I INTRODUCTION  Chemistry, Organic, branch of chemistry in which carbon compounds and their reactions are studied. A wide variety of classes of substances—such as drugs, vitamins, plastics, natural and synthetic fibers, as well as carbohydrates, proteins, and fats—consist of organic molecules. Organic chemists determine the structures of organic molecules, study their various reactions, and develop procedures for the synthesis of organic compounds. Organic chemistry has had a profound effect on life in the 20th century: It has improved natural materials and it has synthesized natural and artificial materials that have, in turn, improved health, increased comfort, and added to the convenience of nearly every product manufactured today.

The advent of organic chemistry is often associated with the discovery in 1828 by the German chemist Friedrich Wöhler that the inorganic, or mineral, substance called ammonium cyanate could be converted in the laboratory to urea, an organic substance found in the urine of many animals. Before this discovery, chemists thought that intervention by a so-called life force was necessary for the synthesis of organic substances. Wöhler's experiment broke down the barrier between inorganic and organic substances. Modern chemists consider organic compounds to be those containing carbon and one or more other elements, most often hydrogen, oxygen, nitrogen, sulfur, or the halogens, but sometimes others as well.

II ORGANIC FORMULAS AND BONDS  
The molecular formula of a compound indicates the number of each kind of atom in a molecule of that substance. Fructose, or grape sugar (C6H12O6), consists of molecules containing 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Because at least 15 other compounds, however, have this same molecular formula, to distinguish one molecule from another, a structural formula is used to show the spatial arrangement of the atoms:

Even an analysis that gives the percentage of carbon, hydrogen, and oxygen cannot distinguish C6H12O6 from ribose, C5H10O5, another sugar in which the ratios of elements are the same, namely 1:2:1.

The forces that hold atoms together in a molecule are chemical bonds, of which there are three types: ionic, covalent, and metallic (see Chemical Reaction; Metals). Ionic bonds are held together by the attraction of opposite electric charges. Covalent bonds are shared pairs of electrons. Wöhler's experiment, for example (see Fig. 1), resulted in a change from ionic bonds in ammonium cyanate to covalent bonds in urea. In ammonium cyanate, the attraction between the group of five atoms in NH4+ bearing a positive charge and the group of three atoms in CNO- bearing a negative charge constitutes an ionic bond. Within the NH4+ group, the four lines—N to H—represent covalent bonds, or electron pairs. Likewise, within the CNO- group and in the molecule of urea the lines represent covalent bonds. The application of heat to ammonium cyanate molecules results in a rearrangement of the bonds. The ability of carbon to form covalent bonds is not unique, as is evident from this example. The bonds between nitrogen and hydrogen are also covalent.

The ability of carbon to form covalent bonds with other carbon atoms in long chains and rings, however, does distinguish carbon from all other elements. Other elements are not known to form chains of greater than eight like atoms. This property of carbon, and the fact that carbon nearly always forms four bonds to other atoms, accounts for the large number of known compounds. At least 80 percent of the 5 million chemical compounds registered as of the early 1980s contain carbon.

III CLASSIFICATION AND NOMENCLATURE  
The consequences of the unique properties of carbon are manifest in the simplest class of organic compounds—the aliphatic, or straight-chain, hydrocarbons.

A Alkanes  
The parent compound of this family, the alkanes, is methane, CH4. The next members of the family are ethane (C2H6), propane (C3H8), and butane (C4H10), so the general formula for any member of this family is CnH2n+2. For compounds containing more than four carbon atoms, Greek prefixes are used with the ending -ane to name the compounds pentane, hexane, heptane, octane, and so on.

The names butane, pentane, and so on, however, do not by themselves specify molecular structure. Two different structural formulas, for example, can be drawn for the molecular formula C4H10. Compounds with the same molecular formula but different structural formulas are called isomers. In the case of butane, the common isomer names are normal butane (written n-butane) and isobutane. Urea and ammonium cyanate are also isomers; they are structural isomers of the molecular formula CH4 N2O.

The formula C8H18 has 18 isomers and C20H42 has 366,319 theoretical isomers. Thus, unsystematic, or trivial, names commonly used when new compounds are discovered must give way to systematic names that can be used in all languages. The International Union of Pure and Applied Chemists (IUPAC) in 1890 agreed on such a system of nomenclature and has revised it to incorporate new discoveries.

In the IUPAC system of nomenclature, the longest chain of carbon atoms is numbered to give the side chains the smallest sum. The three side chains in Fig. 4 are on carbon atoms 2, 2, and 4; if the chain were numbered in the opposite direction, the side chains would be on carbon atoms 2, 4, and 4. Therefore, 2,2,4-trimethylpentane is the correct name because it results in the smaller sum.

