Hückel Aromaticity

     For an aromatic system to be a Huckle, there are two conditions to be met:

1.   The "4n+2" rule
   • This rule is an equation that is set equal to the number of π electrons within the system.  When solving, "n" must be an integer (i.e. 1, 2, 3...).
2.   There must be an even number of phase changes within the orbitals.
   • 0 (zero) is considered an even number
     • A phase change occurs when the + or - phase of an orbital switches to its opposite phase (+ switches to -, or - switches to +).

     Additonal Information:

          Vertex of the compound is drawn at the bottom of a Frost Circle.

     If the "4n+2" rule is not met, the system is termed "not aromatic."  Any pericyclic reaction occuring within a "not aromatic" system is forbidden, and therefore can not take place.  If a system follows the "4n" rule under these conditions, it is considered antiaromatic.

         Figure 1:) Huckle System (dark is + phase and light is - phase)