Chemistry: Organic Chemistry

Introduction

Organic chemistry is the branch of chemistry that focuses on the properties and reactions of compounds that contain carbon atoms. The carbon atom is unique because it is the only element that can bond to itself, forming chains that can contain hundreds of atoms. Carbon can also combine with a wide variety of other elements.

Historical Background and Scientific Foundations

More than a million carbon compounds have been discovered, and new ones are constantly being synthesized. One reason for this large number is isomerism, particularly structural isomerism, first recognized in 1830 by the Swedish chemist Jöns Jakob Berzelius (1779–1848). In this arrangement, two or more compounds contain the same number of carbon and hydrogen atoms arranged in different ways to form unique compounds with distinct chemical and physical properties. A compound formed from five carbons and twelve hydrogen atoms (C₅ H₁₂) will, for example, produce three unique compounds. Other forms of isomerism distinguished by the spatial arrangements of their atoms were discovered later in the nineteenth century.

In the eighteenth and early part of the nineteenth centuries, when many carbon-based compounds were first isolated, they had only been extracted from plants and animals. For example, formic acid had been isolated from ants, salicylic acid had been isolated from willow bark, and urea had been found in urine. This entrenched the idea that organic molecules must be created by (or by the actions of) a living organism. This idea also fit with the doctrine known as vitalism, a theory that developed in reaction to the rise of the mechanistic approach to life being merely a physical process.

The Influence of Liebig and Wöhler

Two German chemists, Justus von Liebig (1803–1873) and Friedrich Wöhler (1800–1882), were responsible for the emergence of organic chemistry in the early nineteenth century. Their quantitative analytical methods helped establish the constitution of newly isolated and synthesized carbon compounds. Both were inspiring teachers who established laboratory work as the basic model for chemical education, teaching students who came from all over Europe and America. The pupils then emulated their methods when they returned home to train the next generation of chemists.

Liebig, the most prominent chemist in nineteenth-century Europe, studied chemistry at the Universities of Bonn and Erlangen, where he obtained his PhD in 1822. After further study in Paris, he chaired the chemistry department at the University of Giessen before moving to the University of Munich, where he spent the remainder of his life. Liebig's major contributions were the development of new methods for the quick and precise measurement of the quantities of carbon, hydrogen, and nitrogen in organic compounds. This allowed Liebig and his students to identify a host of new organic compounds. Much of his work after 1840 was related to agricultural and biological chemistry, including a study of fermentation and methods for increasing soil fertility and yields through the use of artificial fertilizers.

The Synthesis of Urea and the Demise of Vitalism

Wöhler studied medicine at the Universities of Marburg and Heidelberg, obtaining his medical degree in 1823. He went on to study with Berzelius in Stockholm and to hold positions in technical schools in Berlin and Kassel. In 1836 he was appointed professor of chemistry in the medical faculty of Göttingen University, a position he held for the rest of his life. Wöhler's major research interests were in inorganic chemistry, where he isolated the elements boron, silicon, aluminum, cerium, and just missed being the discoverer of vanadium and niobium. Like Liebig, he also established a school of research and teaching, but is best known for synthesizing urea.

In 1828 Wöhler synthesized the organic compound urea in the laboratory using the inorganic compound ammonium cyanate. Urea had previously been found only in urine—that is, from a biological source. While this dealt a blow to vitalism, it did not fully spell its demise. Skeptics believed that compounds associated with living organisms were produced by a “vital force” not available to the chemist. This idea persisted until 1844 when Hermann Kolbe (1818–1884) proved definitively that organic compounds could be produced under laboratory conditions by synthesizing acetic acid from the simple inorganic compounds carbon disulfide and chlorine.

Structural Theory and Its Development

In the early part of the nineteenth century a new problem emerged as chemists tried to classify and bring some order to the ever-increasing number of organic substances. This began a period of confusion and controversy that would last several decades, until the development of the structural theory of organic chemistry.

