Which molecules are aromatic?
An aromatic molecule is one in which electrons form a continuous pi cloud above and below the circular arrangements of atoms. In one representation these atoms are connected by alternating single and double bonds. Another representation is that of the circular pi bond, in which the electron density is evenly distributed through a pi bond above and below the ring. The circulating pi electrons in an aromatic molecule generate significant local magnetic fields that can be detected by NMR techniques. Aromatic molecules typically display enhanced chemical stability, compared to similar non-aromatic molecules.
This commonly seen model of aromatic rings was developed by Friedrich August Kekulé von Stradonitz and was first explained in quantum mechanical terms by Linus Pauling in the 1930s. In 1931, Erich Hückel devised the "4n+2" pi electron rule, valid for planar molecules with a single ring. Molecules having 4n+2 pi electrons (n >= 0) are expected to be aromatic.