Aromatic Compounds (Overview)
Polycyclic Aromatic Compounds
Strictly speaking, the Hückel rule can only be applied to monocyclic compounds though the general concept may also be transferred to polycyclic molecules, such as polycyclic aromatic compounds. These compounds are actually considerably relevant. Benzo[e]pyrene, for instance, which is found in tobacco smoke, is suspected to be carcinogenic.
The formal condensation of a third benzene ring to naphthalene yields anthracene and phenanthrene, respectively. Anthracene and phenanthrene are isomers. However, anthracene is about 25 kJ/mol more stable than phenanthrene.