Aromaticity

Molecules with cyclic delocalised electrons are aromatic. These molecules can be depicted differently. One may use the Kekulé form of the structure with alternating single and double bonds or use the aromatic form with aromatic bonds.

These forms are equivalent but it is not obvious how to convert from the Kekulé form of the structure to the aromatic form. Generally the following conditions are needed:

• Atoms with delocalized electrons should be arranged in one or more rings.
• The structure should be planar at the rings in question.
• The number of π delocalized electrons should be even and follow the Hückel's rule: number of π electrons equals to 4n+2 where n = 0,1,2,3 ...
• Atoms should be connected with alternating single and double bonds. There are some exceptions to this rule, where the double and single bonds are not following each other yet the ring is aromatic. In such ring there is an atom that is capable of donating 2 electrons into the ring's delocalised electron structure. (These atoms: N, O, P, S, As, Se, which can add additional 2 electrons for the ring and B, which does not give additional electrons, only shares its empty molecular orbital.)