Aromaticity of Annulenes | Aromatic, Non-aromatic, Antiaromatic

Annulenes are monocyclic conjugated polyenes. [6] annulene, [14] annulene, [18] annulene, [22] annulene are aromatic, [8] annulene, [10] annulene, [16] annulene are antiaromatic and [4] annulene, [12] annulene, [20] annulene, [24] annulene are antiaromatic. The term aromaticity is originated from aroma. The literal meaning of aroma is fragrance or pleasant smell. The term aromatic was first used in chemistry by Hofmann. In late 18th and the beginning of 19th century, the aromaticity is used for the unsaturated compounds with distinct smell. Some compounds such as terpenes have pleasant smell , but these were not considered as aromatic compounds. At present the unsaturated highly stable compounds are named as aromatic compounds not based on smell but based on their electronic, structural, chemical and magnetic behavior. The aromatic compounds are classified into homo cyclic and heterocyclic. The homo cyclic are classified into benzenoid and non-benzenoid. Benzenoid aromatic compounds are classified into monocyclic and polycyclic aromatic hydrocarbons. The resonance energy of benzenoid compounds decreases as the number of benzene rings increases. Charged aromatic compounds: cyclopropenyl cation, cyclopentadienyl anion and tropylium cation Introduction of Aromaticity    • Aromaticity: Aromatic, Non-aromatic, Antia...   Benzenoid aromatic compounds    • Benzenoid Aromatic compounds: Aromatic, No...   Charged aromatic compounds