CHEMICAL COMPOUND

any substance composed of identical molecules consisting of atoms of two or more elements. There are millions of known compounds, each of which is unique. They range in complexity from water, which has two hydrogen atoms bonded to one oxygen atom, to the nucleic acids, which contain thousands of atoms. Chemical compounds vary in state and in such properties as colour, odour, and chemical behaviour. Most common materials are mixtures of different chemical compounds. Pure compounds can usually be obtained from them by physical methods, such as filtration and distillation, that do not alter the way in which the atoms are combined within the compound. Compounds can be broken down into their constituent elements or transformed into new compounds by chemical changes (reactions). There are several ways of classifying chemical compounds. The most general division is between organic and inorganic compounds. Chemists once believed that carbon-containing compounds could be obtained only from organic-living or formerly living-sources. Although this is now known to be untrue, the branch of chemistry concerned with substances based primarily on carbon atoms is called organic chemistry. any substance composed of identical molecules consisting of atoms of two or more elements. All the matter in the universe is composed of the atoms of more than 100 different chemical elements, which are found both in pure form and combined in chemical compounds. A sample of any given pure element is composed only of the atoms characteristic of that element, and the atoms of each element are unique. For example, the atoms that constitute carbon are different from those that make up iron, which are in turn different from those of gold. Every element is designated by a unique symbol consisting of one, two, or three letters arising from either the current element name or its original (often Latin) name. For example, the symbols for carbon, hydrogen, and oxygen are simply C, H, and O, respectively. The symbol for iron is Fe, from its original Latin name ferrum. The fundamental principle of the science of chemistry is that the atoms of different elements can combine with one another to form chemical compounds. Methane, for example, which is formed from the elements carbon and hydrogen in the ratio four hydrogen atoms for each carbon atom, is known to contain distinct CH4 molecules. The formula of a compound-such as CH4-indicates the types of atoms present, with subscripts representing the relative numbers of atoms (although the numeral 1 is never written). Water, which is a chemical compound of hydrogen and oxygen in the ratio two hydrogen atoms for every oxygen atom, contains H2O molecules. Sodium chloride is a chemical compound formed from sodium (Na) and chlorine (Cl) in a 1:1 ratio. Although the formula for sodium chloride is NaCl, the compound does not contain actual NaCl molecules. Rather, it contains equal numbers of sodium ions with a charge of positive one (Na+) and chloride ions with a charge of negative one (Cl-). (The process for changing uncharged atoms to ions [i.e., species with a positive or negative net charge] will be discussed below.) The substances mentioned above exemplify the two basic types of chemical compounds: molecular (covalent) and ionic. Methane and water are composed of molecules-that is, they are molecular compounds. Sodium chloride, on the other hand, contains ions-it is an ionic compound. The atoms of the various chemical elements can be likened to the letters of the alphabet: just as the letters of the alphabet are combined to form thousands of words, the atoms of the elements can combine in various ways to form a myriad of compounds. In fact, there are millions of chemical compounds known, and many more millions are possible but have not yet been discovered or synthesized. Most substances found in nature-such as wood, soil, and rocks-are mixtures of chemical compounds. These substances can be separated into their constituent compounds by physical methods, which are methods that do not change the way in which atoms are aggregated within the compounds. Compounds can be broken down into their constituent elements by chemical changes. A chemical change (that is, a chemical reaction) is one in which the organization of the atoms is altered. An example of a chemical reaction is the burning of methane in the presence of molecular oxygen (O2) to form carbon dioxide (CO2) and water. CH4 + 2O2 (r) CO2 + 2H2O In this reaction, which is an example of a combustion reaction, changes occur in the way that the carbon, hydrogen, and oxygen atoms are bound together in the compounds. Chemical compounds show a bewildering array of characteristics. At ordinary temperatures and pressures, some are solids, some are liquids, and some are gases. The colours of the various compounds span those of the rainbow. Some compounds are highly toxic to humans, while others are essential for life. Substitution of only a single atom within a compound may be responsible for changing the colour, odour, or toxicity of a substance. So