Analytical methods pioneered by both Liebig and Wöhler were able to determine the content of organic molecules, but did not show how their elements were arranged. The first attempt to solve the problem was known as radical theory. Berzelius proposed a way to understand the formation of inorganic compounds by assuming that some elements had a positive charge and others had a negative charge. Scientists knew that opposite electrical charges would attract; thus the formation of sodium chloride could be explained by assuming that sodium is positively charged and chlorine is negatively charged.

Radical theory was pioneered in the 1830s by Liebig and the French chemist Jean-Baptiste André Dumas (1800–1884). Radicals were thought to be a stable group of elements that were joined together to produce an electropositive group that was joined to an electronegative atom to form a compound. In the case of organic compounds the radical would contain carbon, hydrogen, and various other atoms in combination with an electronegative inorganic partner. Thus ethyl alcohol was represented as C₂H₄ H₂O.

The electropositive radical was capable of transformation but should remain intact. This was demonstrated by Liebig and Wöhler in the case of benzaldehyde, which was extracted from almonds. The fact that compounds made from benzaldehyde all contained the radical C₁₄H₁₀O₂ provided evidence for radical theory.

Radical theory lost its hold on organic chemistry, however, when scientists realized it was possible to substitute one atom for another in what was assumed to be a “stable” radical. This substitution was further shown by Dumas to be an atom of a different charge. For example, in some compounds the substitution of an electronegative chlorine for electropositive hydrogen did not significantly change the character of the product. Dumas was able to convert acetic acid to trichloroacetic acid, and the product still remained an acid.

Radical theory gave way in the 1840s to what became known as type theory. It was developed by two of Dumas's students, Auguste Laurent (1808–1853) and French chemist Charles Frédéric Gerhardt (1816–1856), who proposed that organic (and even inorganic) substances were derived from simple molecules by substitution. Alexander Williamson (1824–1904) experimentally proved the existence of the water type with his synthesis in 1850 of diethyl ether from the potassium salt of ethyl alcohol and ethyl iodide.

In 1845 Laurent also introduced the concept of 2 homologous series—a group of similar compounds that differed by a single unit. The first three members of the alkane series, for example, are methane (CH₄), ethane (CH₆), and propane (C₃H₈); each succeeding molecule contains an additional CH₂ group. The German chemist Hermann Kopp (1817–1892) showed that each additional CH₂ increases the boiling point of each successive group member by a fixed number of degrees.

Three other types were proposed by Gerhardt in 1853, the ammonia (NH₃), hydrogen (H₂), and hydrogen chloride (HCl) types. These types together with the water type could be used to classify the already large number of organic compounds into four distinct groupings. Gerhardt wrote “By exchanging their hydrogens among certain groups, these types give rise to acids, to alcohols, to ethers, to hydrides, to radicals, to organic chlorides, to acetones, to alkalis.” Type theory was thus an advance—but because it still allowed multiple classifications for the same molecule, major problems remained to preoccupy chemists in the latter part of the nineteenth century.

Valences The concept of valence (the combining power of an element) slowly developed in the period from 1850 to 1870 through the work of the English chemist Edward Frankland (1825–1899) and the German August Kekulé (1829–1896). Frankland had begun his chemical studies in London and continued them in Germany with chemist Robert Wilhelm Bunsen (1811–1899) at Marburg. Frankland's view of valence was derived from the earlier radical theory; Kekulé's was based on type theory.

Frankland attempted to synthesize the hydrocarbon radical ethyl (C₂H₅) by reacting ethyl iodide with zinc. This produced not the desired result but butane, with diethyl zinc, the first example of an organometallic compound, as a byproduct. What was crucial about this discovery was that the diethyl zinc (C₂H₅)2Zn always contained twice as much ethyl as zinc. Frankland went on to show that in other organometallic compounds that a definite combining power existed between the organic portion and the metal. In 1852 Frankland proposed that inorganic and organic compounds both had 5 a combining power that came to be known as valence. Frankland also recognized multiple valences and gave examples where the valence was 3 or 5 such as in PCl3 & PCl₅.

