Nomenclature of Aromatic Compounds
Organic compounds containing one or more benzene rings and their functionalized derivatives are called aromatic compounds.
Note : There are ,however, examples of aromatic compounds which do not contain a benzene ring but instead contain other highly unsaturated ring.
Benzene can be represented as either of the following ways :
For IUPAC nomenclature of aromatic compounds, the substituent is placed as prefix to the word benzene.
Many substituted benzene compounds are popular by their common names. Some of them are given below :
Example 1 : Methylbenzene
Common name is Toluene
Example 2 : Methoxybenzene
Common name is Anisole
Example 3 : Aminobenzene
Common name is Aniline
Example 4 : Hydroxybenzene
Common name is Phenol
Disubstituted benzene ring : If a benzene ring is disubstituted i.e. contains two substituents, the position of the substituents is defined by numbering the carbon atoms in such a way that the substituents are located at the lowest number possible.
ortho (o), meta (m), para (p)
In the trivial system of nomenclature, the terms ortho, meta and para are used as prefixes to indicate the relative positions of the substituents present.
ortho is used to indicate that the relative position between the substituents present is 1,2-. o can alternatively be used in place of ortho. For example :
ortho-dibromobenzene = o-dibromobenzene
Example : o-dichlorobenzene
IUPAC name : 1,2-Dichlorobenzene
meta is used to indicate that the relative position between the substituents present is 1,3-. m can alternatively be used to represent meta.
Example : m-dichlorobenzene
IUPAC name : 1,3-Dichlorobenzene
para is used to indicate that the relative position between the substituents present is 1,4-. p can alternatively be used to represent para.
Example : p-dichlorobenzene
IUPAC name : 1,4-Dichlorobenzene
Note : ortho, meta and para cannot be used for tri or higher substituted benzene compounds.
In some cases, common name of benzene derivatives is taken as the base compound. Substituent of the base compound is assigned number 1 and then the numbering is done in such a way that the next substituent gets the lowest number.
Example : 4-Ethyl-2-methylphenol
Example : 2-Chloro-4-methylanisole
When a benzene ring is attached to a side chain which contains a functional group, the benzene ring is considered as a substituent. The name of benzene as a substituent is phenyl.
Example : 3-Phenylpropanoic acid