An Extension of the Hückel 4N + 2 Rule to Polycyclic Non-alternant Conjugated Hydrocarbons

A general rule of aromatic stability of non-alternant polycyclic conjugated hydrocarbons has been detailed and using aromatic stability indices, corroborated. The π-electron system is said to be relatively more stable if the difference between number of 4N + 3-type rings minus number of 4N + 1-type rings is 0 or divisible by 4. If not, the system is assumed to be relatively unstable. The presence of benzene rings does not affect the rule. A further extension for intramolecular H-bonded systems is also suggested. The rule suggested permits prediction of substituent effects on stability as well as the ionic form being most stable if the rule is not fulfilled.