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Aromatic Compounds (Overview)

Aromaticity: Hückel 4n + 2 Rule

The high stability and the characteristic chemical behavior of benzene are called aromatic character or aromaticity, respectively. However, benzene is not the only known aromatic compound; there are many other compounds that also have a considerably high stability and show a similar chemical behavior such as benzene does. The typical qualities of benzene and other aromatic compounds are listed below:

  1. They are cyclic compounds that contain a cyclic, uninterrupted π electron system. That is, they possess cyclic-conjugated double bonds. Each ring atom has a p orbital that is perpendicular to the ring plane.
  2. Aromatic rings are planar. Otherwise, an optimal overlapping of the ring atoms' p orbitals, which form the π electron cloud, is impossible. Optimal overlapping is a prerequisite for the high stability and the characteristic chemical behavior of aromatic compounds.
  3. The number of π electrons which establish the cyclic, uninterrupted π electron cloud amounts to 4n + 2, with n = 0, 1, 2, 3, ... (Hückel's rule).

Cyclobutadiene, for instance, meets the first and the second criteria. However, it does not possess 4n + 2 but 4 π electrons. Therefore it is not aromatic. Compounds that meet the first and second criteria but do not meet Hückel's rule are called antiaromatic. They are much less stable and display a chemical behavior that is in contrast to that of aromatic compounds.

Compounds that meet Hückel's rule but do not fulfill one of the other prerequisites for aromaticity are called non-aromatic. Not only are they less stable than aromatic compounds, but their chemical behavior is quite dissimilar, as well.

Tab.1
Antiaromatic and non-aromatic compounds.
Cyclobutadiene1,3,5-Cycloheptatriene1,3,5,7-Cyclooctatetraene
Fig.1

Cyclobutadiene is an antiaromatic compound.

Fig.2

1,3,5-Cycloheptatriene is a non-aromatic compound.

Fig.3

1,3,5,7-Cyclooctatetraene is a non-aromatic compound.

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