20.2 Annulenes
Annulenes are cyclic hydrocarbons with alternating double and single bonds throughout its structure. Cyclobutadiene, benzene, and cyclooctatetraene are all annulenes.
Cyclobutadiene is very unstable. Benzene is very stable. Cyclooctatetraene has the stability of a normal annulene. All three of these annulenes have other good resonance forms. We clearly see that the unusual stability of benzene does not come from its ability to have another resonance form.
We need another theory beyond resonance forms to explain the unusual stability of benzene. We must use molecular orbital theory to explain it.