Violations of the Hückel (4n + 2) Rule

In polycyclic conjugated hydrocarbons the Hückel (4n + 2)-rule may be violated, so that certain (4n + 2)-membcred rings cause thermodynamic destabilization. The structural conditions for the occurrence of such an effect are analyze d. Eight examples (of which seven are new) of non-b enzenoid hydrocarbons are put forward, in which the Hückel rule is disobeyed.