Aromatic character. The Hückel 4n+2 rule

What properties do all aromatic compounds have in common ?

Aromatic compounds are compounds whose molecular formulas would lead us to expect a high degree of unsaturation, and yet which are resistant to the addition reactions generally characteristic of unsaturated compounds.

From a theoritical standpoint, to be aromatic, a compound must have a molecule that contains cyclic clouds of delocalized pi electrons above and below the plane of the molecule; moreover, the pi clouds must contain a total of (4n+2) pi electrons. Consequently, for the particular degree of stability that characterizes aromatic compounds, the delocalization of pi electrons is not enough. They must a particular number of pi electrons: 2, 6, ….(a whole number). This requirement is called the 4n+2 rule or the Hückel rule, n must be a whole number , which is based on quantum mechanics, and has to do with the filling up of the various orbitals that make up the pi cloud.

For instance: Benzene has six pi electrons, six is a whole number, so if we solved the 4n+2 rule, we have: 4n+2=6 because six pi electrons, consequently 4n=4 so n=1 which is a whole number too.

We can find anion, cation or radical aromatic compouds.

For example: Cyclopentadienyl anion: six pi electrons, n=1. Cyclooctadecanonaene (annulen) which contains 18 pi electrons: 4n+2=18 -> 4n=16 and n=4 ,is also an aromatic compound.

This rule is crucial to know if a compound is aromatic or no.

If you want to see if you have understand, I advice you to consult this website: a little exercise :



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