that some sense can be made out of this great diversity, classification systems have been developed. An example cited above classifies compounds as molecular or ionic. Compounds are also classified as organic or inorganic. Organic compounds, so-called because many of them were originally isolated from living organisms, typically contain chains or rings of carbon atoms. Because of the great variety of ways that carbon can bond with itself and other elements, there are more than nine million organic compounds. The compounds that are not considered to be organic are called inorganic compounds. Within the broad classifications of organic and inorganic are many subclasses, mainly based on the specific elements or groups of elements that are present. For example, among the inorganic compounds, oxides contain O2- ions or oxygen atoms, hydrides contain H- ions or hydrogen atoms, sulfides contain S2- ions, and so forth. Subclasses of organic compounds include alcohols (which contain the -OH group), carboxylic acids (characterized by the -COOH group), amines (which have an -NH2 group), and so on. The various subclasses of organic and inorganic compounds are described in detail later in this article. Additional reading General works Coverage of chemistry at an elementay level is presented in Steven S. Zumdahl, Introductory Chemistry: A Foundation, 2nd ed. (1993); Morris Hein and Susan Arena, Foundations of College Chemistry, 8th ed. (1993); and Morris Hein, et al., Introduction to Organic and Biochemistry (1993). More comprehensive intermediate treatment is available in Steven S. Zumdahl, Chemistry, 3rd ed. (1993); Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten, Chemistry: The Central Science, 6th ed. (1994); Raymond Chang, Chemistry, 5th ed. (1994); Darrell D. Ebbing, General Chemistry, 4th ed. (1993); and John C. Kotz and Keith F. Purcell, Chemistry & Chemical Reactivity, 2nd ed. (1991). N.N. Greenwood and A. Earnshaw, Chemistry of the Elements (1984); and F. Albert Cotton and Geoffrey Wilkinson, Advanced Inorganic Chemistry, 5th ed. (1988), cover the chemistry of the elements in detail. Herman F. Mark et al. (eds.), Encyclopedia of Chemical Technology, 3rd ed., 31 vol. (1978-84), formerly known as Kirk-Othmer Encyclopedia of Chemical Technology, with a 4th edition begun in 1991, treats a broad range of preparations, properties, and uses. Steven S. Zumdahl Inorganic compounds In addition to the standard texts by Greenwood and Earnshaw and by Cotton and Wilkinson noted above, the work by James E. Huheey, Ellen A. Keiter, and Richard L. Keiter, Inorganic Chemistry: Principles of Structure and Reactivity, 4th ed. (1993), is an excellent college-level textbook. An excellent source for in-depth views of the inorganic chemistry of the elements is the monumental Gmelins Handbuch der anorganischen Chemie, 8th ed. (1924- ), with articles in German and English; since 1981 most of the articles have appeared in English, and the volumes now have English titles: Gmelin Handbook of Inorganic Chemistry (1981-89) and Gmelin Handbook of Inorganic and Organometallic Chemistry (1990- ). Another reference work is John C. Bailar, Jr., et al. (eds.), Comprehensive Inorganic Chemistry, 5 vol. (1973). Boranes and carboranes Alfred Stock, Hydrides of Boron and Silicon (1933, reissued 1957), is the classic account of their synthesis, characterization, reactions, and properties. More recent studies include William N. Lipscomb, Boron Hydrides (1963), a detailed survey with emphasis on structure and bonding; Earl L. Muetterties and Walter N. Knoth, Polyhedral Boranes (1968), an extensive survey; Kenneth Wade, Electron Deficient Compounds (1971), not limited to boranes and carboranes; Earl L. Muetterties (ed.), Boron Hydride Chemistry (1975), essays by leading authorities; Herbert C. Brown, Boranes in Organic Chemistry (1972), with an emphasis on hydroboration; Andrew Dequasie, The Green Flame: Surviving Government Secrecy (1991), a personal account of the U.S. military's projects to develop a boron-based fuel; Russell N. Grimes, Carboranes (1970), the first book-length account of these compounds; Geoffrey Wilkinson, F. Gordon A. Stone, and Edward W. Abel (eds.), Comprehensive Organometallic Chemistry, 9 vol. (1982), containing several chapters dealing with carboranes and other heteroboranes; and Catherine E. Housecroft, Boranes and Metalloboranes (1990), basic information on these compounds, uses of physical methods to characterize them, and structural details about borane cages. The text by Greenwood and Earnshaw cited above contains an excellent general account by a leading borane chemist. Coordination compounds John C. Bailar, Jr. (ed.), The Chemistry of the Coordination Compounds (1956), is the first modern book in English surveying the entire field. Jack Lewis and Ralph G. Wilkins (eds.), Modern Coordination Chemistry: Principles and Methods (1960), contains essays on theory, physicochemical techniques, and recent advances. Additional studies include D.P. Graddon, An Introduction to Co-ordination Chemistry, 2nd ed. (1968), a short