“An aromatic hydrocarbon or arene  (or sometimes aryl hydrocarbon) is a hydrocarbon characterized by general alternating double and single bonds between carbons. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent. The configuration of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible such hydrocarbon, benzene.  Aromatic hydrocarbons can be monocyclic (MAH) or polycyclic (PAH).”

“Some non-benzene-based compounds called heteroarenes, which follow Hückel's rule, are also aromatic compounds. In these compounds, at least one carbon atom is replaced by one of the heteroatoms, oxygen, nitrogen, or sulfur. Examples of non-benzene compounds with aromatic properties are furan, a heterocyclic compound with a five-membered ring that includes an oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom.”

(Wikipedia, Aromatic Hydrocarbons, 5/21/2011)

Hückel’s Rule

In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromaticproperties. The quantum mechanical basis for its formulation was first worked out by phycsical chemist Erich Hückel in 1931. The succinct expression as the 4n+2 rule has been attributed to von Doering (1951), although several authors were using this form at around the same time.

A cyclic ring molecule follows Hückel’s rule when the number of its n-electrons equals 4n+2 where n is zero or any positive integer, although clearcut examples are really only established for values of n = 0 up to about n = 6. Hückel's rule was originally based on calculations using the Hückel method, although it can also be justified by considering a particle in a ring system, by the LCAO method and by the Pariser-Parr-Pople method,

Aromatic compounds are more stable than theoretically predicted by alkene hydrogenation data; the "extra" stability is due to the delocalized cloud of electrons, called resonance energy. Criteria for simple aromatics are:

1. follow Huckel's rule, having 4n+2 electrons in the delocalized p-orbital cloud;
2. be able to be planar and are cyclic;
3. every atom in the circle is able to participate in delocalizing the electrons by having a p-orbital or an unshared pair of electrons.

(Wikipedia, Hückel’s Rule, 5/21/2011)

Polyoxazoles    
Polymers /Resins  

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