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Hückel's rule is not valid for many compounds containing more than one ring. For example, pyrene and trans-bicalicene contain 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. Indeed, Hückel's rule can only be ...
Inspection of the Hückel one on the right, plus–minus overlaps are seen between orbital pairs 2-3, 3-4, 4-5, and 6-1, corresponding to an even number (4), as required by a Hückel system. The plus–minus orientation of each orbital is arbitrary since these are just basis set orbitals and do not correspond to any molecular orbital.
The famous Hückel 4n+2 rule for determining whether ring molecules composed of C=C bonds would show aromatic properties was first stated clearly by Doering in a 1951 article on tropolone. [6] Tropolone had been recognised as an aromatic molecule by Dewar in 1945. In 1936, Hückel developed the theory of π-conjugated biradicals (non
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Extended Hückel method, considers also sigma orbitals (whereas the original Hückel method only considers pi orbitals) Hückel's rule (named after Erich Hückel), a method of determining aromaticity in organic molecules; Walter Hückel (1895-1973), German chemist; Wolfgang Hückel (born 1936), German diplomat, Ambassador of the GDR in Chad