Valence bond theory uses Lewis diagrams to depict structure and bonding of covalent entities, such as molecules and polyatomic ions, henceforth molecules. The Lewis diagram of many a molecule, however, is not consistent with the observed properties of the molecule. The Lewis diagram of some molecules suggests a ring bearing a fully conjugated \(\pi\)-electron system, or loop of \(\pi\) electrons, provided each atom in the ring is sp2- or sp-hybridized.
- cyclobutadiene (1)
- benzene (2)
- cyclooctatetraene (3)
- pyridine (4)
- pyrrole (5)
- cyclopentadienyl cation (6)
- cyclopentadienyl anion (7)
As evident from the stability of \(\pi\) electrons however, only some such rings actually have a loop of \(\pi\) electrons. Of the above examples, only the rings in 2, 4, 5, and 7 have a loop of \(\pi\) electrons. Resonance theory can predict that the rings in 2, 4, 5, and 7 have a loop of \(\pi\) electrons.
According to resonance theory, however, the rings in 1, 3, and 6 also should have a loop of \(\pi\) electrons.
Evidently, resonance theory is no more reliable a tool than Lewis diagrams to be used in predicting if a ring has a loop of \(\pi\) electrons. Rings whose Lewis diagram implies a loop of \(\pi\) electrons can be classified into three classes:
- aromatic rings
- antiaromatic rings
- nonaromatic rings
To find whether a ring is aromatic, antiaromatic, nonaromatic, or none of the above, use the following flow chart.
Figure 1.1.1: Flowchart for determining aromatic behavior in cyclic molecules.