Kekulene is apolycyclic aromatic hydrocarbon which consists of 12 fused benzene rings arranged in a circle. It is therefore classified as a [12]-circulene with the chemical formula C48H24. It was first synthesized in 1978,[2] and was named in honor ofAugust Kekulé, the discoverer of the structure of thebenzene molecule.
The nature of theπ 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 sixbenzene-like (aromatic 6 π-electron) rings connected by bridging bonds andvinyl groups in non-aromatic rings, and the "Kekulé" configuration, consisting of two concentric aromatic rings (18 π-electron inner, 30 π-electron outer) linked by radial single bonds.[3][4][5][6][7][8][9]
Proposed electronic configurations
"Kekulé" configuration: Two concentric aromatic rings
"Clar" configuration: Benzene rings alternating with non-aromatic linkers
The synthesis of the compound, first reported in 1978,[5] allowed experimental determination of the electronic structure. In the late 1970s,1H-NMR provided evidence of benzene rings[5] and X-ray analysis determined that the structure had had alternating aromatic and non-aromatic rings,[3] both consistent with the Clar configuration. In 2019, the configuration was determined to be one consisting of benzene-like rings alternating with non-aromatic linkages, by using single moleculeatomic force microscopy to measure the carbon–carbon bond-lengths andbond orders.[10] This configuration is in keeping withClar'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 avoidsteric hindrance among the hydrogen atoms.[10]
^abStaab, Heinz A.; Diederich, François; Krieger, Claus; Schweitzer, Dieter (1983). "Cycloarenes, a New Class of Aromatic Compounds, II. Molecular Structure and Spectroscopic Properties of Kekulene".Chemische Berichte.116 (10):3504–3512.doi:10.1002/cber.19831161022.
^Staab, Heinz A.; Diederich, François (October 1983). "Cycloarenes, a New Class of Aromatic Compounds, I. Synthesis of Kekulene".Chemische Berichte.116 (10):3487–3503.doi:10.1002/cber.19831161021.
^Aihara, Junichi (January 1992). "Is superaromaticity a fact or an artifact? The kekulene problem".Journal of the American Chemical Society.114 (3):865–868.Bibcode:1992JAChS.114..865A.doi:10.1021/ja00029a009.
^abcDiederich, François; Staab, Heinz A. (May 1978). "Benzenoidversus Annulenoid Aromaticity: Synthesis and Properties of Kekulene".Angewandte Chemie International Edition in English.17 (5):372–374.doi:10.1002/anie.197803721.
^Jiao, Haijun; Schleyer, Paul von Ragué (1 November 1996). "Is Kekulene Really Superaromatic?".Angewandte Chemie International Edition in English.35 (20):2383–2386.doi:10.1002/anie.199623831.
^Staab, Heinz A.; Diederich, FrançOis; Krieger, Claus; Schweitzer, Dieter (October 1983). "Cycloarenes, a New Class of Aromatic Compounds, II. Molecular Structure and Spectroscopic Properties of Kekulene".Chemische Berichte.116 (10):3504–3512.doi:10.1002/cber.19831161022.
^Zhou, Zhongxiang (February 1995). "Are kekulene, coronene, and corannulene tetraanion superaromatic? Theoretical examination using hardness indices".Journal of Physical Organic Chemistry.8 (2):103–107.doi:10.1002/poc.610080209.
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