We have already studied in hydrocarbon introduction that aromatic hydrocarbons are a special type of hydrocarbons. Most of the aromatic hydrocarbons contain benzene ring. Benzene is a highly unsaturated hydrocarbon but in a majority of reactions, it does not lose its unsaturation.
When a hydrogen atom of benzene is replaced by any other atom, then benzene becomes a substituted benzene.
Substituted benzene examples
Based upon the number of substituents the benzene is known as monosubstituted (for one), disubstituted (for two), trisubstituted (for three) etc.
In a disubstituted benzene, three different position isomers are possible depending upon the position of one substituent with respect to the other. Ortho (o−) is used to indicate that the relative position of the two substituents is 1,2−. Similarly, meta (m−) and para (p−) are used to indicate the relative positions 1,3− and 1,4− respectively.
ortho, meta and para isomers of dimethylbenzene (xylene)
You may note that both the substituents may be same, as in the above examples, or different.
Benzene is a very stable compound because it exhibits resonance.
Cyclic Polymerisation of Ethyne : We have already discussed in reactions of alkynes that benzene is prepared by cyclic polymerisation of ethyne. The reaction is given below :
Decarboxylation of Aromatic Acids : Sodium salt of benzoic acid on heating with sodalime gives benzene.
Reduction of Phenol : The vapours of phenols on passing over heated zinc dust are reduced to benzene.