Criteria
a. Aromaticity requires a cyclic conjugatedarray of p orbitals. These p orbitals constitute a

delocalizedπmolecular orbital system.

b. The total number of electrons in the in the π-system must equal (4n +2) where n = 0,1,2 ... any

integer. This requirement is known as Huckel's Rule.

c. The molecule mustbe planar so that the p orbitals all overlap in a parallel fashion.

Manifestations - Physical and Chemical

a. Unusual thermodynamic stability. This point is experimentally born out in the dramatically

lower than expected heats of combustion and hydrogenation for aromatic molecules.

b. Aromatic molecules undergo substitution reactions rather than addition reactions in order to

preserve the aromatic πsystem.

c. Special magnetic properties are observed when an aromatic species is placed in a magnetic field.

These properties are the result of a strong "ring current" that is produced in the πcloud. This

ring current strongly influences chemical shifts in the 1H NMR spectrum and leads to shielded

and deshielded regions around the aromatic ring. The regions above and below the plane of the

ring are shielded and the regions around the periphery of the ring are deshielded.