August Kekulé

Friedrich August Kekulé (1829–1896) was born in Darmstadt Germany, a descendent of a noble Bohemian family from Stradonitz, a city near Prague. Kekulé had studied architecture at Geissen, but because of Liebig's influence he changed to chemistry, with additional studies in biological classification. Kekulé obtained his doctorate in Paris in 1851.

As a student in Paris, Kekulé had studied with Dumas and Gerhardt and was naturally drawn to type theory. Further study in London from 1854 to 1855 brought him into contact with two very original thinkers: Williamson and William Odling (1829–1921), chemists who studied chemical structure to understand chemical properties. Kekulé hypothesized that carbon was tetravalent, that is, could combine with four

substituents to form organic molecules. His approach was purely mechanical and may have been influenced by his initial interest in architecture. In 1858 Kekulé proposed that carbon could form chains by using some of its valences to bond to other carbon atoms. Kekulé's representations are not the familiar structural formulas we use today. Those were established by Scottish chemist Archibald Scott Couper (1831–1892) in 1858.

Couper had studied in Paris with the French chemist Charles-Adolphe Wurtz (1817–1884). He had ideas similar to those of Kekulé, but due to a series of mishaps his paper was published after Kekulé's, and thus he failed to receive the credit he should have. In contrast to the conservative and cautious Kekulé, Couper drew formulas for molecules that used dotted lines to represent the bonds between carbon atoms.

The graphical structural formulas that are used today were introduced by Alexander Crum Brown (1838–1922) in 1861. Initially he wrote the elements using the letters such as C, H, O, etc., with circles around them, as Dalton had done in his exposition of the atomic theory in the first decade of the nineteenth century. The circles were connected with solid lines, with the number of lines equal to the valence—four in the case of carbon. The circles were eventually dropped to create the structural formulas we still use today.

Stereochemistry and Aromaticity

The idea that molecules are three dimensional was not realized until the latter part of the nineteenth century. Discoveries made by French scientist Louis Pasteur (1822–1895), Dutch chemist Jacobus van't Hoff (1852–1911), and French chemist Joseph-Achille Le Bel (1847–1930) provided the keys to understanding the three-dimensional nature of many organic molecules.

Pasteur is best known for his microbiological research, but his initial training was in chemistry. In his doctoral research Pasteur studied the puzzling differences between two forms of the same compound: tartaric acid. Pasteur found that natural tartaric acid, isolated from the fermentation of grapes, rotated the polarization plane of light that passed through it. Racemic acid, a synthesized product that had an identical chemical formula, however, did not.

The French physicist Jean-Baptiste Biot (1774–1862) had observed this same phenomenon in 1815 with several organic liquids, including oil of turpentine. Salts of tartaric acid had been studied by other chemists as well, in particular the German chemist Eilhard Mitscherlich (1794–1863), who reported in 1844 that the physical properties of the sodium-ammonium salts of the optically active tartaric acid and the racemic acid were the same in every respect except their interaction with plane-polarized light.

Pasteur found that a comprehensive study of different tartrates showed that the optically active tartrates had an asymmetrical crystalline shape. All the racemic acid crystals were symmetrical. One particular compound, the sodium ammonium double salt of tartaric acid, produced very large well-defined crystals. Using a microscope and tweezers Pasteur was able to isolate two forms of the crystals. When subjected to plane polarized light, one bent the light to the right and the other to the left. The rotation was identical for both except the direction. If mixed in equal amounts there was no rotation at all. This phenomenon came to be known as stereoisomerism. Pasteur proposed that it was possible in nature for molecules to be asymmetric; by extension an association was made by Pasteur between asymmetry and life.

