Hückel's rule
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In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. It was first expressed succinctly as the 4n+2 (actually 2+4n) rule by von Doering in 1951. A cyclic ring molecule follows Hückel's rule when the number of its π 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 6).
[edit] Refinement
Hückel's rule is not valid for many compounds containing more than three fused aromatic nuclei in a cyclic fashion like in pyrene or coronene.
The Pariser-Parr-Pople method is a more precise method of estimating whether a cyclic ring molecule is aromatic.
[edit] Three-Dimensional Rule
In 2000, chemists in Germany formulated a rule to determine when a fullerene would be aromatic. In particular, they found that if there were 2(n + 1)2 π electrons, then the fullerene would display aromatic properties. This follows from the fact that an aromatic fullerene must have full icosahedral (or other appropriate) symmetry, so the molecular orbitals must be entirely filled. This is only possible if there are exactly 2(n + 1)2 electrons, where n is a nonnegative integer. In particular, for example, buckminsterfullerene, with 60 π electrons, is non-aromatic, since 60/2=30, which is not a perfect square.
[edit] See Also
Aromaticity - A much more thorough explanation of what makes a molecule aromatic. Huckel's rule is a simple rule that sometimes fails.
