Kekulene

Kekulene is a polycyclic aromatic hydrocarbon and a circulene with the chemical formula C48H24. It was first synthesized in 1978,[1] and was named in honor of August Kekulé, the discoverer of the structure of the benzene molecule.

Kekulene
Names
Other names
[12]–Coronaphen, [12]Circulene
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
Properties
C48H24
Molar mass 600.720 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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 benzene-like (aromatic 6 π-electron) rings connected by bridging bonds and vinyl groups 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.[2][3][4][5][6][7][8]

  • Proposed electronic configurations
  • "Kékule" configuration: Two concentric aromatic rings
  • "Clar" configuration: Benzene rings alternating with non-aromatic linkers

The synthesis of the compound, first reported in 1978,[4] allowed experimental determination of the electronic structure. In the late 1970s, 1H-NMR provided evidence of benzene rings[4] and X-ray analysis determined that the structure had had alternating aromatic and non-aromatic rings,[2] 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.[9] This configuration is in keeping with Clar's rule, as it has the largest number of disjoint aromatic π sextets.

The whole the 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 among the hydrogen atoms.[9]

References
  1. 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.
  2. Krieger, Claus; Diederich, Francois; Schweitzer, Dieter; Staab, Heinz A. (September 1979). "Molecular Structure and Spectroscopic Properties of Kekulene". Angewandte Chemie International Edition in English. 18 (9): 699–701. doi:10.1002/anie.197906991.
  3. Aihara, Junichi (January 1992). "Is superaromaticity a fact or an artifact? The kekulene problem". Journal of the American Chemical Society. 114 (3): 865–868. doi:10.1021/ja00029a009.
  4. Diederich, 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.
  5. 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.
  6. Schweitzer, D.; Hausser, K.H.; Vogler, H.; Diederich, F.; Staab, H.A. (11 August 2006). "Electronic properties of kekulene". Molecular Physics. 46 (5): 1141–1153. doi:10.1080/00268978200101861.
  7. 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.
  8. 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.
  9. Pozo, Iago; Majzik, Zsolt; Pavliček, Niko; Melle-Franco, Manuel; Guitián, Enrique; Peña, Diego; Gross, Leo; Pérez, Dolores (17 September 2019). "Revisiting Kekulene: Synthesis and Single-Molecule Imaging". Journal of the American Chemical Society. 141 (39): 15488–15493. doi:10.1021/jacs.9b07926. PMID 31525873.
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