The criteria for a compound to be aromatic are specific and must all be satisfied.
1. Must be cyclic
- The compound must have a ring structure.
- Open-chain compounds cannot be aromatic.
2. Must be planar
- All the atoms in the ring must lie in the same plane so that the electrons can move freely over the ring.
- If the ring is twisted or non-planar, it cannot be aromatic.
3. Must have a conjugated system
- The ring must have a continuous system of alternating single and double bonds (or lone pairs that contribute to the electron cloud).
- This allows electrons to be delocalized over the entire ring.
4. Must follow Hückel’s Rule
- The ring must have a specific number of π (pi) electrons that fit the formula: 4n + 2, where n is a whole number (0, 1, 2…).
- This rule ensures extra stability and is called Hückel’s rule.
In short: A compound is aromatic if it is cyclic, planar, fully conjugated, and has 4n+2 π electrons.