Benzene, 399. molecular orbitals

Structure and Stability of Benzene Molecular Orbital Theory 521... 

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. We can construct -tt molecular orbitals for benzene just as we did for 1,3-butadiene in Section 14.1. If six p atomic orbitals combine in a cyclic manner, six benzene molecular orbitals result, as shown in Figure 15.3. The three low-energy molecular orbitals, denoted bonding combinations, and the three high-energy orbitals are antibonding. 

Fig. 20. (a) One component of the bonding et metal-benzene molecular orbital set. Diagrams (b) and (c) illustrate the effect of tilting the p orbitals of the benzene and cyclopropenyl rings. 

Most organic chemists are familiar with two very different and conflicting descriptions of the 7r-electron system in benzene molecular orbital (MO) theory with delocalized orthogonal orbitals and valence bond (VB) theory with resonance between various canonical structures. An attitude fostered by many text books, especially at the undergraduate level, is that the VB description is much easier to understand and simpler to use, but that MO theory is in some sense more fundamental . 

Fig ure 15.10 Energy levels of the six benzene - molecular orbitals. There is a single, lowest-energy orbital, above which the orbitals come in degenerate pairs. 

With the aid of the atomic orbital expansions in Example 7.1 sketch the angular boundary surfaces for the benzene molecular orbitals. 

The rules for constructing the benzene molecular orbitals are straightforward, but require symmetry and orthogonality principles that have not been presented in this book. 

For benzene, molecular orbital theory shows that the six p-orbitals combine to give six molecular orbitals. The three lower-energy molecular orbitals are bonding molecular orbitals, and these are completely filled by the six electrons (which are spin-paired). There are no electrons in the (higher-energy) antibonding orbitals, and hence benzene has a closed bonding shell of delocalised Jt-electrons. 

Calculated benzene molecular orbital resulting from favorable overlap of p orbitals above and below plane of benzene ring... 

---