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 that are applied to the π system (not the molecule as a whole) that need to be met in order 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 of π electrons in aromatic systems : Those numbers are 2, 6, 10, 14 etc. π electrons (note that these numbers equate to 1, 3, 5, 7 etc. π electron pairs). 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. A simpler way to think of the Huckel rule is that it requires an odd number of πelectrons pairs. 
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 (orbital overlap) 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 (a Huckel number) where n=1.




To aid in counting the electrons the following factors may help:
What about the terms "nonaromatic" and "antiaromatic" ?
If a system satisfies all of the first three criteria (i.e. it has a cyclic, planar and conjugated π system), but has 4n π electrons in the cyclic conjugated π system (n = 0, 1, 2, 3 etc. which is equivalent to an even number of πelectrons pairs), then the system will be antiaromatic. Note that antiaromatic compounds are rare because they tend to be highly unstable.
If a system fails to satisfy at least one of the first three criteria (has a cyclic, planar and conjugated π system), then the system will be nonaromatic.
Similarly, systems with odd numbers of
π electrons in the cyclic conjugated π system (i.e. not 4n+2 or 4n systems) are typically nonaromatic.
© Dr. Ian Hunt, Department of Chemistry 