Introduction to aromaticity
Aromaticity is a concept in organic chemistry that explains the stability of many compounds and why they don't often involve themselves in reactions.
Rules for aromaticity
For a molecule to be aromatic, it must be:
- Cyclic – The molecule must form a ring.
- Planar – The ring must be flat.
- Conjugated – Every atom in the ring must have a p-orbital.
- (4n + 2) π electrons – The ring must have 2, 6, 10, ...., pi electrons.
This last rule is known as Hückel’s Rule, where n is any non negative integer.
Examples of Aromatic Compounds
| Molecule | π Electrons | Aromatic? |
|---|---|---|
| Benzene | 6 | Yes |
| Pyrrole | 6 | Yes |
| Furan | 6 | Yes |
| Cyclobutadiene | 4 | No |
| Cyclooctatetraene | 8 | No |
Some exceptions: Antiaromatic vs Non-Aromatic
- Antiaromatic: compounds which follow all aromaticity rules but instead of (4n + 2) π, they have 4n π electrons
- Non-aromatic: does not follow more than one criteria.