Illustrated Glossary of Organic Chemistry

Huckel's Rule (4n+2 rule): In order to be aromatic, a molecule must have a certain number of pi electrons (electrons with pi bonds, or lone pairs within p orbitals) within a closed loop of parallel, adjacent p orbitals. The pi electron count is defined by the series of numbers generated from 4n+2 where n = zero or any positive integer (i..e, n = 0, 1, 2, etc.). The most common case in six pi electrons (n = 1) which is found for example in benzene, pyrrole, furan, and pyridine.

Aromatic Molecules

Cyclopropenyl cation Two pi electrons n = 0

Benzene Six pi electrons n = 1

Pyrrole Six pi electrons n = 1

Furan Six pi electrons n = 1

Pyridine Six pi electrons n = 1

Imidazole Six pi electrons n = 1

Naphthalene Ten pi electrons n = 2

Nonaromatic and Antiaromatic Molecules