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Aromatic Compounds (Overview)

Annulenes

Annulenes
Annulenes are monocyclic compounds containing alternating ring double bonds, such as benzene, but of different sizes. According to systematic nomenclature, benzene is a [6]annulene, while cyclobutadiene is [4]annulene, whereas cyclooctatetraene is an [8]annulene, for instance.
Fig.1
[4]Annulene.
Fig.2
[6]Annulene.
Fig.3
[8]Annulene.

Though [10]annulene possesses 4n+2 π electrons, it is not aromatic because the ring is not plane, due to bonding angle strains. Nevertheless, annulenes of large ring sizes may also be aromatic. [18]Annulene, for instance, is aromatic. Its carbon-carbon bond lengths are of nearly the same size and it posseses 4n+2 π electrons (with n = 4).

Fig.4
[18]Annulene.

Exercise

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