural isomers. If three or more different types of atoms are present in a molecule, the number of isomers is even larger. The flat structural formulas depicted above suggest that the alkanes consist of flat chains of carbon atoms. The four bonds around each carbon atom are actually in a tetrahedral arrangement, however, so these chains are not flat but have a three-dimensional structure.

The alkanes are very unreactive, but they will undergo substitution reactions in which one or more hydrogen atoms are replaced by other atoms or groups of atoms. The most common substitution reactions involve halogenation. Methane, for example, reacts readily with chlorine gas to form methyl chloride. A similar group of saturated hydrocarbons comprises the cycloalkanes in which the carbon atom chain forms a ring. Another homologous series is the alkenes (or olefins). Alkenes contain a carbon-carbon double bond, and since this means that all the carbon atoms are not bonded to four other atoms, the alkenes are known as unsaturated hydrocarbons. The simplest alkenes are ethylene (or ethene) and propylene (propene). The presence of the double bond makes alkenes more reactive than alkanes. The alkynes characterized by the presence of a carbon-carbon triple bond and, make up another homologous series of unsaturated hydrocarbons. The simplest alkyne is acetylene. The triple bond in alkynes makes these compounds very reactive.

The characteristic reaction of unsaturated hydrocarbons is addition: atoms or groups of atoms add to an alkene or alkyne by disrupting a double or triple bond. Chlorine, for example, adds to ethylene to form dichloroethane. Under certain conditions alkenes will add to themselves, forming large compounds known as addition polymers. Many of the familiar plastics are polymers. Polyethylene, for example, is an addition polymer formed when many ethylene molecules add together. The alkanes, alkenes, and alkynes are collectively called aliphatic hydrocarbons. Another important group of compounds are the aromatic hydrocarbons, the simplest of which is benzene. The formula suggests that benzene is highly unsaturated, and it would therefore be expected to be very reactive. Instead, it is found to be fairly inert, and the explanation for this behavior lies in its structure. Benzene is a planar cyclic molecule represented by two so-called resonance structures where each corner of the hexagon represents a CH group. The pi-electrons involved in the double bonds are actually delocalized around the carbon ring, giving stability to the molecule and decreasing its reactivity. Aromatic molecules, many of which are based on benzene, are generally characterized by their tendency to undergo substitution reactions rather than the addition reactions expected for unsaturated molecules.

There are a number of hydrocarbon derivatives containing other elements as well as carbon and hydrogen. These compounds are usually classified by functional group, the group of atoms in the molecules giving them their characteristic properties. Several classes of molecules contain one or more oxygen atoms. One such class is the alcohols, in which an H atom in the corresponding hydrocarbon has been replaced by the hydroxy group --OH. The simplest alcohol is methanol. In general, alcohols can be represented as R--OH, where R stands for a hydrocarbon group. Two alcohols can react together and eliminate a water molecule to form an ether, which is characterized by an O atom bonded to two hydrocarbon groups. Another common functional group containing oxygen is the carbonyl group. When the carbon atom of the carbonyl group is bonded to two hydrocarbon groups, the molecule is called a keytone. The simplest keytone is acetone, which is commonly used as a solvent. When the carbonyl group is attached to a hydrogen atom and a hydrocarbon group, the compounds are called aldehydes.

The carbonyl and hydroxyl group are combined to form a carboxylic acid group, and compounds containing this group, are known as carboxylic acids. Carboxylic acids and alcohols also react together and eliminate a water molecule to form another type of molecule, an esther. Many esters are found in fruits and give them their characteristic aromas. Methyl butyrate, shown above, is responsible for the aroma of pineapples. A number of esters also exist that can be derived from an alcohol plus an inorganic acid. Phosphoric acid groups, for example, are important constituents of many organic molecules found in plant and animal tissue.

A number of organic compounds contain nitrogen. One class of nitrogen-containing compounds includes the amines.

All amines have unpleasant odors, and many smell like decaying fish. Another nitrogen-containing compound, an amide, is formed when the hydroxyl group of a carboxylic acid is replaced by an amino group. Other organic groups can also be attached to the nitrogen atom. One group of atoms is called an amide linkage and is found in such diverse molecules as proteins and nylons, which belong to a class of synthetic fibers known as condensation polymers. Nylons are polyamides that form when diacids (molecules containing two carboxylic acid groups) combine with diamines with the elimination of water molecules. Nitrogen is also present in organic compounds as the inorganic nitro, nitrite, and nitrate groups. Compounds containing these groups are unstable, and all conventional explosives are organic nitro and nitrate compounds. Two examples are trinitrotoluene (\TTNT\t) and glycerol trinitrate (nitroglycerine).

Halogen atoms are often found in organic molecules, forming the halocarbons R CI, R--F, and R--Br. Another element often present in functional groups is sulfur. Many organic molecules contain more than one functional group, and it is these functional groups that determine the chemical behavior of a particular molecule. A knowledge of the functional groups and their properties is fundamental to an understanding of organic chemistry.

