Orbital Model for Benzene

Kekules model for the structure of benzene is nearly, but not entirely, correct. Kekules two structures for benzene differ only in the arrangement of the electrons all of the atoms occupy the same positions in both structures. This is precisely the requirement for resonance (review Sec. 1.12). Kekule s formulas represent two identical contributing structures to a single resonance hybrid structure of benzene. Instead of writing an equilibrium symbol between them, as Kekul did, we now write the double-headed arrow ( - ) used to indicate a resonance hybrid  

Benzene is a resonance hybrid of these two contributing structures. 

To express this model another way, all benzene molecules are identical, and their structure is not adequately represented by either of Kekule s contributing structures. Being a resonance hybrid, benzene is more stable than either of its contributing Kekule structures. There are no single or double bonds in benzene—only one type of carbon-carbon bond, which is of some intermediate type. Consequently, it is not surprising that benzene does not react chemically exactly like alkenes. 

Ball-and-stick and space-filling models of benzene. 

PROBLEM 4.4 How does the resonance model for benzene explain the fact that there are only three isomers of dibromobenzene  

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