Benzene, bond energy terms resonance

The postulated transition state of the tt—a bond conversion occurs when the plane of the rotating benzene molecule is approximately at 45° to the catalyst surface. The electronic hybridization changes involved in this process (Fig. 4) are similar to those proposed by Melander 22) for homogeneous substitution reactions and show how resonance interactions may achieve a lowering in the activation energy. This aspect has previously been discussed in terms of the Lennard-Jones theory where the activation energy is lowered more effectively by resonance effects... 

The resonance energies, or perhaps better, stabilization energies, are really not wholly satisfactory for the bond lengths in cyclohexene and benzene are not the same. There is an additional term, the "compression energy" required to make the bond lengths the same. 

In the hands of Pauling this treatment has been successful in correlating many facts in terms of a number of possible structural i ormulae, and it has been widely developed by Wheland and many others. Sometimes the valence bond structures betv een which resonance is considered to take place are natural and plausible, such as the two Kekule forms of benzene on other occasions it is tiecessary to postulate less plausible structures of liigh energy such as the ionic forms of carbon dioxide see Long i ). The... 

The weak reactivity of benzene is in part attributed to this additional stabiliza- j tion. That stabilization energy is often called the delocalization energy or reso- nance energy of benzene and is taken as a measure of the aromaticity of the j molecule. The expression resonance energy comes from the description of] the delocalized tt system in valence-bond theory in terms of the resonant hybrid... 

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