7.4: Naming Aromatic Compounds

Nomenclature of Disubstituted Benzenes

With disubstituted benzenes there are three distinct positional isomers which can occur and must be identified in the compounds name. Although numbering can be used to indicate the position of the two subsituents it is much more common for the compounds to be named using prefixes. These prefixes are italicized and are often abbreviated with a single letter. They are defined as the following:

• ortho- (o-): 1,2- (next to each other in a benzene ring)
• meta- (m): 1,3- (separated by one carbon in a benzene ring)
• para- (p): 1,4- (across from each other in a benzene ring)

Figure \(\PageIndex{2}\). If any do appear then the compound is not named as a benzene but with a different parent name. These compounds are named as such: Position prefix-Name of the substituent + Name of parent chain.

Figure \(\PageIndex{2}\) the compound is named as such: Position prefix-Names of the substituents in alphabetical order + benzene. Remember if two of the same substituent appears then the prefix di- is used before the substituent's name.

Nomenclature of Benzenes with Three or more Substituents

When three or more substituents are present the ortho, meta, para positional prefixes become inadequate and a numbering system for the ring must be applied. Here again it is important to check if any of the substituents are listed in Figure \(\PageIndex{2}\). If a substituent from Figure \(\PageIndex{2}\) is present it is given the parent name in the nomenclature. Also, this substituent is given position one in the numbering system. The other substituents are numbered such that they get the lowest possible sum. In the compound's name the subsituents are given their position number and listed alphabetically. Remember that di-, tri, tetra- prefixes are still used to indicate multiple of the same substituent being present but are ignored for alphabetical listing.

Figure \(\PageIndex{2}\), the benzene ring is name in the same fashion as cycloalkanes. The lowest possible number is given to the substituents present. This is best done by arbitrarily giving a substituent position one and then numbering the rest of the substituents. Then this process is repeated with the other substituents. Which ever iteration provides the lowest overall sum of numbers will be used in the compound's name. The substituents are assigned a location number, listed alphabetically and the suffix -benzene is added.