There are four sets of equivalent carbon atoms (see equivalent ligands) in the benzene ring of a monosubstituted benzene.
- A = any substituent
- C2 ≡ C6
- C3 ≡ C5
They are labeled for identification purposes as follows.
A disubstituted benzene could exist as three constitutional isomers (1-3), which differ from one another in the relative positions of the substituents on the benzene ring and can be identified using the descriptors otho, meta, and para.