Kekulene Explained

Kekulene is a polycyclic 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,^[1] and was named in honor of August Kekulé, the discoverer of the structure of the benzene 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 benzene-like (aromatic 6 π-electron) rings connected by bridging bonds and vinyl 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.^{[2] [3] [4] [5] [6] [7] [8]}

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 Clar 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.

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 among the hydrogen atoms.^[9]

Notes and References

1. Staab . Heinz A. . Diederich . François . Cycloarenes, a New Class of Aromatic Compounds, I. Synthesis of Kekulene . Chemische Berichte . October 1983 . 116 . 10 . 3487–3503 . 10.1002/cber.19831161021.
2. Krieger . Claus . Diederich . Francois . Schweitzer . Dieter . Staab . Heinz A. . Molecular Structure and Spectroscopic Properties of Kekulene . Angewandte Chemie International Edition in English . September 1979 . 18 . 9 . 699–701 . 10.1002/anie.197906991.
3. Aihara . Junichi . Is superaromaticity a fact or an artifact? The kekulene problem . Journal of the American Chemical Society . January 1992 . 114 . 3 . 865–868 . 10.1021/ja00029a009.
4. Diederich . François . Staab . Heinz A. . Benzenoidversus Annulenoid Aromaticity: Synthesis and Properties of Kekulene . Angewandte Chemie International Edition in English . May 1978 . 17 . 5 . 372–374 . 10.1002/anie.197803721.
5. Jiao . Haijun . Schleyer . Paul von Ragué . Is Kekulene Really Superaromatic? . Angewandte Chemie International Edition in English . 1 November 1996 . 35 . 20 . 2383–2386 . 10.1002/anie.199623831.
6. Schweitzer . D. . Hausser . K.H. . Vogler . H. . Diederich . F. . Staab . H.A. . Electronic properties of kekulene . Molecular Physics . 11 August 2006 . 46 . 5 . 1141–1153 . 10.1080/00268978200101861.
7. Staab . Heinz A. . Diederich . FrançOis . Krieger . Claus . Schweitzer . Dieter . Cycloarenes, a New Class of Aromatic Compounds, II. Molecular Structure and Spectroscopic Properties of Kekulene . Chemische Berichte . October 1983 . 116 . 10 . 3504–3512 . 10.1002/cber.19831161022.
8. Zhou . Zhongxiang . Are kekulene, coronene, and corannulene tetraanion superaromatic? Theoretical examination using hardness indices . Journal of Physical Organic Chemistry . February 1995 . 8 . 2 . 103–107 . 10.1002/poc.610080209.
9. Pozo . Iago . Majzik . Zsolt . Pavliček . Niko . Melle-Franco . Manuel . Guitián . Enrique . Peña . Diego . Gross . Leo . Pérez . Dolores . Revisiting Kekulene: Synthesis and Single-Molecule Imaging . Journal of the American Chemical Society . 17 September 2019 . 141 . 39 . 15488–15493 . 10.1021/jacs.9b07926. 31525873 . 6786662 . free .