but scholarly introduction; A.A. Grinberg, An Introduction to the Chemistry of Complex Compounds (1962; originally published in Russian, 2nd ed., 1951), a comprehensive treatment emphasizing history and experimental data, especially the work of the Russian school; Christian K. Jrgensen, Inorganic Complexes (1963), a survey of recent progress emphasizing spectra and static properties; Mark M. Jones, Elementary Coordination Chemistry (1965), a readable, comprehensive, introductory textbook; George B. Kauffman (trans. and ed.), Classics in Coordination Chemistry, 3 vol. (1968-78), annotated translations of the most important contributions from 1798 to 1935; George B. Kauffman (ed.), Werner Centennial (1967), a collection of papers surveying historical and research aspects, and Coordination Chemistry: A Century of Progress (1994), a collection of historical, review, and research papers; George B. Kauffman, Inorganic Coordination Compounds (1981), a survey with historical emphasis; Geoffrey Wilkinson, Robert D. Gillard, and Jon A. McCleverty (eds.), Comprehensive Coordination Chemistry, 7 vol. (1987), an up-to-date compendium of synthesis, reactions, properties, and applications; Arthur E. Martell and Melvin Calvin, Chemistry of the Metal Chelate Compounds (1952), the first monograph on the chelate effect; Fred Basolo and Ralph G. Pearson, Mechanisms of Inorganic Reactions, 2nd ed. (1967), a study of metal complexes in solution; B.F.G. Johnson (ed.), Transition Metal Clusters (1980); and Michael Thor Pope, Heteropoly and Isopoly Oxometalates (1983). George B. Kauffman Organometallic compounds Introductory surveys of organometallic chemistry may be found in several textbooks of inorganic chemistry: Duward F. Shriver, Peter Atkins, and Cooper H. Langford, Inorganic Chemistry, 2nd ed. (1994); Gary L. Miessler and Donald A. Tarr, Inorganic Chemistry (1991); and in the works cited above by Cotton and Wilkinson, by Huheey, Keiter, and Keiter, and by Greenwood and Earnshaw. Specific aspects are explored in Christoph Elschenbroich and Albrecht Salzer, Organometallics: A Concise Introduction, 2nd, rev. ed. (1992; originally published in German, 2nd ed., 1988), a well-illustrated discussion of s-, p-, and d-block organometallics, with emphasis on organometallic reactions; Akio Yamamoto, Organotransition Metal Chemistry, trans. from Japanese (1986), a very good presentation of the fundamentals and their application of d-block organometallic chemistry to synthesis and catalysis; Robert H. Crabtree, The Organometallic Chemistry of the Transition Metals, 2nd ed. (1994), stressing concepts of structure and reactions for d-block organometallics; James P. Collman et al., Principles and Applications of Organotransition Metal Chemistry, new ed. (1987), a highly authoritative and detailed presentation of d-block organometallic chemistry, with emphasis on principles, mechanisms, and the utility of organometallics in organic synthesis and catalysis; D. Michael P. Mingos and David J. Wales, Introduction to Cluster Chemistry (1990), emphasizing structure and bonding in d-block organometallic clusters; Duward F. Shriver, Herbert D. Kaesz, and Richard D. Adams (eds.), The Chemistry of Metal Cluster Complexes (1990), covering the structures and chemistry of d-block organometallic clusters, by authorities in the field; and George W. Parshall and Stephen D. Ittel, Homogeneous Catalysis, 2nd ed. (1992), treating commercial and laboratory homogeneous catalytic processes, most involving organometallic chemistry. The multivolume work edited by Wilkinson, Stone, and Able cited above contains detailed chapters covering s-, p-, d-, and f-block organometallic chemistry, with excellent coverage of the primary literature. D.F. Shriver Jerome David Odom Organic compounds Comprehensive introductions to the chemistry of organic compounds are available in a wide variety of well-illustrated university and college-level textbooks, such as K. Peter C. Vollhard and Neil E. Schore, Organic Chemistry, 2nd ed. (1994); Andrew Streitwieser, Clayton H. Heathcock, and Edward M. Kosower, Introduction to Organic Chemistry, 4th ed. (1992); John McMurry, Organic Chemistry, 3rd ed. (1992); Seyhan N. Ege, Organic Chemistry, 3rd ed. (1994); Francis A. Carey, Organic Chemistry, 2nd ed. (1992); Robert Thornton Morrison and Robert Nielson Boyd, Organic Chemistry, 6th ed. (1992); T.W. Graham Solomons, Organic Chemistry, 5th ed. (1992); L.G. Wade, Jr., Organic Chemistry, 2nd ed. (1991); and G. Marc Loudon, Organic Chemistry, 2nd ed. (1988). Advanced textbooks that cover reactions and mechanisms of all important classes of organic compounds are Jerry March, Advanced Organic Chemistry, 4th ed. (1992); and Francis A. Carey and Richard J. Sundberg, Advanced Organic Chemistry, 3rd ed., 2 vol. (1990). Organic molecules common to everyday life are discussed in an entertaining way in P.W. Atkins, Molecules (1987, reprinted 1991). An extensive compilation of chemical compounds and properties is David R. Lide and G.W.A. Milne (eds.), CRC Handbook of Data on Organic Compounds, 3rd ed., 7 