20.10 Naming Aromatic Compounds

Monosubstituted Benzenes

The simplest aromatic compounds are benzene rings with one substituent replacing one of the hydrogen atoms. If this substituent is an alkyl group, it is named first, followed in one word with "benzene". The molecule shown below is therefore called ethylbenzene.

Substituents can be groups other than alkyl groups. If a chlorine atom were substituted for a hydrogen, the name becomes chlorobenzene. An $$\ce{-NH_2}$$ group is called an amino group, so the corresponding molecule is called aminobenzene, often referred to as an aniline. An $$\ce{-NO_2}$$ group is called a nitro group and so the third example below is nitrobenzene.

Disubstituted Benzenes

If more than one substituent is present, their location relative to each other can be indicated by numbering the positions on the benzene ring.

The number of the carbon location then precedes the name of the substituent in the overall name, with the numbers separated by a comma. As with branched alkanes, the system requires that the numbers be the lowest possible and that prefixes be used for more than one of the same substituent. If there are different substituents, the first in alphabetical order is given the lower number and listed first. The structures below are called 1,2-dimethylbenzene and 1-ethyl-4-methylbenzene.

An alternate system for naming di-substituted benzene rings uses three different prefixes: ortho, meta, and para. If two groups are in the ortho position, they are on adjacent carbon atoms. The meta positioning refers to being in a 1,3 arrangement. The para positioning refers to being in a 1,4 arrangement. Shown below are the three possibilities for dimethylbenzene, also called xylene.

Lastly, a benzene ring missing one hydrogen atom $$\left( \ce{-C_6H_5} \right)$$ can itself be considered the substituent on a longer chain of carbon atoms. That group is called a phenyl group and so the molecule below is called 2-phenylbutane.

Contributors

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.