Pasteur and Kekulé had speculated that stereoisomerism might be caused by either the tetravalency of carbon itself or the way carbon is oriented in space as a result. This question was answered in 1874 by van't Hoff and Le Bel, independently of each other.

At this time van't Hoff was a professor in the veterinary school in Utrecht, Holland. His initial proposal, written in Dutch, attracted little attention. An extended form, La chimie dans l'espace (Chemistry in space) published in French in 1875, attracted much interest and controversy. He explained stereoisomerism by proposing that the four carbon valences were on the apexes of a tetrahedron. Four different substituents bonded to the central carbon atom could produce two structures that were mirror images of each other; this would then produce the asymmetry in carbon compounds.

Thus two mirror images could exist that were identical in all their properties except for the way they affected polarized light. These mirror images came to be known as enantiomers. Pasteur found that if a mixture contains equal amounts of each type, they cancel each other; this is why racemic acid did not affect polarized light and the natural tartaric acid did. Any mixture with equal numbers of enantiomers is now known as a racemic mixture.

Le Bel's explanation was similar, but he started with a carbon that had four identical groups and looked at what happened with successively substituted atoms. With four different substituents, an asymmetry is produced in which the mirror image is no longer super-imposable on the original. Molecules that have more than one center of asymmetry (that is, more than one stereogenic center) will have 2ⁿ stereoisomers, where n is the number of sterogenic centers. In organic chemistry, each carbon atom to which four other atoms (or groups of atoms) are bonded constitutes a stereogenic center. Not every carbon atom in an organic molecule is necessarily a stereogenic center.

An example of how this rule applies is the glucose molecule (C 6H12 O 6), whose stereoisomers were first studied by German chemist Emil Fischer (1852–1919). Each glucose molecule has three centers of asymmetry, which allows the atoms to combine into 2 ⁿ = 8 (eight) unique compounds, which Fischer identified. One of the major achievements of twentieth-century synthetic organic chemistry has been to develop methods to produce particular stereoisomers when multiple routes are possible.

One major structural problem that remained to be solved was the structure of benzene (C 6 H6) and aromatic compounds in general. Benzene was a formidable challenge because its unusual properties could not be based on conventional ideas of structure. Many chemists had tried and failed to solve the benzene problem before Kekulé devised the first rational structure in 1865.

In a story of dubious authenticity, Kekulé is said to have had a daydream in which he envisioned a snake catching its tail; this led him to visualize benzene's hexagonal structure, in which six carbon atoms alternate in single and double bonds. This model predicted that four unique isomers existed in disubstituted benzene molecules (benzene molecules containing two substituted atoms or groups of atoms), but only three were known. Kekulé proposed that one of these isomers has two forms that differ only by the locations of the single and double bonds. If an equilibrium exists between these two forms, all the carbon atoms become equivalent. This explanation became the basis of aromatic chemistry until the development of molecular orbital theory in the 1930s.

The Birth of the Synthetic Organic Chemical Industry

Organic compounds such as ethyl alcohol, found in beer and wine, and acetic acid, used in vinegar, have been produced since antiquity. Other naturally occurring organic compounds have been used for millennia as dyes and medicinal agents. But the production of organic substances in manufacturing plants did not being in earnest until the 1850s. The synthetic organic chemical industry began when a process was developed in the late-eighteenth century to produce gas from coal. This produced coal tar, a waste product that was considered a nuisance as it had only limited use, mainly as a water-proofing agent.

Organic chemists began to examine the constituents of coal tar in earnest in the 1840s. One of the primary investigators was the German chemist August Wilhelm Hofmann (1818–1892). A student of Liebig, he became the first professor of chemistry at the newly founded Royal College of Chemistry in London in 1844. Hofmann had begun to analyze coal tar in Liebig's laboratory and continued this work when he moved to London.

Hofmann found that coal tar's major components were aromatic hydrocarbons, a class of compounds based on the benzene molecule, first isolated by Michael Faraday (1791–1867) in 1825 from compressed oil gas. Hofmann and his students isolated at least 20 different substances from coal tar, the most important being aniline (C6 H5NH 2), an organic analog of ammonia, and phenol (C6 H5OH), which was used as one of the first antiseptics.

