There are two classifications of aromatic compounds.Nuclear Substituted Compounds
When the functional group or any substituent, in aromatic compounds is directly attached to the benzene ring, it is a called nuclear substituted compound. Such compounds are named as the derivatives of benzene under the IUPAC system. However, the common names of many such compounds are retained by IUPAC.
Sidechain Substituted Compounds
Aromatic compounds where the functional group is present in the sidechain of the ring are called sidechain substituted compounds. Sidechain substituted compounds are named as the phenyl derivatives of the corresponding aliphatic compounds.
Naming Benzene Derivatives
Only one kind of monosubstituted derivatives are possible in benzene rings as all six hydrogen atoms are equivalent. For example, there is only one toluene. It does not matter where the methyl group is attached because all the following arrangements are equivalent.
When two same or different monovalent substituents, are present on a benzene ring, the following three arrangements are possible.
For the same substituent (A)
ortho (or, 1,2-) meta-(or, 1,3-) para- (or, 1,4)
For different substituents (A and B)
ortho (or, 1,2-) meta-(or, 1,3-) para- (or, 1,4)
These arrangements are named as follows:
The compound containing the two groups on the adjacent sites is called 'ortho'; it is denoted as 'o-'. In the IUPAC system, the ortho position is designated as 1,2-.
The compound containing the two groups on alternate sites is called 'meta': it is denoted as 'm-'. In the IUPAC system,the meta position is designated as 1,3-.
The compound containing the two groups diagonally opposite to each other is called 'para': denoted as 'p-'. In the IUPAC system, the para position is designated as 1,4-.
For example, the three xylenes are named as,
o-xylene m-xylene p- xylene
(1,2-dimethylbenzene) (1,3-dimethylbenzene) (1,3-dimethylbenzene)
In the case of trisubstituted derivatives, the nature of the substituted groups determine the number of arrangements. When the three substituent groups are identical (say, A), three arrangements are possible. These are termed as follows.
vicinal (vic-) unsymmetrical (unsym-) symmetrical (sym-)
For a trisubstituted product, if the two substituents are identical and the third different, then six products are possible. When all the three groups are different, ten products are possible. Since, naming each individual compound is not possible, it was found convenient to indicate the position of any substituent by the numeral indicating the serial number of the carbon atom in the ring, to which that substituent is attached.
Numbering the Carbon Atoms in the Ring
The numbering of carbon atoms in the ring (or nucleus) is done as follows.
* When there is only one substituent on the ring, there is only one compound possible. Thus, numbering of the carbon atoms of the nucleus does not arises.
* If there are two or more substituents, then numbering is in the alphabetical order of the substituents on the carbon atoms. The prefixes such as 'di', 'tri', 'cyclo', 'iso', etc., are ignored while arranging the substituents alphabetically.
* When two or more functional groups are present, then the principal functional group is assigned the number 1. The order of priority of the functional groups is the same as done for aliphatic polyfunctional compounds.
For the sake of convenience, the ring is oriented in such a way that position 1 is at the top and numbering is done in a clockwise or anticlockwise manner whichever gives lower numbers to the other substituents.
This is illustrated through the following example. The IUPAC names are written in bold letters.
(o-xylene) (m-xylene) (p-xylene)
1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene
Names of some typical aromatic compounds are given below:
Aromatic hydrocarbons (arenes)
toluene o-xylene m-xylene p-xylene
Methylbenzene 1,2-dimethylbenzene 1,3-dimethylbenzene 1,4-dimethylbenzene
1,3,5-trimethylbenzene 2-phenylpropane(cumene) phenylethene (styrene)
Anthracene Phenonthrene
Halogen derivatives
2-chlorotoulene 1,2-dichlorobenzene phenyl chloromethane
Hydroxy derivatives:
Phenols and aromatic alcohols
2-methyl phenol phenylmethanol 1,4-dihydroxy benzene 2,4,6-trinitrophenol(o-cresol) (picric acid)
Aldehydes and Ketones
Nuclear substituted
Benzaldehyde Methyl phenyl ketone Diphenyl ketone(Acetophenone) (Benzophenone)
Carboxylic acids
Nuclear substituted
benzoic acid 2-methylbenzoic acid 2-hydroxybenzoic acid 1,4-benzenedicarboxylic acid(o-toluic acid) (o-salicylic acid) (terephthalic acid)
Acid derivatives
Benzoyl chloride Benzamide Phenyl benzoate
Benzoic anhydride Ethyl-4-bromobenzoate
Alkoxy derivatives
Methoxy benzene 1-methoxy- phenoxybenzene4-nitrobenzene
Amines
Aminobenzene 2-Amino toluene Benzyl amine
Nitro derivatives
nitrobenzene 1,3-dinitrobenzene 2,4,6-trinitrotoluene(m-nitrobenzene)
Nitriles and Carbylamines
Benzonitrile Phenylcarbylamine(phenyl isocyanide) (phenyl cyanide)
Sulphonic acids
benzenesulphonic acid benzene-1,3-disulphonic acid
Vriti Edustore