RESEARCH IN ORGANIC CHEMISTRY

There are several main areas of organic chemical research. One of these is the synthesis of organic molecules, which involves converting available substances into a desired product molecule. Such a process often requires a whole series of carefully controlled reactions. Many new molecules that have never appeared in nature are created in this way. Examples include plastics, synthetic fibers, and pharmaceuticals. Many chemists are also involved in synthesizing natural products, and it is now possible to synthesize some very complex molecules.

Another important area of research is the determination of the structure of organic molecules. When molecules are synthesized or natural products are isolated, their structures must be characterized. Often a careful study of the chemical reactivity and physical properties of a particular compound gives clues to its structure. Spectroscopic techniques, which take advantage of each molecule's characteristic interactions with light, also play an important role in structure determination. Often the structure of a particular molecule can be determined by X-ray diffraction. Still, a knowledge of the electronic structure is also necessary to really understand a molecule, and many scientists are engaged in research aimed at a more accurate theoretical description of chemical bonds in organic molecules.

Another field of research is the study of reaction mechanisms, the pathways through which chemical reactions proceed. It is possible to measure both the rate at which a particular reaction proceeds and the effects on the rate of such factors as temperature and concentration of reactants. Such measurements provide valuable insight into how a reaction or type of reaction proceeds, and this kind of information often leads to new synthetic methods.

A large number of organic chemists are also active in the area known as biochemistry. This is the branch of organic chemistry focused on molecules that make up living organisms. The chemical elements of major importance to living systems (in order of abundance) are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Many other elements, including metals, are also present in trace amounts. Most biologically important molecules exist as macromolecules known as biopolymers, which can be divided into three main groups. The first two groups are the proteins and carbohydrates, which, along with fats and oils, are the major sources of energy in the food supply of animals. The third group of biopolymers is made up of the nucleic acids, which store genetic information determining the form of a living system and regulate its reproduction and development.

ORGANIC CHEMISTRY AND THE ENVIRONMENT

Organic research has produced many compounds that have benefited society as a whole. At the same time many of these synthetic substances have been found to have deleterious effects not only on the Earth's environment but on animals and human beings as well.

An ever-increasing number of organic substances have proved to be carcinogenic or cancer-causing. Since the induction period between exposure to a carcinogen and the onset of cancer can often be years, determining whether or not a substance is a carcinogen is not always a simple matter. Certain classes of compounds are known carcinogens, however. A particularly dangerous group of compounds includes the aromatic amines, which have been widely used in the production of dyes. An example is 2-naphthylamine, which has been banned in many parts of the United States because it is known to produce cancer in at least 50% of the people exposed to it.

Another group of carcinogens, also important in the dye industry, includes some of the azo dyes, which contain a doubly bonded pair of nitrogen atoms attached to aromatic rings. An example of a carcinogenic azo dye is methyl (butter) yellow (4-Dimethylaminoazobenzene), used in the past to color butter.

A widely publicized example of a carcinogenic organic molecule is diethylstilbestrol (\TDES\t), a synthetic female sex hormone whose use has now been severely restricted. \TDES\t was commonly used in the United States as an addition to cattle feed because it caused cattle to fatten faster, even though it had been known since 1940 to cause cancer in animals. \TDES\t was also taken by a number of pregnant women to prevent miscarriage, and it has now been found that \TDES\t can produce a rare form of cancer in these women's daughters.

A group of chemicals that are not carcinogens but are still dangerous includes various pesticides. The use of pesticides has served to increase the world's food supply and decrease disease-carrying pests, but many of these pesticides, particularly the polychlorinated hydrocarbons, have generated controversy because of their persistence in the environment. An example of such a "hard" insecticide is 2,2-di(p-chlorophenyl)-1,1,1-trichloroethane (\TDDT\t).

DHT is extremely stable and persists for years in the environment. Consequently, it moves through the food chain and gradually builds up in plant and animal tissues. Although there is no evidence that normal use of \TDDT\t has ever harmed humans, it has proved harmful to fish and birds of prey, and the use of \TDDT\t has now been severely limited.

Two other classes of pesticides are now widely used and are known as "soft" insecticides because they decompose more rapidly in the environment than the polychlorinated hydrocarbons. These groups are the organophosphates, an example of which is Malathion, and the carbamates, an example being Sevin. Even these must be used with care, however, since they are highly toxic.

There are also a number of organic chemicals that are not directly injurious to animals or humans but whose effect on the environment could well prove to be detrimental. One example is the synthetic detergents, which have all but replaced soap (over 3 billion kg/7 billion lb of detergents are produced annually in the United States). These detergents have a high phosphate content, which increases their cleaning efficiency but creates an environmental problem when they are discharged into streams and lakes from sewage systems. The phosphates increase the water's nutrient content, thus promoting excess growth of algae and weeds and depleting the oxygen supply in the water. This process is known as eutrophication. Members of the chemical industry are engaged in developing suitable replacements for phosphates.