In 1856 William Henry Perkin (1838–1907), one of Hofmann's students at the Royal College of Chemistry, tried to synthesize quinine, a drug used to treat malaria. At that time quinine could only be extracted from the bark of the cinchona, a tree that was native to the Dutch East Indies. Hofmann thought that quinine might be the aromatic hydrocarbon naphthalene. Perkin took a different approach and used an impure form of an aniline derivative called allyl toluidine and treated it with potassium dichromate. The reaction failed to produce quinine, and Perkin repeated the experiment using aniline itself.

This produced a brown substance which, when dissolved in alcohol, produced a vivid purple solution. Perkin was only 18 at the time, but he had the insight to see that this might be useful as a dye, especially since purple is a color with few natural sources. When Perkin's samples were sent to a dyer, he found that it dyed silk a subdued purple that became known as mauve.

In partnership with his father and brother, Perkin built a plant to manufacture this synthetic dye on a commercial scale. Perkin's genius was his ability to convert a laboratory process into a commercial product. His success led many English competitors to develop their own lines of aniline-based dyes. Using various derivatives of aniline and reaction conditions, the whole spectrum of colors became available, eliminating the need for natural dyes and dooming that industry.

Britain became the center of the synthetic organic chemical industry for the next two decades. The dye industry, in particular, became a magnet for many highly trained German organic chemists. The experience they gained in Britain had two distinct consequences. First, German chemists became masters of laboratory synthesis, developing synthetic replacements for the natural red dye alizarin and blue dye indigo. Second, Germany became the center of the organic chemical industry, founding of such companies as BASF, AGFA, Bayer, and Hoechst. Partnerships that developed between academia and the industry made Germany the world leader in chemical production by 1914. This became a problem for the United States during World War I, when exports of German organic chemicals were stopped by the British blockade.

Although synthetic dyes had been developed, pharmaceuticals were still derived mainly from natural sources. One key breakthrough was the synthesis of salicylic acid by Kolbe in 1853. A natural product derived from willow bark, salicylic acid was known for its pain-relieving abilities. Although one of Kolbe's students converted the laboratory synthesis into a commercial product, salicylic acid taken internally produced unwanted side effects. The acetyl derivative prepared in 1897 by a chemist working for the Bayer Company in Germany was found to produce the same pain relief without the side effects. This compound was sold as aspirin beginning in 1899 and has remained a staple product for over a century.

In the latter part of the nineteenth century, German physician and bacteriologist Robert Koch's (1843–1910) postulates proved the validity of the germ theory of disease and ushered in a more rational approach to the treatment of bacterial infections. Although synthetic dyes had previously been used to stain cells for medical studies, scientists such as German medical researcher Paul Ehrlich (1854–1915) thought that some dyes might actually destroy bacteria also. The dye methylene blue, for example, killed the parasite associated with malaria.

Ehrlich believed that a drug must be linked in a rational and systematic manner to its target. Ehrlich's best-known work was on syphilis, which is caused by a spirochete, a type of bacterium. Ehrlich had been studying organic compounds that contained arsenic as a treatment against certain tropical diseases, only to find that most of these, although somewhat effective, had toxic side effects. Based on his knowledge of organic chemistry, Ehrlich synthesized a series of organoarsenic compounds, going through 605 before the next one, marketed under the trade name Salvarsan, proved successful. An even more effective compound called Neosalvarsan was produced in 1912; this remained the standard treatment until the beginning of the antibiotic era in the late 1940s.

