Notice that the layout of the energy levels in benzene is a regular hexagon with its apex point ing downwards. It turns out that the energy level diagram for the molecular orbitals resulting from the combination of any regular cyclic arrangement of p orbitals can be deduced from the appropriately sided polygon with an apex pointing downwards. The horizontal diameter (the red line) represents the energy of a carbon p orbital and any energy levels on this line repre sent non-bonding molecular orbitals. All molecular orbitals with energies below this line are bonding; all those above are antibonding.

It’s worth noting a few points about these energy level diagrams: n = 8

• The method predicts the energy levels for the molecular orbitals of planar, cyclic arrangements of identical atoms (usually all C) only.

• There is always one single molecular orbital lower in energy than all the others. This is because there is always one molecular orbital where all the p orbitals combine in phase.

• If there is an even number of atoms, there is also a single molecular orbital highest in energy; otherwise, there will be a pair of degenerate molecular orbitals highest in energy.

• All the molecular orbitals come in degenerate pairs except the one lowest in energy and (for even-numbered systems) the one highest in energy.

مواضيع ذات صلة

Organic compounds are more soluble in water as ions

Heterocyclic aromatic compounds

Hückel’s rule tells us if compounds are aromatic

Molecular orbitals and aromaticity

Benzene has six π molecular orbitals

Varying the number of electrons

Heats of hydrogenation of benzene and cyclooctatetraene

? What makes benzene special

Aromaticity

Conjugation and reactivity: looking forward to Chapter 10

The amide group

Delocalization over three atoms is a common structural feature The carboxylate anion