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## Hückel's rule
## Additional recommended knowledgeIn organic chemistry, 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 - (1) follow Huckel's rule, having 4n+2 electrons in the delocalized cloud, (2) are 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. ## RefinementHückel's rule is not valid for many compounds containing more than three fused aromatic nuclei in a cyclic fashion.. For example, pyrene contains 16 conjugated electrons (8 bonds), and coronene contains 24 conjugated electrons (12 bonds). Both of these polycyclic molecules are aromatic even though they fail the 4n+2 rule. The Pariser-Parr-Pople method is a more precise method of estimating whether a cyclic ring molecule is aromatic. ## Three-Dimensional RuleIn 2000, chemists in Germany formulated a rule to determine when a fullerene would be aromatic. In particular, they found that if there were 2( |

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hückel's_rule". A list of authors is available in Wikipedia. |