Because little was known about how drugs actually worked, very few breakthroughs occurred between 1920 and 1940. If a drug proved effective it was purely by chance in most cases, not by design. A good example of this was the discovery of sulfa drugs by the Bayer division of I.G. Farben in Germany. Thousands of compounds were synthesized by Bayer chemists before one, whose structure was similar to a class of dyes known as azo compounds, showed any medicinal potential. The drug Prontosil was patented in 1932 and was the first synthetic antibacterial effective against streptococcal and staphylococcal bacteria (but not against enterobacteria).

What made Prontosil effective was that it decomposed in vivo to a compound called sulfanilamide, a relatively simple compound first synthesized in 1908. Thousands of sulfanilamide analogs were created in the 1930s, and several proved effective. This was the only antibacterial therapy available in any quantity through World War II. Penicillin, an antibiotic first used in 1942, was much more effective, but it was difficult to produce in large quantities, and much of what was made was intended for military use.

The era after 1945 marked the golden age of organic chemistry and the discovery of new drugs. New synthetic techniques and, most importantly, a revolution in instrumentation that included nuclear magnetic resonance, mass spectrometry, and various types of infrared, visible, and ultraviolet spectroscopy made it possible to determine the structures of both natural and synthetic products rapidly. Tremendous strides were made in finding medicinal agents to deal with a variety of conditions.

Industrial Advances

Prior to 1939 insecticides were mainly inorganic in nature and posed many health hazards, especially when used in the agricultural sector. In 1939 the organic insecticide DDT (dichloro-diphenyl-trichloroethane) was found to be very effective against many pests, especially those that carry diseases such as malaria. Spraying DDT saved an enormous number of lives during World War II because of its ability to prevent outbreaks of malaria and other tropical diseases. Various other chlorinated hydrocarbon pesticides were introduced when effectiveness and safety concerns made the use of DDT less desirable. These second-generation insecticides such as Chlordane, Lindane, and Dieldrin also had their problems and were replaced with an entirely new class of organophosphates.

In the post-World War II era, organic herbicides came to be an important addition to agriculture. One of the earliest commercial products, 2,4-D (2,4-dichloro-phenoxyacetic acid), highly effective against broad-leaf plants, improved agricultural yields. Agent orange is a 50–50 mixture of 2,4-D and 2,4,5,T (2,4,5-trichloro-phenoxyacetic acid) that was used extensively as a defoliant during the Vietnam war, sometimes creating serious health effects in those who came into contact with it. Trace amounts of dioxin, a highly carcinogenic material, are produced in the manufacture of 2,4,5-T. The manufacture of 2,4-D does not produce dioxin and is still used to control unwanted vegetation. Many new types of herbicides have been produced since then that are different in structure than the chloroacetic acids, and many are sold for use in both agriculture and for the home gardener.

Petroleum and Petrochemicals

The manufacture of chemicals from petroleum in the nineteenth century was spurred by the increasing need for new fuel sources to replace plant oils and animal tallow. The discovery that coal could be converted into a gas was promising, but mass distribution required large facilities for production and an infrastructure for distribution. A more convenient fuel was needed.

In the mid-nineteenth century a process devised by Scottish chemist James Young (1811–1883) heated certain forms of coal to produce a liquid that, after distillation and purification, proved to be an excellent illuminant. Sold under the trade name kerosene, it became the dominant method of lighting worldwide for the balance of the nineteenth and early part of the twentieth century.

In the United States, kerosene was mass-produced in plants using the Young process. However, the large amounts of industrial waste produced, the need to import special coal, and the required royalty payments led to the search for an alternative.

Crude oil seeps had been found in various parts of the United States, including one in Titusville, Pennsylvania. A New York-based business syndicate thought that it might be possible to drill into the earth, pump this crude oil to the surface, then distill it into a product similar to kerosene. The process worked, and after petroleum was found in other states and other countries, oil eventually became the world's dominant energy source.

German chemists had pioneered the production of petrochemicals from both coal and natural gas (methane) from 1900 to 1930, devising processes to produce phenol, ethylene, ethylene oxide, acetone, vinyl acetate, and vinyl chloride. In the 1920s American chemists began to use the byproducts of the crude-oil refining process to produce many of the same products that German chemists had been able to make from coal.

