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Aromatic Hydrocarbons

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Aromatic Hydrocarbons - Lesson Summary

Aromatic Hydrocarbons are so named as they possess a pleasant smell. Hydrocarbons containing conjugated cyclic rings are called aromatic hydrocarbons.
These aromatic hydrocarbons are also called arenes.
Aromatic hydrocarbons can be further classified as Benzenoids and non-Benzenoids on the basis of the presence or absence of benzene ring.
Aromatic hydrocarbons containing isolated or one or more fused benzene rings are known as Benzenoids aromatic Hydrocarbons. Benzene is the parent compound in Benzenoids. It is an aromatic hydrocarbon with six carbon hydrogen bonds arranged in a hexagon.
Ex: Benzene, Naphthalene, Anthracene…etc

Certain other aromatic hydrocarbons such as azulene and tropolone which do not contain benzene ring are known as non-Benzenoids.

All six hydrogen atoms in benzene are equivalent. Therefore replacing a hydrogen atom with another alkyl group yields only one type of mono-substituted product. Thus, the mono-substituted products of benzene do not show isomerism.
When two hydrogen atoms of a benzene ring are replaced with two similar or different alkyl groups, there is a possibility of formation of three isomers. These differ in the position of the alkyl group. Thus, di-substituted product of benzene shows position isomerism.
The three position isomers of a  di-substituted benzene  are ortho (1,2 or 1,6), meta (1,3 or 1,5) and para (1,4).
Thus, the three position isomers of dimethylbenzene or xylene are:
  •  1, 2-Di methyl benzene or ortho-xylene,
  •  1, 3-Di methyl benzene or meta-xylene and
  •  1, 4-Di methyl benzene or para-xylene


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