Delocalized Electrons An Explanation Based on Molecular Orbital Theory

11-6 Delocalized Electrons An Explanation Based on Molecular Orbital Theory 

In Section 11, we discussed localized tt bonds, such as those in ethylene, C2H4. Some molecules, such as benzene (C6H6) and substances related to it— aromatic compounds—have a network of rr bonds that extends over several nuclei. In this section, we will combine ideas from the bonding theories we have studied thus far to consider bonding in benzene and a few ofher polyatomic molecules. We will describe the bonding in these molecules by using valence bond theory for the tr bonds and molecular orbital theory for the rr bonds. 

In general, the application of molecular orbital theory to polyatomic molecules is rather complicated. This is because the molecular orbitals (more precisely, the combinations of atomic orbifals contributing to the various molecular orbitals) are not easily deduced. Thus, we will not attempt to deduce them. Instead, we will simply present the molecular orbitals as needed and use them to help us understand the bonding in each case. 

A The term aromatic relates to the fragrant aromas associated with some (but by no means all) of these compounds. 

A Dame Kathleen Lonsdale first determined the X-ray crystal structure of benzene. Her experiment demonstrated that the benzene molecule is flat, as predicted by theorists. 

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Delocalized Electrons An Explanation Based on Molecular Orbital Theory—Bonding in the benzene molecule, C H, is partly based on the concept of delocalized molecular orbitals. These are regions of high electron density that extend over several atoms in a molecule (Fig. 11-31). Delocalized molecular orbitals also provide an alternative to the concept of resonance in other molecules and ions. 

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