A system by which a unique and unambiguous name or other designation is assigned to a given organic molecular structure. A set of rules adopted by the International Union of Pure and Applied Chemistry (IUPAC) is the basis for a standardized name for any organic compound. Common or nonsystematic names are used for many compounds that have been known for a long time. The latter names have the advantage of being short and easily recognized, as in the examples below. However, in contrast to systematic names, common or trivial names do not convey information from which the structure can be written by reference to prescribed rules.
In the IUPAC system, a name is formed by combination of a parent alkyl chain or ring with prefixes and suffixes to denote substituents. For aliphatic compounds, the parent name is a stem that denotes the longest straight chain in the structure with the ending -ane. The first four members of the alkane series are methane, ethane, propane, and butane. From five carbons on, the names follow the Greek numerical roots, for example, pentane and hexane. Changing the ending -ane to -yl gives the name of the corresponding radical or group; for example, CH3CH2 is the ethyl radical or ethyl group. Branches on an alkyl chain are indicated by the name of a radical or group as a prefix. The location of a branch or other substituent is indicated by number; the chain is numbered from whichever end results in the lowest numbering. See also Alkane.
Double or triple bonds are indicated by the endings -ene or -yne, respectively. The configuration of the chain at a double bond is denoted by E- when two similar groups are on opposite sides of the plane bisecting the bond, and Z-when they are on the same side. See also Alkene;
A compound containing a functional group is named by adding to the parent name a suffix characteristic of the group. If there are two or more groups, a principal group is designated by suffix and the other(s) by prefix.
The same general principles apply to cyclic compounds. For alicyclic rings, the prefix cyclo- is followed by a stem indicating the number of carbon atoms in the ring, as is illustrated in the structure below.
The names of aromatic hydrocarbons have the ending -ene, which denotes a ring system with the maximum number of noncumulative double bonds. Each of the simpler polycyclic hydrocarbons has a different parent name. To number the positions in a polycyclic aromatic ring system, the structure must first be oriented with the maximum number of rings arranged horizontally and to the right. The system is then numbered in clockwise sequence starting with the atom in the most counterclockwise position of the upper right-hand ring, omitting atoms that are part of a ring fusion. See also Aromatic hydrocarbon.
Heteroatom | Prefix | Ring size | Suffix |
|---|---|---|---|
O | ox(a)- | 4 | -ete |
S | thi(a)- | 5 | -ole |
N | az(a)- | 6 | -ine |
7 | -epine |
Systematic names for rings containing a heteroatom (O, S, N) are based on a combination of a prefix denoting the heteroatom(s) and a suffix denoting the ring size, as indicated in the table. See also Chemical symbols and formulas; Heterocyclic compounds; Organic chemistry.
