Abstract:
Benzene has been regarded as the parent hydrocarbon of the aromatic series since the beginning of systematic organic chemistry. It has long been a matter of interest whether other cyclic conjugated polyolefinic systems would show the characteristic aromatic properties of nucleus stability and diminished unsaturation. Since Kekulé's original formulation of benzene in 1865, the concept of the aromatic state has undergone considerable extension. Modern advances have chiefly been along the theoretical interpretation of the fine structure of aromatic compounds in terms of the electronic theory, and the discovery and elucidation of new aromatic systems.
A molecule may be said to be aromatic if it is a cyclic system containing conjugated double bonds in the ring when represented by the conventional symbols, and in which these bonds interact to a greater or less extent, thus bringing about a certain stabilisation of the molecule by resonance, which will in consequence be more saturated than if the double bonds were fixed and purely olefinic in character. Benzenoid aromatic compounds contain one or more six-membered rings with the equivalent of three double bonds, as in benzene, naphthalene and pyridine. Aromatic compounds not of the benzene type may be described as non-benzenoid (Jackson “Organic Reactions”, Vol.II, pp 341). Heterocyclic aromatic compounds such as furan, thiophene, and pyrrole are well-known. Other non-benzenoid aromatic systems are possible and have in recent years been discovered and studied. Some of the more interesting and important ones are discussed briefly in the following sections.
