Aromatic Hydrocarbons Introduction, Isomerism and Preparation

Aromatic Hydrocarbons

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.

Substituted Benzene

When a hydrogen atom of benzene is replaced by any other atom, then benzene becomes a substituted benzene.

Substituted benzene examples

aromatic hydrocarbons : substituted benzene
aromatic hydrocarbons : substituted benzene

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)

ortho isomer of Xylene
meta isomer of Xylene
para isomer of 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 :

Preparation of Benzene by polymerisation

Decarboxylation of Aromatic Acids : Sodium salt of benzoic acid on heating with sodalime gives benzene.

Preparation of benzene by decarboxylation

Reduction of Phenol : The vapours of phenols on passing over heated zinc dust are reduced to benzene.

Preparation of benzene by reduction of phenol