Orbital energies for conjugated ring systems of 3-9 carbon atoms

The pattern of two half-filled degenerate levels for An systems persists in larger rings. In contrast, 4 +2 systems are all predicted to have all electrons paired in bonding MO s. This provides the theoretical basis of the Hiickel rule. 

A modification of the method for calculation of the energy of the hypothetical localized molecules results in a better correlation between the delocalization energy calculated by HMO theory and experimentally observed stability. A series of 

At present, then, aromaticity is best defined in terms of stability derived from the delocalization of bonding electrons. An aromatic molecule is characterized by appreciable stabilization relative to a noncyclic polyene. An antiaromatic molecule is one that is destabilized relative to a polyene model, and the term nonaromatic can be applied to molecules for which the calculated energy and energy of the polyene model are comparable. Cyclobutadiene, with an estimated destabilization energy of 15-20 kcal/mol, is a good example of an antiaromatic species. 

The term annulene has been coined to refer to the completely conjugated monocyclic polyenes. The synthesis of annulenes has now been extended well beyond the first two members of the series, [4]annulene (cyclobutadiene) and [6]annulene (benzene). The generality of the Hiickel rule can be tested by considering the properties of the members of the annulene series. 

The smallest member, cyclobutadiene, has been the objective of synthetic efforts for many years, but success was achieved only relatively recently. It was first detected by trapping with various reagents after release from an iron complex  

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Fig. 9.1. Orbital energies for conjugated ring systems of 3-9 carbon atoms.

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