Hückel's rule
Hückel's rule is not valid for many compounds containing more than one ring. For example, pyrene and trans-bicalicene contain 16 conjugated electrons (8 bonds), and coronene contains 24 conjugated electrons (12 bonds). Both of these polycyclic molecules are aromatic, even though they fail the 4"n" + 2 rule. Indeed, Hückel's rule can only be theoretically justified for monocyclic systems.