Organic Chemistry

I

Introduction

Organic Chemistry, 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 modern life: 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.

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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 C_nH_2n+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 C_nH_2n, and the IUPAC names are consistent with those of the alkanes.

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