Hückel's rule

In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. It was formulated by physical chemist Erich Hückel in 1931. A cyclic ring molecule follows Hückel's rule when the number of its pi electrons equals <math>4n + 2<math> where <math>n<math> is zero or any positive integer.

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.