# Aromatic

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.

• benzene (2)

• cyclooctatetraene (3)

• pyridine (4)

• pyrrole (5)

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:

1. aromatic rings
2. antiaromatic rings
3. 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.