Aromaticity: Aromatic or not?
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:
- 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.
- 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.
- 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.