vol. (1994). Rodd's Chemistry of Carbon Compounds, 2nd ed. edited by S. Coffey (1964- ), is still useful. R. Panico and W.H. Powell, A Guide to IUPAC Nomenclature of Organic Compounds, ed. by Jean-Claude Richer (1993), is a comprehensive exposition of international nomenclature recommendations from the International Union of Pure and Applied Chemistry. Spectroscopic properties of molecules are the focus of Joseph B. Lambert et al., Introduction to Organic Spectroscopy (1987). Richard A.Y. Jones, Physical and Mechanistic Organic Chemistry, 2nd ed. (1984); and Thomas H. Lowry and Kathleen Schueller Richardson, Mechanism and Theory in Organic Chemistry, 3rd ed. (1987), specialize in the mechanisms of organic reactions. Richard C. Larock, Comprehensive Organic Transformations (1989); and Stanley R. Sandler and Wolf Karo, Organic Functional Group Preparations, 2nd ed., 3 vol. (1983-89), contain listings of many methods for the synthesis of organic compounds. Strategies for the synthesis of molecules are discussed in E.J. Corey and Xue-min Cheng, The Logic of Chemical Synthesis (1989); and Ari L. Horvath, Molecular Design (1992). Hydrocarbons The structure, nomenclature, synthesis, physical properties, and chemical reactions of hydrocarbons are presented in most of the comprehensive introductory textbooks of organic chemistry, especially the works by Carey, by McMurry, by Morrison and Boyd, by Solomons, and by Wade cited above. More detailed discussions may be found in Derek Barton and W. David Ollis (eds.), Comprehensive Organic Chemistry, vol. 1, Stereochemistry, Hydrocarbons, Halo Compounds, Oxygen Compounds ed. by J.F. Stoddart (1979). Ernest L. Eliel et al., Conformational Analysis (1965, reprinted 1981), remains the classic work in its field. The effects of strain on structure and reactivity are discussed in Arthur Greenberg and Joel F. Liebman, Strained Organic Molecules (1978). Alkenes and alkynes are covered in depth in Saul Patai and Jacob Zabicky (eds.), The Chemistry of Alkenes, 2 vol. (1964-70); and Saul Patai (ed.), The Chemistry of the Carbon-Carbon Triple Bond, 2 vol. (1978). The concept of "special stability" in aromatic hydrocarbons is covered in Peter J. Garratt, Aromaticity (1971, reissued 1986); and much of the most important chemistry of arenes is presented in George A. Olah (ed.), Friedel-Crafts and Related Reactions, 4 vol. in 6 (1963-65). Buckminsterfullerene was proclaimed the "molecule of the year" and reviewed in Science, 254(5039):1705-1707 (Dec. 20, 1991). Industrial organic chemicals and the processes by which they are prepared are described in H. Harry Szmant, Organic Building Blocks of the Chemical Industry (1989). Stanley R. Sandler and Wolf Karo, Polymer Syntheses, 2nd ed., vol. 1 (1992), presents information on techniques of hydrocarbon polymerization. Francis A. Carey Alcohols, phenols, and ethers These topics are addressed by all the organic chemistry texts cited above, especially those by Wade, by Solomons, by McMurray, and by Morrison and Boyd. Carl R. Noller, Chemistry of Organic Compounds, 3rd ed. (1965), is a classic textbook using an older style of teaching organic chemistry, with heavy emphasis on history, industrial chemistry, and nomenclature. More specialized treatments appear in Derek Barton and W. David Ollis (eds.), Comprehensive Organic Chemistry, vol. 1, Stereochemistry, Hydrocarbons, Halo Compounds, Oxygen Compounds, ed. by J.F. Stoddart (1979), part 4, "Alcohols, Phenols, Ethers, and Related Compounds," pp. 577-939; John A. Monick, Alcohols: Their Chemistry, Properties, and Manufacture (1968); and Saul Patai (ed.), The Chemistry of Ether Linkage (1967), The Chemistry of the Hydroxyl Group, 2 vol. (1971), and Chemistry of Ethers, Crown Ethers, Hydroxyl Groups, and Their Sulphur Analogues, 2 vol. (1980). Leroy G. Wade, Jr. Aldehydes and ketones Saul Patai (ed.), The Chemistry of the Carbonyl Group, 2 vol. (1966-70); Saul Patai (ed.), The Chemistry of Double-Bonded Functional Groups, 2 vol. (1977, reissued 1989); and Saul Patai and Zvi Rappoport (eds.), The Chemistry of Enones, 2 vol. (1989), are collections of detailed essays covering a wide range of aspects of carbonyl chemistry. C. David Gutsche, The Chemistry of Carbonyl Compounds (1967), a short treatise written for undergraduates, covers the subject in a systematic manner. J. Frederic Walker, Formaldehyde, 3rd ed. (1964, reprinted 1975), covers all aspects of this important compound. William P. Jencks, "Carbonyl- and Acyl-Group Reactions," in his Catalysis in Chemistry and Enzymology (1969, reissued 1987), pp. 463-554, is a detailed discussion of the mechanisms of these reactions. Carboxylic acids and their derivatives Studies of the subject are F.D. Gunstone, An Introduction to the Chemistry and Biochemistry of Fatty Acids and Their Glycerides, 2nd ed. (1968); Anderson W. Ralston, Fatty Acids and Their Derivatives (1948); Saul Patai (ed.), The Chemistry of Carboxylic Acids and Esters (1969), The Chemistry of Acid Derivatives (1979), and The Chemistry of Acyl Halides (1972); and Jacob Zabicky (ed.), The