Another family of hydrocarbons, the cyclanes, has a cyclic or ring structure; the smallest ring contains three carbon atoms. The cyclanes have the general formula CnH2n, and the IUPAC names are consistent with those of the alkanes.

B Alkenes and Alkynes  Isomeric with the cyclanes, or cycloalkanes as they are sometimes called, is the family of alkenes, also represented by the general formula CnH2n. This family of hydrocarbons is characterized by one or more double bonds between carbon atoms. Propene and cyclopropane, for example, are isomers, as are 1,3-dimethylcyclohexane and 3,4-dimethyl-2-hexene. (The location of the double bond is indicated by the 2-hexene part of the name.) Double bonds may also occur in cyclic compounds, for example, in á-pinene, a constituent of turpentine, and vitamin A.

Chemists commonly use a shorthand notation when writing the structural formulas of cyclic organic compounds. The apex of the angles in these formulas represents a carbon atom. Each carbon atom is understood to have 2, 1, or 0 hydrogen atoms bound to it, depending on whether there are 2, 3, or 4 bonds to other (usually carbon) atoms. For example, see Fig. 8 for the fully notated structural formula for á-pinene.

Alkynes, or acetylenes, the third major family of aliphatic hydrocarbons, have the general formula CnH2n-2 and contain still fewer hydrogen atoms than alkanes or alkenes. Acetylene, HCCH, the most common example, is termed ethyne in the IUPAC system.

C Functional Groups  
Other atoms, such as chlorine, oxygen, and nitrogen, may be substituted for hydrogen in an alkane, providing the correct number of chemical bonds is allowed—chlorine forming one bond to other atoms, oxygen forming two bonds to other atoms, and nitrogen three bonds. The chlorine atom in ethyl chloride, the OH group in ethyl alcohol, and the NH2 group in ethyl amine are called functional groups. Functional groups determine many of the chemical properties of compounds. Other functional groups are shown in Table 2 with general formulas, prefixes or suffixes that are added to names, and an example of each class.

D Optical and Geometric Isomers  The tetrahedral nature of the shapes of carbon bonds dictates some properties of organic compounds that can be accounted for only by spatial relationships. When four different groups of atoms are attached to a central carbon atom, two different molecules can be constructed in space. The compound lactic acid (see Fig. 9), for example, exists in two forms—a phenomenon called optical isomerism. The optical isomers are related the same way as an object and its mirror image are related. A mirror placed between the two structures of lactic acid would reflect like groups: CH3 of one reflecting the position of CH3 in the other, OH reflecting OH, and so on—just as a mirror placed next to a right-hand glove reflects an image of a left-hand glove.

Optical isomers have exactly the same chemical properties and all of the same physical properties except one: the direction that each type of isomer turns a plane of polarized light (see Optics). Dextro-lactic acid, or D-lactic acid, turns the plane of polarized light to the right and levo-lactic acid, or L-lactic acid, to the left. Racemic lactic acid (a 1:1 mixture of D- and L-lactic acid that is found in sour milk) exhibits zero rotation because left and right rotations cancel each other.

Double bonds in carbon compounds give rise to geometric isomerism (not related to optical isomerism) if each double bond has different groups attached. A molecule of 2-heptene, for example, may be arranged two ways in space because rotation about the double bond is restricted. When the like groups, hydrogen atoms in this case, are on opposite sides of the double bonded carbon atoms, the isomer is called trans and when the hydrogens are on the same side, the isomer is called cis.

E Saturation  Compounds containing double or triple bonds are said to be unsaturated. Unsaturated compounds can undergo addition reactions with various reagents that cause the double or triple bonds to be replaced with single bonds. Addition reactions cause unsaturated compounds to become saturated. Although saturated compounds are generally more stable than unsaturated compounds, two double bonds in the same molecule cause less instability if they are separated by a single bond. Isoprene, the building block for natural rubber, has this so-called conjugated structure, as does retinal, a compound derived from vitamin A.

Complete conjugation in a six-membered carbon ring has a more profound effect, a stabilizing influence so strong that the compound is no longer unsaturated. Benzene, C6H6, and the family of cyclic compounds called aromatic hydrocarbons, do not add the reagents that react with isoprene, alkanes, and alkenes. In fact, the properties of aromatic compounds are so different that a more appropriate symbol for benzene is the hexagon on the extreme right of Fig. 13 rather than the other two. The circle inside the hexagon suggests that the six electrons represented as three conjugated double bonds belong to the entire hexagon and not to individual carbons at the corners of the hexagon. Other aromatic compounds are shown in Fig. 14.

