Ring Systems Description

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

The spectroscopic features of the heterocyclic ring systems may be inferred from the data noted in the correlation tables (Appendices 2, 3 and 4). Since the range of one- and multi-heteroatom systems is so diverse, a generalised discussion of these features would be too extensive for the scope of this book. Illustrative and descriptive accounts are included in appropriate preparative sections. 

The ring system, which has an axis of symmetry, consists of two interchangeable ortho, two interchangeable meta protons, and one para proton, all in the characteristic region for ring protons. The alert student, having absorbed the concept of chemical-shift equivalence, (Section 3.8.3) and the description of the... 

In the Aq case, 6 is not an important contributor to the ground state description of the properties of either 4 or 5. However, with 4 there are alternative modes of homoconjugation possible that involve the external cyclopropane bonds This is shown in Scheme 2 for the bicyclo[3.1.0]hexenyl cation. This alternative mode of conjugation of a cyclopropane in a Aq situation, an option not available to the parent Aq antiaromatic unsaturated ring systems, leads to a fundamentally different set of properties and reactions of these systems as compared to the potentially homoaromatic 4 + 2 cases. 

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