Ever since Kekultf's intuitive assignment of the hexagon structure to
benzene, organic chemists have been captivated by the idea that there
might exist other cyclically conjugated hydrocarbons besides bemcne
exhibiting aromatic properties. Efforts to clarify this question
experimentally have remained inclusive until fairly recently due to the
lack of proper synthetic methods. It was thus left to the molecular
orbital theory of unsaturated systems developed by E. Hückel to give an
answer. According to this theory, aromaticity is not confined to the 6
JI-electron system, but is gener- ally to be expected for cyclically
conjugated systems with (411 -1 2) JI- electrons, provided that they
possess planar or near-planar ring skeletons. Hückel's predictions have
inspired over the last twenty years numcrous experimental
investigations, which in brilliant verification of the theory have led
to the discovery of a host of nonbenzenoid aromatic compounds. A few
years ago a consideration of molccular models made us conclude that the
classical aromatic hydrocarbons naphthalcne, anthracene, naphtha- ce ne
etc. may be transformed into approximately planar (4 n] 2) JI-clcctron
systems by the basically simple device of bridging. As will be
delineated in this lecture, an entirely new class of aromatic
compounds - tcrmcd "Hückel-Aromatics" in honour of Iiiickel-- has
emergcd from this conccpt.