The nomenclature of aromatic compounds is divided into three main categories, depending on the number of substituents on the benzene ring.
Compounds that have only one substituent on the benzene ring are addressed as monosubstituted. When naming these compounds, the rule is to have the name of the substituent followed by the word benzene. Typical examples are chlorobenzene, nitrobenzene and bromobenzene. In this category, we can see most of the common names, like phenols and toluenes.
Derivatives of benzene with two substituents are called disubstituted compounds. In this case, it’s first necessary to choose the main substituent, which usually is the one that forms a common name, like OH- for phenol, and then specify the position of the second substituent with a prefix. There are different prefixes depending on where the substituent is on the ring:
Ortho: The substituent is on the carbon adjacent to the main one.
Meta: The substitute is on the second carbon from the main one.
Para: The substituent is on the carbon opposite to the main one.
Trisubstituted or more
When three or more substituents are present, the use of prefixes results inadequate and it’s switched to numbers. The first step is to identify a substituent that forms a common name, if present. This will form the parent name and become the point where the numbering starts. The other substitutents are assigned the lowest value possible and placed in alphabetic order. An example is: 5-nitro-2-propylaniline.