Hückel’s rule is a simple guideline to determine whether a cyclic, conjugated compound is aromatic.
What it says
A compound is aromatic if the number of electrons in the conjugated system (the π electrons in the ring) fits the formula:
4n + 2
- n is a whole number: 0, 1, 2, 3…
- Only compounds with this number of electrons are aromatic.
Key Points
- It applies only to cyclic compounds with a continuous conjugated system.
- The electrons that count are those that are delocalized over the ring.
- Compounds that do not follow this rule are either anti-aromatic (less stable) or non-aromatic.
Simple way to remember
- If the ring has 2, 6, 10, 14… delocalized electrons, it is aromatic.
- If it has 4, 8, 12… electrons, it is anti-aromatic.
In short:
Hückel’s rule says that a cyclic, conjugated compound is aromatic if it has a special number of delocalized electrons that gives it extra stability, and that number follows 4n + 2.