Four rules to be said for a compound as aromatic.
- First, it must be cyclic.
- Second, every atom in the ring must be conjugated.
- Third, the molecule must have [4n+2] pi electrons
- Fourth, the molecule must be flat (if 1 and 3 conditions are obeyed)
Benzene and cyclooctatetraene are both cyclic and conjugated, but benzene is aromatic and cyclooctatetraene is not. The difference is that benzene has 6 pi electrons, and cyclooctatetraene has 8. The condition that aromatic molecules must have [4n+2] pi electrons is sometimes called “Hückel’s rule”.
Each p orbital contains a single electron. We can verify the total number of pi electrons in benzene by counting the pi bonds: 3 pi bonds times two electrons = 6 pi electrons total.