Many other compounds also exhibit aromatic characteristics. Some of the most common have two or more benzene rings fused together. Such compounds are called polycyclic benzenoid aromatic compounds. A typical example of this type of molecule is naphthalene, C 10H 8. Structurally, naphthalene looks like this:
This structure is a hybrid of the following four resonance structures.
The resonance energy associated with naphthalene is 61 kcal/mole. Thus naphthalene is more stable than benzene. The benzenoid or benzene‐like ring should have 36 kcal per mole of resonance energy, and the nonbenzenoid ring should have 25 kcal per mole of resonance energy. Because there is total delocalization of the electrons in the system, the benzenoid and nonbenzenoid rings cannot be identified at any given time.
Other benzenoid structures include anthracene, phenanthrene, and pyrene.