Another group of chemicals that appears to be harmful to our environment includes chlorfluoromethanes, which have been widely used as aerosol propellants and refrigerant gases. Their concentration in the upper atmosphere has been steadily increasing, and there is evidence that these compounds play a role in reactions that tend to destroy the Earth's ozone layer. Ozone is an important component of the Earth's atmosphere in that it absorbs high-energy ultraviolet radiation from the sun's rays, thus serving to shield the Earth from this radiation. Destruction of the ozone layer would result in higher levels of ultraviolet radiation reaching the Earth's surface, which would in turn result in an increased incidence of skin cancer among persons exposed to sunlight. These compounds serve as an example of a group of apparently harmless chemicals that after years of widespread use may prove to be very dangerous. Examples such as these are making organic chemists aware that they must consider the possible harmful effects of the chemicals they produce in the laboratory.

ORGANIC CHEMICAL INDUSTRY

The major sources of organic chemicals are petroleum and natural gas. The earliest organic raw materials suitable for industrial processes came from heating coal in the absence of air to yield coke and coal tar. Coal tar is a mixture of volatile compounds, including aromatics such as benzene, toluene, and naphthalene. These compounds are well suited as starting materials in the synthesis of dyes, and for this reason the synthetic dye industry was one of the first major organic chemical industries. The chemical industry developed first in Europe, but during World War I the United States became aware of how dependent it was on Europe for important chemicals. This awareness provided the impetus for the development of the chemical industry in the United States, and in the last 50 years the organic chemical industry has expanded rapidly. Important products are plastics, dyes, synthetic fibers, detergents, and pharmaceuticals. Industries maintain large research laboratories where new products are developed and production processes are improved. Industrial processes must be economical and carried out on a large scale.

Ninety percent of all organic chemicals are now made from starting materials derived from petroleum and natural gas. These are called petrochemicals. Crude petroleum is a mixture of hydrocarbons, including primarily alkanes as well as some cycloalkanes and aromatics. These are separated by distillation into a series of fractions according to boiling point, the higher boiling point belonging to molecules containing a larger number of carbon atoms. The different fractions include mixtures of hydrocarbons we know as gasoline, kerosene, fuel oils, and asphalt. Since compounds do not occur in petroleum in the proportion that is most in demand, methods have been developed to break down larger molecules into smaller molecules and to combine smaller molecules to form larger ones. These processes are known as refining. In one of the refining processes, catalytic reforming, alkanes and cycloalkanes are converted to aromatics by passing them over suitable catalysts. Over 380 million liters (100 million gallons) of benzene are produced annually in this way. Major products of the refining process are the components of gasoline. Since branched alkanes burn more smoothly in gasoline than straight-chain alkanes, much of the refining process is aimed at producing branched alkanes. During a process known as isomerization, straight-chain alkanes are passed over catalysts and converted to branched-chain alkanes. In the cracking process large alkanes are heated to a high temperature in the absence of oxygen and are broken down into smaller alkanes suitable for gasoline.

A large proportion of the components of petroleum are eventually converted into polymers such as plastics, synthetic fibers, and synthetic rubbers. Over five million tons of these products are produced annually in the world, and the production of polymers is second only to the production of steel as a measure of the vitality of a nation's economic system.

Organic chemicals also play an important role in the food industry where they are used to color, flavor, preserve, and fortify the nutritional quality of foods. More than 1.8 million kg (4 million lb) of synthetic dyes are used annually to color food, and at least 750 synthetic flavorings are in use. As the physiological effects of some of these food additives become known, the advisability of their widespread use is beginning to be questioned. Many products are artificially sweetened, for example, and for some years popular sweeteners were sodium and calcium cyclamates. These were banned in 1970, however, when they were shown to produce bladder cancer in rats. Saccharin has since been the primary artificial sweetener, but it too has now been banned as a carcinogen. A great deal of current research is aimed at finding a safe economical replacement for these sweeteners.

As stated above, the basic starting materials for the organic chemical industry come from petroleum and natural gas.  In the past these have been abundant and cheap, but this situation is rapidly changing.  As the supply of petroleum dwindles and its cost increases, the effects will be felt throughout the world. Society becomes more heavily dependent upon these petrochemicals as each year passes, and present and future chemists must either face the prospect of a decreased supply of starting materials or attempt to find new sources of carbon-containing raw materials.  One attractive alternative may be the commercial generation of useful chemicals using living organisms.  The relatively new field of molecular biology and its related technologies explore the use of cellular media and bacteria for the biochemical synthesis of compounds.  SUZANNE RABITZ

Bibliography:  Fessenden, Ralph J.  and Joan S., Organic Chemistry, 3d ed.  (1986);  Morrison, Robert T., and Boyd, Robert N., Organic Chemistry, 5th ed.  (1987);  Pine, S, H., et al., Organic Chemistry, 5th ed.  (1987);, Streitwieser, Andrew, Introduction to Organic Chemistry, 3d ed.  (1985);  Ternay, A, L., Contemporary Organic Chemistry, 2d ed.  (1979).

See also:  \Tbiochemistry\t;  \Tchemical industry\t;  \Tchemistry\t; \Tpetrochemicals\t.