Aromaticity in Heterocyclic Compound

Aromaticity is a manifestation of electron delocalization in closed circuits, either in two or in three dimensions. This results in energy lowering, often quite substantial, and a variety of unusual chemical and physical properties. These include a tendency toward bond length equalization, unusual reactivity, and characteristic spectroscopic features. Since aromaticity is related to induced ring currents, magnetic properties are particularly important for its detection and evaluation.Aromaticity is a notion that appeared in the mid-nineteenth century to differentiate between unsaturated hydrocarbons and formally unsaturated benzene .At the
end of the nineteenth century it seemed that cyclicity was a necessary condition for differentiation between the two, but at the beginning of the twentieth century it turned out that the above assumption was not correct because cyclooctatetraene exhibited typical properties known for polyenes . The essential property of benzene- like compounds, often identified with aromatic compounds, was low reactivity. Hence thermodynamic stability was defined as resonance energy and was the first quantitative measure of aromaticity.

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