Chemistry of Amides (1970). Jerry March Amines David Ginsburg, Concerning Amines, Their Properties, Preparation, and Reactions (1967), treats most aspects of amine chemistry at an introductory to intermediate level. Derek Barton and W. David Ollis (eds.), Comprehensive Organic Chemistry, vol. 2, Nitrogen Compounds, Carboxylic Acids, Phosphorus Compounds, ed. by I.O. Sutherland (1979), contains a comprehensive section on amines at an intermediate level. Advanced chapters on most fundamental features of amine chemistry may be found in Saul Patai (ed.), The Chemistry of the Amino Group (1968), while his The Chemistry of Amino, Nitroso, and Nitro Compounds and Their Derivatives, 2 vol. (1982), contains chapters at an advanced level on the structure of amines, inversion phenomena, and oxidation. Barry M. Trost and Ian Fleming (eds.), Comprehensive Organic Synthesis, vol. 6, Heteroatom Manipulation (1991), chapter 1, includes a substantial section on synthesis of amines and ammonium salts at an intermediate to advanced level. Peter A.S. Smith Organic sulfur compounds Eric Block, Reactions of Organosulfur Compounds (1978); Shigeru Oae (ed.), Organic Chemistry of Sulfur (1977); and Shigeru Oae, Organic Sulfur Chemistry: Structure and Mechanism (1991), are general overviews of the field. Specialized treatments, listed according to sulfur functional groups, include Saul Patai (ed.), The Chemistry of the Thiol Group, 2 vol. (1974); Saul Patai and Zvi Rappoport (eds.), The Chemistry of Sulphonic Acids, Esters, and Their Derivatives (1991), and The Chemistry of Sulphur-Containing Functional Groups (1993); Saul Patai, Zvi Rappoport, and Charles Stirling (eds.), The Chemistry of Sulphones and Sulphoxides (1988); Saul Patai (ed.), The Chemistry of Sulphenic Acids and Their Derivatives (1990); N.S. Simpkins, Sulphones in Organic Synthesis (1993); and Barry M. Trost and Lawrence S. Melvin, Jr., Sulfur Ylides (1975). The biochemistry of sulfur and the chemistry of sulfur compounds in garlic, onion, and related plants is described in the studies by Ryan J. Huxtable, Biochemistry of Sulfur (1986); and Eric Block, "The Chemistry of Garlic and Onions," Scientific American, 252(3):114-119 (March 1985), and "The Organosulfur Chemistry of the Genus Allium and Its Importance to the Organic Chemistry of Sulfur," Angewandte Chemie (International Edition in English), 31(9):1135-1178 (1992). Collections of essays on special topics include Chryssostomos Chatgilialoglu and Klaus-Dieter Asmus (eds.), Sulfur-Centered Reactive Intermediates in Chemistry and Biology (1991); L.I. Belen'kii (ed.), Chemistry of Organosulfur Compounds (1990); and Eric Block (ed.), Advances in Sulfur Chemistry, vol. 1 (1994). Eric Block Heterocyclic compounds Modern textbooks of heterocyclic chemistry include T.L. Gilchrist, Heterocyclic Chemistry, 2nd ed. (1992); and John A. Joule and George F. Smith, Heterocyclic Chemistry, 2nd ed. (1978). A simplified treatment is given in David T. Davies, Aromatic Heterocyclic Chemistry (1992). Alan R. Katritzky, Handbook of Heterocyclic Chemistry (1985); and Alan R. Katritzky and Charles W. Rees (eds.), Comprehensive Heterocyclic Chemistry, 8 vol. (1984), are more detailed. Advances in Heterocyclic Chemistry (irregular); The Chemistry of Heterocyclic Compounds (irregular); and Progress in Heterocyclic Chemistry (annual) are continuing series of reviews. Alan Roy Katritzky Organohalogen compounds Organic halogen compounds are prominently featured in all introductory organic chemistry textbooks, such as the ones cited above. More extensive treatments are available in the work by Stoddart cited above; and in Saul Patai, The Chemistry of the Carbon-Halogen Bond (1973). The preparation of alkyl halides from alcohols and their use in functional-group transformations are reviewed in the text by Carey and Sundberg cited above. One of the chief uses of organohalogen compounds in organic synthesis lies in their conversion to Grignard reagents, a topic that forms the basis of a chapter in Frank R. Hartley and Saul Patai (eds.), The Chemistry of the Metal-Carbon Bond, vol. 4, The Use of Organometallic Compounds in Organic Synthesis (1987). Organohalogen compounds feature prominently in texts devoted to specific reaction mechanisms, most notably S.R. Hartshorn, Aliphatic Nucleophilic Substitution (1973); and William H. Saunders, Jr., and Anthony F. Cockerill, Mechanisms of Elimination Reactions (1973). The unique properties of organofluorine compounds are described in R.D. Chambers, Fluorine in Organic Chemistry (1973). Gordon W. Gribble, "Naturally Occurring Organohalogen Compounds-A Survey," Journal of Natural Products, 55(10):1353-1395 (1992), is an authoritative review. Objective summaries of environmental issues concerning organohalogen compounds can be found in several articles in Chemical and Engineering News, a special issue devoted to dioxin, vol. 61, no. 23 (June 6, 1983); Bette Hileman, "The Great Lakes Cleanup Effort," 66(6):22-39 (Feb. 8, 1988); and Pamela S. Zurer, "Ozone Depletion's Recurring Surprises Challenge Atmospheric