Kekulene is a

polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...

which consists of 12 fused benzene rings arranged in a circle. It is therefore classified as a 2

circulene A circulene is a macrocyclic arene in which a central polygon is surrounded and fused by benzenoids. Nomenclature within this class of molecules is based on the number of benzene rings surrounding the core, which is equivalent to the size of the ...

with the chemical formula C₄₈H₂₄. It was first synthesized in 1978, and was named in honor of

August Kekulé Friedrich August Kekulé, later Friedrich August Kekule von Stradonitz ( , ; 7 September 1829 – 13 July 1896), was a German organic chemist. From the 1850s until his death, Kekulé was one of the most prominent chemists in Europe, especially ...

, the discoverer of the structure of the

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

molecule.

Geometry and electronic structure

The nature of the π bonding within the molecule was long debated, as several distinctly different arrangements were possible. The two most significant proposals are the "Clar" configuration, consisting of six

-like (aromatic 6 π-electron) rings connected by bridging bonds and

vinyl group In organic chemistry, a vinyl group (abbr. Vi; IUPAC name: ethenyl group) is a functional group with the formula . It is the ethylene (IUPAC name: ethene) molecule () with one fewer hydrogen atom. The name is also used for any compound contai ...

s in non-aromatic rings, and the "Kékule" configuration, consisting of two concentric aromatic rings (18 π-electron inner, 30 π-electron outer) linked by radial single bonds. File:Kekulene (kekule conjugation).png, "Kékule" configuration: Two concentric aromatic rings File:Kekulene (linked benzenes).png, "Clar" configuration: Benzene rings alternating with non-aromatic linkers The synthesis of the compound, first reported in 1978, allowed experimental determination of the electronic structure. In the late 1970s, ¹H-NMR provided evidence of benzene rings and X-ray analysis determined that the structure had had alternating aromatic and non-aromatic rings, both consistent with the Kékule configuration. In 2019, the configuration was determined to be one consisting of benzene-like rings alternating with non-aromatic linkages, by using single molecule atomic force microscopy to measure the carbon–carbon bond-lengths and bond orders. This configuration is in keeping with Clar's rule, as it has the largest number of disjoint aromatic π sextets. Though the whole structure is essentially planar, it only has three-fold symmetry rather than six-fold. The carbon–hydrogen bonds in the center of the ring have a slight alternating tilt out of the plane to avoid

steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

among the hydrogen atoms.

References

{{PAHs Polycyclic aromatic hydrocarbons Substances discovered in the 1970s