Kekule polycyclic hydrocarbons

We decided to include this Biphenylenes Revisited section in this review article on aromaticity for two reasons (1) to draw attention to the inadequacy of the concept of parity of Kekule valence structures and the inconsistencies of the approaches based on the algebraic count of Kekule structures for discussion of conjugated polycyclic hydrocarbons, which is still practiced and (2) to raise awareness in theoretical circles of a need for reliable computations of the molecular energy and RE of non-benzenoid conjugated hydrocarbons. This is desirable not only so that we can obtain better sets of parameters needed for calculation of the resonance energies of non-benzenoid hydrocarbons but also so that we can get better insight into the roles of various Kekule valence structures in representation of molecules and find out to what extent the ideas of Clar can be extended to non-benzenoid systems. If we find out that indeed only Kekule valence structures of maximal degree of... 

Because some polycyclic hydrocarbons raise the of double-stranded DNA, it has been suggested that intercalation (Section 10.3b) is a step in carcinogenesis. However, X-ray diffraction shows that even highly carcinogenic hydrocarbons are not flat [see e.g. Iball (1964)] an anomaly partly due to the overcrowding of substituents, partly to the presence of non-Kekule rings, of which (12.41) has two. 

In general, the most stable resonance structure for a polycyclic aromatic hydrocarbon is the one with the greatest number of rings that correspond to Kekule formulations of benzene. Naphthalene provides a fairly typical exanple ... 

It is one of the standard results of theoretical organic chemistry that benzenoid hydrocarbons (and polycyclic conjugated molecules in general) have no unique classical structural formula. The conjugation models of their 7r-electrons must be visualized by means of several so-called Kekule structures [22, 25, 32, 33], none of which correctly describes the actual situation. An attempt to overcome this century-long problem was proposed by Clar in the 1950 s and eventually elaborated in due detail in his seminal book [18]. 

Mishra, R.K. and Patra, S.M. (1998). Numerical Determination of the Kekule Structure Count of Some Symmetrical Polycyclic Aromatic Hydrocarbons and their Relationship with n-Electron-ic Energy (A Computational Approach). J.Chem.Inf.Comput.Scl, 38,113-124. 

Balaban, A.T. and Tomescu, 1. (1984) Chemical graphs. XL. Three relations between the Fibonacci sequence and the numbers of Kekule structures for non-branched cata-condensed polycyclic aromatic hydrocarbons. Croat. Chem. Acta, 57, 391-404. 

Of polycyclic non-benzenoid hydrocarbons by far the most numerous and the most investigated series consists of the azulenes. The parent compound is azulene itself, bicyclo[5,3,0]decapentaene (VI). Like benzene it can be drawn as a polyene in two equivalent Kekule forms. It has a peripheral conjugated system involving 10 TT-electrons. Its structure is discussed on the following pages. 

Konishi A, Hirao Y, Nakano M, Shimizu A, Botek E, Champagne B, Shiomi D, Sato K, Takui T, Matsumoto K, Kurata H, Kubo T (2010) Synthesis and characterization of teranthene a singlet biradical polycyclic aromatic hydrocarbon having Kekule structures. J Am Chem Soc 132 11021-11023... 

Ohkami N, Motoyama A, Yamaguchi T, Hosoya H, Gutman I (1981) Graph—Theoretical Analysis of the Clar s Aromatic Sextet — Mathematical Pro rties of the Set of the Kekule Patterns and the Sextet Polynomial for Polycyclic Aromatic Hydrocarbons. Tetrahedron 37 1113... 

Kekule valence structures continue to offer an alternative basis for description of benzenoid and non-benzenoid conjugated hydrocarbons, which complements characterization of molecules by quantum chemistry, which tends almost exclusively to consider molecular orbital approaches as the frame for discussion of molecular properties. We have already seen applications of conjugated circuits (which are extracted from individual Kekule valence structures) for clarification of the notion of aromaticity in polycyclic conjugated hydrocarbons and characterization of the relative magnitudes of molecular resonance energies and will also later consider the use of conjugated... 

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