Scientists," 71(21):8-18 (May 24, 1993). Francis A. Carey Coloured compounds and dyes Overviews include Society of Dyers and Colourists and American Association of Textile Chemists and Colorists, The Colour Index, 3rd ed. (1971- ); Society of Dyers and Colourists, Colour Terms and Definitions, rev. ed. (1979); John Shore (ed.), Colorants and Auxiliaries, 2 vol. (1990); Daniel M. Marmion, Handbook of U.S. Colorants: Food, Drugs, Cosmetics, and Medical Devices, 3rd ed. (1991); and T.P. Coultate, Food: The Chemistry of Its Components, 2nd ed. (1989), which includes a section on food colorants for the general reader. A detailed history of the beginnings of the dye industry is Anthony S. Travis, The Rainbow Makers: The Origins of the Synthetic Dyestuffs Industry in Western Europe (1993). K. Venkataraman, The Chemistry of Synthetic Dyes, 8 vol. (1952-78), is an authoritative series on the dyestuff industry. Monographs on the organic chemistry of colorants include P.F. Gordon and P. Gregory, Organic Chemistry in Colour (1983); and Heinrich Zollinger, Color Chemistry, 2nd rev. ed. (1991), with more than 1,500 references. Procedures for dye syntheses are detailed in David R. Waring and Geoffrey Hallas (eds.), The Chemistry and Application of Dyes (1990); and C.L. Bird and W.S. Boston (eds.), The Theory of Coloration of Textiles (1975). J.R. Aspland, "Textile Color Application Processes," Color Research and Applications, 10(4):205-214 (1983), outlines practical aspects of industrial textile dyeing. Industrial preparations of dyestuff intermediates are explored in H. Harry Szmant, Organic Building Blocks of the Chemical Industry (1983); George Britton, The Biochemistry of Natural Pigments (1983); and D.H. Solomon and D.G. Hawthorne, Chemistry of Pigments and Fillers (1983). Susan Budavari (ed.), The Merck Index, 11th ed. (1989), lists more than 300 common colorants, giving key references and physical properties. Ben Selinger, Chemistry in the Marketplace, 4th ed. (1989), provides an overview of chemistry in the household, including the common colorants. J.B. Stothers Carbohydrates Chapters in the organic chemistry texts by Wade and by Morrison and Boyd cited above discuss carbohydrates in a readable style. More in-depth treatment is provided by Derek Barton and W. David Ollis (eds.), Comprehensive Organic Chemistry, vol. 5, Biological Compounds, ed. by E. Haslam (1979), part 26, "Carbohydrate Chemistry," pp. 685-830; John F. Kennedy (ed.), Carbohydrate Chemistry (1988); and Hassan S. El Khadem, Carbohydrate Chemistry: Monosaccharides and Their Oligomers (1988). Leroy G. Wade, Jr. Amino acids, peptides, and proteins Overviews are found in chapters in the works by Ege and by Carey cited above; and in Donald Voet and Judith G. Voet, Biochemistry (1992). More specific monographs include G.C. Barrett (ed.), Chemistry and Biochemistry of Amino Acids (1985); Edwin J. Cohn and John T. Edsall, Proteins, Amino Acids, and Peptides as Ions and Dipolar Ions (1943, reissued 1965); Theodor Wieland and Miklos Bodanszky, The World of Peptides (1991); Miklos Bodanszky, Principles of Peptide Synthesis, 2nd rev. ed. (1993); Miklos Bodanszky, Yakir S. Klausner, and Miguel A. Ondetti, Peptide Synthesis, 2nd ed. (1976); and Gary E. Means and Robert E. Feeney, Chemical Modification of Proteins (1971). Lipids The work by Gunstone cited above is a classic review and a source for original references. The textbooks by Carey, by Streitwieser, Heathcock, and Kosower, and by Voet and Voet, also cited above, contain particularly good sections on lipids. Robert Barker, Organic Chemistry of Biological Compounds (1971), is an intermediate-level university text; chapter 7 deals with lipids. Advanced texts include Ronald Kluger, "Mechanisms of Enzymatic Carbon-Carbon Bond Formation and Cleavage," in The Enzymes, vol. 20, Mechanisms of Catalysis, ed. by David S. Sigman (1992), pp. 271-317, focusing on how the enzymes involved in producing the fundamental structure of lipids accomplish the required catalysis; David E. Cane, "The Enzymology of the Biosynthesis of Natural Products," in Colin J. Suckling (ed.), Enzyme Chemistry, 2nd ed. (1990), pp. 265-305, on the details of production of complex terpenoids in plants; William L. Alworth, Stereochemistry and Its Application in Biochemistry (1972); and Marcel Florkin and Elmer H. Stotz (eds.), Comprehensive Biochemistry, vol. 33A, A History of Biochemistry: Part V: The Unravelling of Biosynthetic Pathways (1979), chapter 59, discussing the biosynthesis of fatty acids and glycerides at an advanced level. Ronald H. Kluger Melvyn C. Usselman Inorganic compounds Organometallic compounds General considerations Defining characteristics Figure 22: The periodic table of the elements, showing the group numbers and the s, p, d, 1/4 A compound is regarded as organometallic if it contains at least one metal-carbon (M-C) bond where the carbon is part of an organic group. Typically, an organic group contains carbon-hydrogen (C-H) bonds; examples of common organic groups found in organometallic compounds are given in Table 19. These