15.4 • Aromatic Ions
According to the Hückel criteria for aromaticity, a molecule must be cyclic, conjugated (nearly planar with a p orbital on each atom), and have 4n + 2 π electrons. Nothing in this definition says that the number of π electrons must be the same as the number of atoms in the ring or that the substance must be neutral. In fact, the numbers can differ and the substance can be an ion. Thus, both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic even though both are ions and neither contains a six-membered ring.
To understand why the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic, imagine starting from the related neutral hydrocarbons, 1,3-cyclopentadiene and 1,3,5-cycloheptatriene, and removing one hydrogen from the saturated CH2 carbon in each. If that carbon then rehybridizes from sp3 to sp2, the resultant products would be fully conjugated, with a p orbital on every carbon. There are three ways in which the hydrogen might be removed.
- The hydrogen can be removed with both electrons (H:–) from the C–H bond, leaving a carbocation as product.
- The hydrogen can be removed with one electron (H·) from the C–H bond, leaving a carbon radical as product.
- The hydrogen can be removed with no electrons (H+) from the C–H bond, leaving a carbanion as product.
All the potential products formed by removing a hydrogen from 1,3-cyclopentadiene and from 1,3,5-cycloheptatriene can be drawn with numerous resonance structures, but Hückel’s rule predicts that only the six-π-electron cyclopentadienyl anion and cycloheptatrienyl cation can be aromatic. The other products are predicted by the 4n + 2 rule to be unstable and antiaromatic (Figure 15.7).
In practice, both the four-π-electron cyclopentadienyl cation and the five-π-electron cyclopentadienyl radical are highly reactive and difficult to prepare. Neither shows any sign of the stability expected for an aromatic system. The six-π-electron cyclopentadienyl anion, by contrast, is easily prepared and remarkably stable (Figure 15.8a). In fact, the anion is so stable and easily formed that 1,3-cyclopentadiene is one of the most acidic hydrocarbons known, with pKa = 16, a value comparable to that of water!
In the same way, the seven-π-electron cycloheptatrienyl radical and eight-π-electron anion are reactive and difficult to prepare, while the six-π-electron cycloheptatrienyl cation is extraordinarily stable (Figure 15.8b). In fact, the cycloheptatrienyl cation was first prepared more than a century ago by reaction of 1,3,5-cycloheptatriene with Br2, although its structure was not recognized at the time.