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Chemical Compounds

Table of Contents
Nomenclature.
The necessity of giving each compound a unique name requires a richer variety of terms than is available with descriptive prefixes such as n- and iso-. The naming of organic compounds is facilitated through the use of formal systems of nomenclature. Nomenclature in organic chemistry is of two types: common and systematic. Common names for organic compounds originate in many different ways but share the feature that there is no necessary connection between name and structure. The name that corresponds to a specific structure must simply be memorized, much like learning the name of a person. Systematic names, on the other hand, are keyed directly to molecular structure according to a generally agreed upon set of rules. The most widely used standards for organic nomenclature have evolved from suggestions made by a group of chemists assembled for that purpose in Geneva in 1892 and have been revised on a regular basis by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC rules govern all classes of organic compounds but are ultimately based on alkane names. Compounds in other families are viewed as derived from alkanes by appending functional groups to, or otherwise modifying, the carbon skeleton.

The IUPAC rules assign names to unbranched alkanes according to the number of their carbon atoms. Methane, ethane, and propane are used for CH{sub 4} , CH{sub 3}CH{sub 3} , and CH{sub 3}CH{sub 2} CH{sub 3}, respectively. The unbranched alkane CH{sub 3} CH{sub 2}CH{sub 2} CH{sub 3} is defined as butane, not n-butane as given above. (The n- prefix is not used in systematic IUPAC nomenclature.) Beginning with five-carbon chains, the names of unbranched alkanes consist of a Latin or Greek stem corresponding to the number of carbons in the chain followed by the suffix -ane. Some examples are given in Table 22. A group of compounds, such as the unbranched alkanes, that differ from one another by successive introduction of CH{sub 2} groups constitute what is called a homologous series.

Alkanes with branched chains are named on the basis of the name of the longest continuous chain of carbon atoms in the molecule, called the parent. The alkane shown has seven carbons in its longest continuous chain and is therefore named as a derivative of heptane, the unbranched alkane that contains seven carbon atoms. The position of the CH{sub 3} (methyl) substituent on the seven-carbon chain is specified by a number (3-), called a locant, obtained by successively numbering the carbons in the parent chain starting at the end nearer the branch. The compound is therefore called 3-methylheptane.

When there are two or more identical substituents, replicating prefixes (di-, tri-, tetra-, etc.) are used, along with a separate locant for each substituent. Different substituents, such as an ethyl (-CH{sub 2} CH{sub 3}) and a methyl (-CH{sub 3}) group, are cited in alphabetical order. Replicating prefixes are ignored when alphabetizing. In alkanes numbering begins at the end nearest the substituent that appears first on the chain so that the prefix numbers are as low as possible.

Methyl and ethyl are examples of alkyl groups. IUPAC nomenclature rules for naming alkyl groups beyond these two extend to cover even very complex structures. The IUPAC rules are unambiguous in the sense that there is no possibility that two different compounds will have the same name.

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