What makes a structure aromatic?

Valence bond theory uses Lewis diagrams to depict structure and bonding of covalent entities, such as molecules and polyatomic ions, henceforth referred to just as 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 π-electron system, or loop of π electrons, provided each atom in the ring is sp²– or sp-hybridized.  Note that in cases like 5 that contain sp3 atoms with lone pairs, these atoms (usually N, O or S) can rehybridize to sp² and make the remaining p-orbital available for bonding.  A fully bonded sp³ carbon cannot do this, and thus cannot form part of a fully conjugated system.

• cyclobutadiene (1)

• benzene (2)

• cyclooctatetraene (3)

• pyridine (4)

• pyrrole (5)

• cyclopentadienyl cation (6)

• cyclopentadienyl anion (7)

As evident from the stability of π electrons however, only some such rings actually have a loop of π electrons. Of the above examples, only the rings in 2, 4, 5, and 7 have a loop of π electrons. Resonance theory can predict that the rings in 2, 4, 5, and 7 have a loop of π

electrons.

According to resonance theory, however, the rings in 1, 3, and 6 also should have a loop of π

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 π electrons. Rings whose Lewis diagram implies a loop of π 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.

Flowchart for determining aromatic behavior in cyclic molecules.