Aromatic Hydrocarbons Introduction, Isomerism and Preparation
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.
Isomerism in Aromatic Hydrocarbons
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.
Stability of Benzene
Benzene is a very stable compound because it exhibits resonance.
Preparation of Benzene
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.