The molecule must be cyclic, planar with uninterrupted cloud of π electrons above and below the plane of the ring. It should have 4n+2 π electrons.

Here every atom in the ring must have a p orbital and the delocalization should result in an uninterrupted cyclic cloud of π electrons above and below the plain of the ring.

The German Chemist Erich Hückel was the first one to recognize that an aromatic compound must have an odd number of pairs of electrons, which can mathematically be written as 4n+2 (n = 0,1,2,3 etc). Molecules which obey these rules are aromatic and those which follow these rules partially fall in the category of anti-aromatic and non aromatic compounds.The new definition is

The ability of the compound to sustain diamagnetic ring current in NMR is said to be diatropic is said to be aromatic.

Based on the properties of compounds, there are FOUR criteria about the pi system that need to be met in order for the "special" aromatic stabilisation to be observed:

• 1.Conjugated (there needs to one "p" orbital from each atom in the cycle, so each atom must be either sp2or sp hybridised)
• 2.Cyclic (Linear systems are not aromatic)
• 3.Planar (so there is good overlap / interaction between the "p" orbitals.... not always easy to consider)
• 4.The Huckel Rule..... 4n+2 pi electrons ( this is equivalent to an odd number of pi-electrons pairs) in the cyclic conjugated system.