An Orbital Hybridization View of Bonding in Benzene

Heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, a hypothetical 1,3,5-cyclohexatriene, and benzene. All heats of hydrogenation are in kilojoules per mole. 

The precise value of the resonance energy of benzene depends, as comparisons with 1,3,5-cyclohexatriene and (Z)-l,3,5-hexatriene illustrate, on the compound chosen as the reference. What is important is that the resonance energy of benzene is quite large, six to ten times that of a conjugated triene. It is this very large increment of resonance energy that places benzene and related compounds in a separate category that we call aromatic. 

The heats of hydrogenation of cycloheptene and 1,3,5-cycloheptatriene are 110 kJ/mol (26.3 kcal/mol) and 305 kJ/nnol (73.0 kcal/nnol), respectively. In both cases cycloheptane is the product. What is the resonance energy of 1,3,5-cycloheptatriene How does it compare with the resonance energy of benzene  

Higher level molecular orbital theory can provide quantitative information about orbital energies and how strongly a molecule holds its electrons. When one compares aromatic and nonaromatic species in this way, it is found that cyclic delocalization causes the TT electrons of benzene to be more strongly bound (more stable) than they would be if restricted to a system with alternating single and double bonds. 

We ll come back to the molecular orbital description of benzene later in this chapter (Section 11.19) to see how other conjugated polyenes compare with benzene. 

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