Cyclic molecules may contain atoms of elements other than carbon. The most common so-called hetero atoms are sulfur (S), nitrogen (N), and oxygen (O), although others—for example, boron (B), phosphorus (P), and selenium (Se)—are known.

IV SOURCES OF ORGANIC COMPOUNDS  
Coal tar was once the only source of aromatic and some heterocyclic compounds. Petroleum was the source of aliphatic compounds that contain such substances as gasoline, kerosene, and lubricating oil. Natural gas supplied methane and ethane. These three categories of natural substances are still the major sources of organic compounds for most countries. When petroleum is not available, however, a chemical industry can be based on acetylene, which in turn can be synthesized from limestone and coal. During World War II, Germany was forced into just that position when it was cut off from reliable petroleum and natural-gas sources.

Table sugar from cane or beets is the most abundant pure chemical from a plant source. Other major substances derived from plants include carbohydrates such as starch and cellulose, alkaloids, caffeine, and amino acids. Animals feed on plants and other animals to synthesize amino acids, proteins, fats, and carbohydrates.

V DETERMINATION OF STRUCTURE  
The use of chemical reactions to identify the structures of organic compounds has been replaced largely by instrumental methods since 1940. Infrared spectra are used to identify functional groups, and ultraviolet spectroscopy can distinguish aromaticity and certain kinds of unsaturation in a molecule. A nuclear magnetic resonance (nmr) spectrum gives the largest amount of information about the structure of a compound; infrared and ultraviolet spectra complement rather than duplicate such data. Proton resonance spectroscopy is sometimes used to determine the nature of the local environment of the hydrogen atoms in a molecule and it can often simultaneously supply the ratios of types of hydrogen. More recently, carbon-13 nuclear magnetic resonance spectroscopy has been used to derive complementary information to the proton data. Also, an X-ray spectrum may be necessary to determine three-dimensional aspects of structure in a complex organic molecule. See Chemical Analysis.

VI PHYSICAL PROPERTIES OF ORGANIC COMPOUNDS  In general, covalent organic compounds are distinguished from inorganic salts by low melting points and boiling points. The ionic compound sodium chloride (NaCl), for example, melts at about 800° C (about 1470° F), but the strictly covalent molecule, carbon tetrachloride (CCl4), boils at 76.7° C (170° F). Between these temperatures an arbitrary line may be drawn at about 300° C (about 570° F) to distinguish most covalent from most ionic compounds. A large fraction of organic compounds melt or boil below 300° C, although exceptions exist. Organic compounds generally dissolve in nonpolar solvents (liquids that do not have localized electric charges) such as gasoline or carbon tetrachloride, or solvents of low polarity such as alcohols, acetic acid, and acetone. Organic compounds are often insoluble in water, a strongly polar solvent.

Hydrocarbons have low densities, often about 0.8 compared to water, 1.0, but functional groups may increase the densities of organic compounds to 1.0. Only a few organic compounds have densities greater than 1.2, generally those containing multiple halogen atoms.

Functional groups capable of forming hydrogen bonds generally increase viscosity (resistance to flow) in molecules. For example, the viscosities of ethanol, ethylene glycol, and glycerol increase in that order. These compounds contain one, two, and three OH groups, respectively, which form strong hydrogen bonds.

VII CHEMICAL REACTIONS  Chemists ordinarily design organic reactions to be carried out at optimum conditions for the particular reaction in order to produce maximum yields. To do so necessarily means that the chemist must be aware of desirable catalysts, whether or not the reaction is reversible, and how to take advantage of equilibrium positions. As an example, two different products may be obtained in sulfonating naphthalene (adding the SO3H functional group to a naphthalene molecule) by taking advantage of the reversibility of this reaction (Fig. 16).

At a temperature of 80° C (176° F) the rate of reaction at the á-position is faster than the rate at the â-position. Consequently, a 91 percent yield of á-naphthalenesulfonic acid can be obtained at 80°; at a higher temperature, the â-isomer predominates. At 160° C (320° F) the á-isomer desulfonates more rapidly than the â-form, so an 85 percent yield of â-naphthalenesulfonic acid, the more stable isomer, is obtained.

Catalysts are frequently essential for rapid chemical reactions. Water, for example, will not add to unsaturated compounds unless a small amount of a strong acid (represented in Fig. 17 by H+) is present. See Acids and Bases; Catalysis; Chemical Reaction.

Contributed By:
K. P. Müller
H. Reinhard

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