include the simple methyl group, CH3, and larger homologs such as the ethyl group, C2H5, which attach to a metal atom through only one carbon atom. (Simple alkyl groups such as these are often abbreviated by the symbol R.) More elaborate organic groups include the cyclopentadienyl group, C5H5, in which all five carbon atoms can form bonds with the metal atom. The term metallic is interpreted broadly in this context; thus, when organic groups are attached to the metalloids such as boron (B), silicon (Si), germanium (Ge), and arsenic (As), the resulting compounds are considered to be organometallic along with those containing true metals such as lithium (Li), magnesium (Mg), aluminum (Al), and iron (Fe). As illustrated in Figure 22, the "metal" in an organometallic compound can include most elements, with the exception of nitrogen (N) and phosphorus (P) in group 15 and all the elements in groups 16 (the oxygen group), 17 (halogens), and 18 (noble gases). One example of an organometallic compound is trimethylboron, B(CH3)3, which contains three B-C bonds. Another is ferrocene, Fe(C5H5)2, which has a more elaborate structure with the iron atom sandwiched between two C5H5 rings (see Figure 23). Some compounds with metal-carbon bonds are not regarded as organometallic, because the constituent carbon atom is not part of an organic group; two examples are metal carbides-such as Fe3C, a hard solid that is a component of cast iron-and metal cyanide compounds-such as the deep blue paint pigment Prussian blue, KFe2(CN)6. Historical developments The first synthetic organometallic compound, K[PtCl3(C2H4)], was prepared by the Danish pharmacist William C. Zeise in 1827 and is often referred to as Zeise's salt. At that time Zeise had no way of determining the structure of his new compound, but it is now known that the structure contains an ethene molecule (H2C=CH2) attached through both carbon atoms to the central platinum (Pt) atom. The platinum atom also is bonded to three chlorine (Cl) atoms. The potassium ion, K+, is present to balance the charge. The attachment of the ethene carbon atoms to the central platinum atom qualifies Zeise's salt as an organometallic compound. A development with a more immediate impact on the field of chemistry was the discovery in 1849 by the German-trained British chemist Edward C. Frankland of diethylzinc, H5C2-Zn-C2H5, which he showed is very useful in organic synthesis. Since then, an ever-increasing variety of organometallic compounds have been utilized in organic synthesis in both the laboratory and industry. Figure 25: A schematic structure for vitamin B12 coenzyme, which contains five 1/4 Another milestone in the development of the field was the discovery of tetracarbonylnickel (Figure 24) by the German-educated British industrial chemist Ludwig Mond and his assistants in 1890. More recently, the discovery of the remarkably stable compound ferrocene (shown above, Figure 23) occurred at a time (1951) when techniques for the determination of structures were becoming widely available. Since the 1950s, organometallic chemistry has become a very active field, marked by the discovery of new organometallic compounds along with their detailed structural and chemical characterization and their application as synthetic intermediates and catalysts in industrial processes. Two organometallics encountered in nature are vitamin B12 coenzyme (Figure 25), which contains a cobalt-carbon (Co-C) bond, and dimethylmercury, H3C-Hg-CH3, which is produced by bacteria to eliminate the toxic metal mercury. However, organometallic compounds are generally unusual in biological processes. d- and f-block organometallic compounds Compounds with metal-carbon bonds Alkene and alkyne ligands Inorganic compounds Coordination compounds General considerations Coordination compounds are substances with characteristic chemical structures in which a central metal atom is surrounded by nonmetal atoms (or groups of atoms), called ligands, joined to it by chemical bonds. The class includes a number of important biological materials, such as vitamin B12 and hemoglobin, the red colouring matter of blood. It also includes a number of industrially important materials used as dyestuffs and pigments; as agents for extracting, purifying, and analyzing metals; and as catalysts for preparing such useful organic substances as the polyethylene plastics. Coordination compounds have been studied extensively because of what they reveal about molecular structure and chemical bonding, as well as because of the unusual chemical nature and the useful properties of certain coordination compounds. The general class of coordination compounds-or complexes, as they are sometimes called-is extensive and diverse. The substances in the class may be composed of electrically neutral molecules or of positively or negatively charged species (ions). Among the many coordination compounds having neutral molecules is uranium hexafluoride (UF6). The structural formula of the compound represents the actual arrangement of atoms in the molecules: In this formula