ABSTRACT
Although Kekulé was interested in the structural origin of aromaticity, Erlenmayer, who a year later proposed Kekulé valence structures for naphthalene, advocated that the characterization of aromaticity be based on properties of compounds. However, such a point of view has inherent difculties when the approach is extended to molecules that show only partial aromatic properties. The structural approach to aromaticity, which was advocated by Kekulé, on one hand was crowned with the success of the Hückel’s 4n + 2 rule in resolving the fundamental difference between benzene and cyclooctatetraene. At the same time, the structural approach to aromaticity was frustrated by a lack of progress in generalizing this simple and elegant solution for monocyclic systems from monocyclic systems to polycyclic conjugated hydrocarbons. It is not that the problem did not attract the attention of a number of theoreticians, as it did, but the generalization appeared elusive for so long a time that, eventually, theoreticians, around the 1960s, apparently gave up in searching for a potentially nonexistant solution-a solution that was, however, found to be good in the mid-1970s but remained unrecognized by most chemists until the present time.
