OrganicChemistry, 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: natural materials have been improved and natural and artificial materials have been synthesized, improving health, increasing comfort, and adding to the convenience of many products manufactured today.
The advent of organic chemistry is often associated with the discovery in 1828 by the German chemistFriedrich Wohler 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. Wohler’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, sulphur, or the halogens, but sometimes others as well.
Organic Formulae 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. At least 15 other compounds have this same molecular formula, so to distinguish one molecule from another a structural formula is used to show the spatial arrangement of the atoms:
An analysis that gives the percentage of carbon, hydrogen, and oxygen cannot distinguish C6H12O6 from ribose, C5H10O5, another sugar in which the ratios of these three elements are the same, namely 1:2:1. The empirical formula, which shows the simplest ratios of the elements that are present, is CH2O for both substances.
The forces that hold atoms together in a molecule are chemical bonds, of which there are three types: ionic, covalent, and metallic. The ability of carbon to form covalent bonds with other carbon atoms in long chains and rings distinguishes carbon from all other elements. Other elements hardly ever form chains of more than eight 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, running into many millions.
Classification and Nomenclature
The consequences of the unique properties of carbon are manifest in the simplest class of organic compounds—the aliphatichydrocarbons. Hydrocarbons consist of the elements hydrogen and carbon only. Aliphatic hydrocarbons are those in which the carbon atoms form a single straight chain, or a branching one. There are three main types: alkanes, alkenes, and alkynes.
The simplest alkane 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 CncH2n+2. For compounds containing more than four carbon atoms, Greek or Latin prefixes are used with the ending “-ane” to name the compounds: pentane, hexane, heptane, octane, and so on.
A formula such as C4H10 does not by itself specify molecular structure. Two different structural formulae, for example, can be drawn for the molecular formula C4H10. Compounds with the same molecular formula but different structural formulae are called isomers. In the case of C4H10, the common isomer names are butane and methylpropane (formerly 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 for new compounds when they were discovered must give way to systematic names that can be used in all languages. The International Union of Pure and Applied Chemistry (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 found. This may have side chains. Two side chains (methyl groups) on the first molecule shown in figure 4 are on carbon atom 2, and another is on carbon atom 4; this can be represented by the prefix 2,2,4-. If the chain were numbered in the opposite direction, the side chains would be on carbon atoms 2, 4, and 4, and would be represented by 2,4,4-. In fact the prefix 2,2,4- is used, because it has the smaller number at the first point of difference. This prefix is attached to the name “trimethylpentane”: the “trimethyl-” signifies the three methyl groups, and “-pentane” signifies a straight hydrocarbon chain with five carbon atoms. The second molecule shown in figure 4 has a single ethyl (CH3CH2-) side chain attached to the third carbon atom of a straight five-carbon hydrocarbon chain, and is therefore called 3-ethylpentane.
Another family of hydrocarbons, the cycloalkanes or cyclanes, has a cyclic or ring structure; the smallest possible ring contains three carbon atoms. The cycloalkanes have the general formula CnH2n, and the IUPAC names are consistent with those of the alkanes (figure 5).
Alkenes and Alkynes
Isomeric with the cycloalkanes are the alkenes, also represented by the general formula CnH2n. This family of hydrocarbons is characterized by one or more double bonds between carbon atoms. Figure 6 shows two examples. Propene and cyclopropane 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 a-pinene, a constituent of turpentine and vitamin A (figure 7).
Chemists commonly use a shorthand notation when writing the structural formulae of cyclic organic compounds. The apex of the angles in these formulae 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 figure 8 for the full structural formula for a-pinene.
Alkynes, or acetylenes, another major family of 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.
Other atoms, such as chlorine, oxygen, and nitrogen, may be substituted for hydrogen in an alkane, providing that 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 chloroethane (ethyl chloride), the OH group in ethanol (ethyl alcohol), and the NH2 group in aminoethane (ethylamine) are called functional groups. Functional groups determine most of the chemical properties of compounds. Other functional groups are shown in the accompanying table with general formulae, prefixes or suffixes that are added to names, and an example of each class.
Optical and Geometric Isomers
When four different groups of atoms are attached to a central carbon atom, they lie at the corners of a tetrahedron (a figure with four triangular faces). Two different forms of such a molecule can exist. For example, the compound lactic acid, 2-hydroxypropanoic acid (see figure 9), exists in two forms—a phenomenon called optical isomerism. The optical isomers are related in the same way as an object and its mirror image are related: 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 in which each type of isomer turns the plane of polarization of plane-polarized light. Dextro-lactic acid or (+)-lactic acid (formerly called d-lactic acid), turns the plane ofpolarized light to the right and laevo-lactic acid, or (-)-lactic acid (formerly called l-lactic acid), turns it to the left. Racemic lactic acid (a 1:1 mixture of (+)- and (-)-lactic acid) 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 in 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.
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 do retinal and vitamin A.
Complete conjugation in a six-membered carbon ring has a more profound effect. Benzene, C6H6, and the family of cyclic compounds called aromatic hydrocarbons, do not undergo addition reactions with the reagents that react with isoprene 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 figure 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 figure 14.
Cyclic molecules containing atoms of elements other than carbon are called heterocyclic compounds (figure 15). The most common so-called hetero atoms are sulphur, nitrogen, and oxygen.
Sources of Organic Compounds
Coal tar was once the only source of aromatic and some heterocyclic compounds. Petroleum was the source of alkanes that contain such substances as petrol, kerosene, and lubricating oil. Natural gas supplied mainly methane. 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 ethyne, 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.
Sugar (sucrose) from cane or beet 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.
Physical Properties of Organic Compounds
In general, covalent organic compounds are distinguished from inorganic salts by low melting and boiling points. The ionic compound sodium chloride (NaCl), for example, melts at about 800° C (1470° F), but the covalent molecule tetrachloromethane (carbon tetrachloride, CCl4) boils at 76.7° C (170° F). Between these temperatures an arbitrary line may be drawn at about 300° C (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 without polar groups generally dissolve in non-polar solvents (liquids that do not have localized electric charges) such as octane or tetrachloromethane and are often insoluble in water, a strongly polar solvent.
Hydrocarbons have low densities, often about 0.8 of that of water, but functional groups may increase the densities of organic compounds. Only a few organic compounds have densities greater than 1.2, generally those containing multiple halogen atoms.
Functional groups capable of forming hydrogen bonds generallyincrease viscosity (resistance to flow) in molecules. For example, the viscosities of ethanol, ethane-1,2-diol (glycol), and propane-1,2,3-triol (glycerol) increase in that order. These compounds contain one, two, and three OH groups, respectively, which form strong hydrogen bonds.