Aromaticity: Aromatic or not?
In order to be aromatic, a molecule must be cyclic and planar and must also have an uninterrupted cyclic π electron cloud. In addition, it must fulfill Hückel's rule (4n + 2 π electrons). However, the number of p orbitals that participate in the π system do not need to be equal to the number of π electrons. Thus, anions, cations, and radicals may be aromatic too and Hückel's rule may also be applied to them accordingly.
Due to the -hybridized ring carbon, cyclopentadiene (= 16) is not aromatic, as it does not possess an uninterrupted cyclic π electron cloud.
- If a hydride anion () is removed from the -hybridized ring carbon, the cyclopentadienyl cation is formed, and the hybridization of the -hybridized ring carbon is altered to . Consequently, the cyclic cyclopentadienyl cation is planar and it possesses a cyclic uninterrupted π electron cloud. However, it does not meet Hückel's rule, as it has 4 π electrons. Therefore, it is antiaromatic.
- Abstraction of a hydrogen atom from the -hybridized ring carbon yields the cyclopentadienyl radical. The hybridization of the -hybridized ring carbon is transformed to , as well. Analogous to the cyclopentadienyl cation, the cyclopentadienyl radical does not meet Hückel's rule, as it possesses five π electrons. Thus, it is not aromatic.
- The abstraction of a proton () from the -hybridized ring carbon yields the cyclopentadienyl anion. The hybridization of the -hybridized ring carbon is consequently altered to , as well. The cyclic cyclopentadienyl anion is planar, it possesses a cyclic uninterrupted π electron cloud, and it meets Hückel's rule, as it has 4*1 + 2 (n = 1) π electrons. Therefore, the cyclopentadienyl anion is a relatively stable aromatic species. As a result, the acidity of cyclopentadiene may be compared to that of water (= 16). In contrast to the cyclopentadienyl cation and the radical, it can easily be obtained through treatment with a base. The cyclopentadienyl cation and radical are considerably instable and reactive species.