A Resonance Picture of Benzene

Other alkenes and required severe conditions. Benzene was clearly a remarkably stable compound, and the cyclohexatriene structure couldn t account for this. 

The structure expected for 1,3,5-cyclohexatriene (bond length differences exaggerated) 

5-cyclohexatriene structure must contain alternating long single bonds and short double bonds (shown here with exaggerated short and long bonds). Such is not the case for benzene, which is a regular hexagon with a uniform carbon—carbon bond distance of 1.39 A. 

PROBLEM 13.2 One early problem with the Kekule stmcture was that only one 1,2-disubstituted isomer of benzene could he found for any given substituent. That is, there is only one 1,2-dimethylbenzene (o-xylene). Eixplain why this observation would have been hard for Kekule to explain. 

Perhaps the remarkable stability of benzene can be explained just by realizing that delocalization of electrons is strongly stabilizing. There are really two Kekule forms for benzene. They are resonance forms, differing from each other only in the distribution of electrons but not in the positions of atoms. An orbital picture for cyclohexatriene should have influenced us in this direction. Each carbon is hybridized and there is, therefore, a 2p orbital on every carbon. The stmcture in Rgure 13.8a 

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