Chapter 11 : Arenes and Aromaticity 
Qu: How can we recognise systems that are aromatic ?Based on the properties of aromatic compounds, there are FOUR criteria about the π system that need to be met inorder for the "special" aromatic stabilisation to be observed:Ans: By applying the criteria for aromaticity outlined below.
In order for a compound to be aromatic, all FOUR of these criteria must be met.
Study Tips: The Huckel rule....just think of "n" as an integer 0, 1, 2, 3... etc. it's just a number to feed into the simple formula to generate a set of numbers that correspond to the numbers π electrons in aromatic systems : Those numbers are 2, 6, 10, 14 etc. π electrons. So, if a cyclic, conjugated, planar π system contains one of these numbers of π electrons, then it's an aromatic system. For example, benzene is a 6 π electron system. "n" does not really relate to a simple structural property in the molecule. 
The most important and well known aromatic system is benzene. It consists of a set of 6 sp^{2} C arranged in a ring. This creates a set of 6 p orbitals, no gaps in the π system so we have a cyclic, conjugated π system. The planarity of the ring means that the interaction of the adjacent p orbitals is optimal. The 3 C=C in benzene mean that we have 3 pairs of π electrons = 6 π electrons = a 4n+2 number where n=1.




To aid in counting the electrons the following factors may help:
© Dr. Ian Hunt, Department of Chemistry 