Many of the products produced from petroleum form the basis of synthetic macromolecules. For example, ethylene can be converted directly into polyethylene, or transformed into styrene to produce polystyrene or vinyl chloride to produce polyvinyl chloride. Oil and natural gas have proved to be some of the most valuable of all organic substances for producing either other organic materials or intermediates. The petrochemical industry became the leading chemical industry in the United States after 1945; it remains an important part of the economy.

Organic Chemistry in the Twentieth Century

While the focus in the nineteenth century was on the synthesis of new molecules and the structure of natural products, research in the twentieth century centered on the reactions of organic molecules. By studying the processes involved in converting a reactant to a product, scientists hoped to follow the same path using a different reactant. A major step in this direction occurred with the development of the Lewis-Langmuir theory of covalent bonding, based upon the concept of the electron pair as the basis of the chemical bond as formulated by American physical chemist Gilbert N. Lewis (1875–1946) and Irving Langmuir (1881–1967).

This began the era of the electronic interpretation of reaction mechanisms. For a new generation of chemists, particularly those in Great Britain such as Robert Robinson (1886–1975) and Christopher Ingold (1893–1970), the Lewis-Langmuir theory became a way to understand the unique structure of molecules such as benzene and how reactions produced certain products and not others.

The tools of physical chemistry, such as thermodynamics and kinetics (the measurement of the rates of reactions) were used to validate electronic interpretation. This hybrid combination of organic and physical chemistry became known as physical organic chemistry, a term coined by the American physical chemist Louis Hammett (1894–1987) for his 1940 pioneering textbook in the new type of study.

Many significant investigations had been performed before 1940 by British and American chemists, including the British investigators Arthur Lapworth (1872–1941) and Kennedy J.P. Orton (1872–1930) and the Americans James Bryant Conant (1893–1978), Howard Lucas (1885–1963), and Frank C. Whitmore (1887–1947) among others. After 1945 American chemists became the preeminent practitioners of physical organic chemistry. Many reactions whose mechanisms had remained a

mystery for decades were revealed by applying the techniques pioneered by individuals in the decades and centuries before.

Modern Cultural Connections

In the latter part of the twentieth century organic chemistry focused increasingly on the chemistry of life and biochemical processes. This usually involved developing methods to synthesize complex molecules, especially those with many centers for optical isomerism, and the structural elucidation of natural products found to be active against various conditions such as cancer.

Organic chemistry provides a host of useful products that have made the world more colorful with synthetic dyes, improved public health through pesticides and drugs, and increased the crop yields with fertilizers and pesticides. Synthetic fibers and engineered materials are also the products of organic chemistry.

See Also Chemistry: Biochemistry: The Chemistry of Life; Chemistry: Chemical Bonds; Chemistry: Chemical Reactions and the Conservation of Mass and Energy; Chemistry: Molecular Structure and Stereochemistry; Chemistry: The Practice of Alchemy; Chemistry: States of Matter: Solids, Liquids, Gases, and Plasma.

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Aftalion, Fred. A History of the International Chemical Industry. Translated by Otto Theodor Benfey. Philadelphia: University of Pennsylvania Press, 1991.

Brock, William H. The Chemical Tree: A History of Chemistry. New York: W.W. Norton, 2000.

Haber, L.F. The Chemical Industry During the Nineteenth Century: Study of the Economic Aspect of Applied Chemistry in Europe and North America. London: Oxford University Press, 1958.

Ihde, Aaron J. The Development of Modern Chemistry. New York: Dover, 1984.

Spitz, Peter H. Petrochemicals: The Rise of an Industry. New York: John Wiley & Sons, 1988.

Tarbell, Dean S., and Ann Tracy Tarbell, Essays on the History of Organic Chemistry in the United States, 1875–1955. Nashville: Folio Publishers, 1986.

Martin Saltzman