Applying the Criteria for Aromaticity

Cyclobutadiene has two pairs of tt electrons, and cyclooctatetraene has four pairs of TT electrons. These compounds, therefore, are not aromatic because they have an even number of pairs of tt electrons. There is an additional reason why cyclooctatetraene is not aromatic—it is not planar, it is tub-shaped (see page 335). Because cyclobutadiene and cyclooctatetraene are not aromatic, they do not have the unusual stabihty of aromatic compounds. 

Now let s look at some other compounds and determine whether they are aromatic. Cyclopentadiene is not aromatic because it does not have an unintermpted ring of p orbitalbearing atoms. One of its ring atoms is sp hybridized, and only sp and sp carbons have p orbitals. Therefore, cyclopentadiene does not fulfill the first criterion for aromaticity. 

The cyclopentadienyl cation is not aromatic either, because although it has an uninterrupted ring of p orbital-bearing atoms, its tt cloud has an even number (two) pairs of tt electrons. The cyclopentadienyl anion, on the other hand, is aromatic it has an uninterrupted ring of p orbital-bearing atoms, and the tt cloud contains an odd number (three) pairs of delocalized tt electrons. 

How do we know that the cyclopentadienyl anion s lone-pair electrons are tt electrons There is an easy way to determine this if a lone pair can be used to form a tt bond in the ring of a resonance contributor of the compound, then the lone-pair electrons are TT electrons. 

The resonance hybrid shows that all the carbons in the cyclopentadienyl anion are equivalent. Each carbon has exactly one-fifth of the negative charge associated with the anion. 

---