Clar Aromatic Sextet Revisited

The famous Hiickel 4n + 2 rule [19], which explained the unusual stability of mono-cyclic systems, is widely and well known. In contrast, Clar s rule on the great stability of benzenoid systems having 6n % electrons, which is much broader as it applies to polycyclic conjugate systems, appears not to be so widely known. According to 

The fully benzenoid hydrocarbons have 6n % electrons, n being an integer. It becomes obvious that the most stable hydrocarbons do not follow HUckeTs rule (2 + 4w) x electrons [19]. Therefore the latter must be strictly limited to monocyclic systems. 

FIG U RE 11.4 Clar formulas for smaller benzenoid hydrocarbons having migrating sextets. 

Fully benzenoid hydrocarbons are those for which Clar s formulas contain only sextet rings and anpty rings and no CC double bonds. The smallest fully benzenoid hydrocarbon is triphenylene. 

It is unclear how such a profound anpirical prediction, which characterizes the most stable benzenoid systems, could have been overlooked or ignored for such a long time by so many theoretical chanists. Clar s approach to novel representation of benzenoid systems may have appeared to some as a phantasm, but Clar, as we have already mentioned, has reported on a number of experimental results that support his 

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Randic, M. Clar s Ji-Aromatic Sextet Revisited. In Valence Bond Theory, Cooper, D. L., Ed. Elsevier Amsterdam, 2002 Chapter 16. pp 503-533. 

Suresh CH, Gadre SR (1999) Clar s aromatic sextet theory revisited via molecular electrostatic potential topography. J Org Chem 64 2505-2512... 

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