the solid lines, which represent bonds between atoms, show that four of the fluorine (F) atoms are bonded to the single atom of uranium (U) and lie in a plane with it, the plane being indicated by dotted lines (which do not represent bonds), whereas the remaining two fluorine atoms (also bonded to the uranium atom) lie above and below the plane, respectively. An example of an ionic coordination complex is the hydrated ion of nickel, (Ni), [Ni(H2O)6]2+, the structure of which is shown below. In this structure, the symbols and lines are used as above and the brackets and the "two plus" (2+) sign show that the double positive charge is assigned to the unit as a whole. The central metal atom in a coordination compound itself may be neutral or charged (ionic). The coordinated groups-or ligands-may be neutral molecules such as water (in the above example), ammonia (NH3), or carbon monoxide (CO); negatively charged ions (anions) such as fluoride (in the first example above) or cyanide ion (CN-); or, occasionally, positively charged ions (cations) such as the hydrazinium (N2H5+) or nitrosonium (NO+) ion. Complex ions-that is, the ionic members of the family of coordination substances-may exist as free ions in solution, or they may be incorporated into crystalline materials (salts) with other ions of opposite charge. In such salts, the complex ion may be either the cationic (positively charged) or the anionic (negatively charged) component (or, on occasion, both). The hydrated nickel ion (above) is an example of a cationic complex. An anionic complex is the hexacyanide of the ferric iron (Fe) ion, 3-, or Crystalline salts containing complex ions include potassium ferricyanide, K3, and the hexahydrate of nickel chloride, [Ni(H2O)6]Cl2. In each case the charge on the complex ion is neutralized by ions of opposite charge. In the case of potassium ferricyanide, three positively charged potassium ions, K+, neutralize the charge on the complex, and in the nickel complex the charges are neutralized by two negative chloride ions, Cl-. The distinction between coordination compounds and other substances is, in fact, somewhat arbitrary. The designation coordination compound, however, is generally restricted to substances whose molecules or ions are discrete entities and in which the central atom is metal. Accordingly, molecules such as sulfur hexafluoride (SF6) and carbon tetrafluoride (CF4) are not normally considered to be coordination compounds, because sulfur (S) and carbon (C) are nonmetallic elements. Yet there is no great difference between these compounds and, say, uranium hexafluoride. Furthermore, such simple ionic salts as sodium chloride (NaCl) or nickel difluoride (NiF2) are not considered to be coordination compounds, because they consist of continuous ionic lattices rather than discrete molecules. Nevertheless, the arrangement (and bonding) of the anions surrounding the metal ions in these salts is similar to that in coordination compounds. Coordination compounds generally display a variety of distinctive physical and chemical properties, such as colour, magnetic susceptibility, solubility and volatility, an ability to undergo oxidation-reduction reactions, and catalytic activity. A coordination compound is characterized by the nature of the central metal atom or ion; the oxidation state of the latter (that is, the gain or loss of electrons in passing from the neutral atom to the charged ion); and by the number, kind, and arrangement of the ligands. Because virtually all metallic elements form coordination compounds, sometimes in several oxidation states and usually with many different ligands, a large number of coordination compounds is known. Coordination number The coordination number is the term proposed by the Alsatian-born Swiss chemist Alfred Werner to denote the total number of bonds from the ligands to the metal atom. Coordination numbers generally range between 2 and 12, with 4 (tetracoordinate) and 6 (hexacoordinate) being the most common. Werner referred to the central atom and the ligands surrounding it as the coordination sphere. Inorganic compounds Inorganic compounds are most often classified in terms of the elements or groups of elements that they contain. Oxides, for example, can be either ionic or molecular. Ionic oxides contain O2- (oxide) ions and metal cations, while molecular oxides contain molecules in which oxygen (O) is covalently bonded to other nonmetals such as sulfur (S) or nitrogen (N). When ionic oxides are dissolved in water, the O2- ions react with water molecules to form hydroxide ions (OH-), and a basic solution results. Molecular oxides react with water to produce oxyacids, such as sulfuric acid (H2SO4) and nitric acid (HNO3). In addition, inorganic compounds include hydrides (containing hydrogen atoms or H- ions), nitrides (containing N3- ions), phosphides (containing P3- ions), and sulfides (containing S2- ions). Transition metals form a great variety of inorganic compounds. The most important of these are coordination compounds in which the metal atom or ion is surrounded