Many compounds obtained from natural sources like resins, balsams, oil of wintergreen etc. Possessing pleasant fragrance (aroma) were named as Aromatic compounds. In greek the word aroma means pleasant smell. Benzene is one such important aromatic compound.
Benzene and compounds that resemble benzene in their chemical behaviour are nowday called as aromatic compound. Many compounds that resemble benzene do not have pleasant odour e.g. anthracene.
Many compounds that have pleasant odour do not resemble benzene e.g. methyl acetate, chloroform, ethyl acetate etc. Therefore, early relation of aromatic compounds with odour no longer exists.
Benzene (C6H6) :
Benzene is parent compound of most of the Aromatic compounds. It is a colourless liquid having a characteristic odour, boiling point 353K. coaltar is an important source of benzene. Benzene is also present in petroleum. Benzene was synthesized by Berthelot (1870) from acetylene. Benzene was originally called phene and hence C6H5 was called phenyl group. Aromatic hydrocarbons are also called arenes. Aryl group is represented as -Ar.
* Characteristic of Aromatic compounds
All Aromatic compounds exhibit following three important characteristics.
i) Higher percentage of carbon
These compounds contain Higher percentage of carbon content than corresponding aliphatic hydrocarbon. As a result, these compounds burn with sooty flame.
ii) Thermal stability
All Aromatic compounds are exceptionally stable as compared with alkanes. Hence on combustion, aromatic compounds liberate comparatively lesa amount of heat.
iii) Chemical behaviour
a) Despite the presence of unsaturation, aromatic compounds do not undergo addition reaction similar to alkenes and alkynes under normal condition. For example, benzene does not decolouries alkaline potassium permanganate.
b) Despite the presence of a number of double bonds, Aromatic compounds undergo substitution reactions, which are characteristic of aromatic compounds. For example, benzene undergo halogenation like alkanes.
Types of aromatic compounds : Aromatic compounds are of two types.
i) Benzenoid : these compounds contain one or more benzene rings in their structure. e.g. benzene, benzaldehyde, benzoid acid, naphthalene, anthracene etc.
ii) Non - Benzenoid : these compounds do not contain any benzene rings in their structure e.g. furan, thiophene, pyridine etc.
