Benzene, bond energy terms

In Fig. 9.1, orbitals below the dashed reference line are bonding orbitals when they are filled, the molecule is stabilized. The orbitals that fall on the reference line are nonbonding placing electrons in these orbitals has no effect on the total bonding energy of the molecule. The orbitals above the reference line are antibonding the presence of electrons in these orbitals destabilizes the molecule. The dramatic difference in properties of cyclobutadiene (extremely unstable) and benzene (very stable) is explicable in terms of... 

The order of reactivity of different alkyl-substituted benzenes is toluene > ethylbenzene > isopropylbenzene, which is opposite to the general reactivity expected from the C—H bond energies. This was explained in terms of an initiating electron transfer equilibrium between Co(OAc)3 and the arene as the rate-determining... 

In the previous section, we met three situations where the localized bond model fails. In each case, we had to represent bonding in terms of MOs covering three or more atoms, formed by overlapping of AOs on them. Now the general rules for formation of such many-center MOs are basically the same as for normal two-center bonds. The AOs involved must be of comparable energies (i.e., all from valence shell AOs of the participating atoms) and they must overlap in space. How they overlap is not important. There is no basic distinction between the 7r-type overlap of p AOs in benzene (Fig. 1.37), the linear a-type overlap in the transition state of Fig. 1.39, and the nonlinear a-type overlap of AOs in diborane (Fig. 1.36). All three situations are equivalent in terms of MO theory. The distinction between them arises solely from quantitative considerations of the efficiency of overlap and the energies of the orbitals involved. 

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