kekulene
{{chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 427467941
| ImageFileL1 = kekulene.svg
| ImageSizeL1 = 140px
| ImageAlt = Skeletal formula of kekulene
| ImageFileR1 = Kekulene 3D spacefill.png
| ImageSizeR1= 140px
| ImageAlt1 = Space-filling model of the kekulene molecule
| PIN = 15,23:16,22-Di(metheno)naphtho[2,3-a]tetrapheno[3,2-o]pentaphene
| OtherNames =
|Section1={{Chembox Identifiers
| Abbreviations =
| CASNo_Ref = {{cascite|correct|??}}
| CASNo=15123-47-4
| PubChem = 5460755
| SMILES = C1=CC2=CC3=C4C=C2C5=CC6=C(C=CC7=CC8=C(C=C76)C9=CC2=C(C=CC6=C2C=C2C(=C6)C=CC6=C2C=C4C(=C6)C=C3)C=C9C=C8)C=C51
| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}
| ChemSpiderID = 4574217
| InChI = 1/C48H24/c1-2-26-14-28-5-6-30-16-32-9-10-34-18-36-12-11-35-17-33-8-7-31-15-29-4-3-27-13-25(1)37-19-39(27)41(29)21-43(31)45(33)23-47(35)48(36)24-46(34)44(32)22-42(30)40(28)20-38(26)37/h1-24H
| InChIKey = TYPKKLUFDMGLAC-UHFFFAOYAP
| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
| StdInChI = 1S/C48H24/c1-2-26-14-28-5-6-30-16-32-9-10-34-18-36-12-11-35-17-33-8-7-31-15-29-4-3-27-13-25(1)37-19-39(27)41(29)21-43(31)45(33)23-47(35)48(36)24-46(34)44(32)22-42(30)40(28)20-38(26)37/h1-24H
| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
| StdInChIKey = TYPKKLUFDMGLAC-UHFFFAOYSA-N
| RTECS =
| MeSHName =
| ChEBI_Ref = {{ebicite|changed|EBI}}
| ChEBI = 32987
}}
|Section2={{Chembox Properties
| C=48 | H=24
| Appearance=
| MeltingPt=
| BoilingPt=
| Solubility=
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| Section3 = {{Chembox Structure
| CrystalStruct =
| SpaceGroup = monoclinic, C2/c
| PointGroup =
| LattConst_a = 2795.1(4) pm
| LattConst_b = 457.9(1) pm
| LattConst_c = 2268.0(2) pm
| LattConst_alpha =
| LattConst_beta = 109.64(1)
| LattConst_gamma =
| LattConst_ref =
| LattConst_Comment =
| UnitCellVolume =
| UnitCellFormulas = 4
| Coordination =
| MolShape =
| OrbitalHybridisation =
| Dipole =
}}
|Section8={{Chembox Hazards
| MainHazards=
| FlashPt=
| AutoignitionPt =
}}
}}
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,{{cite journal |last1=Staab |first1=Heinz A. |last2=Diederich |first2=François |title=Cycloarenes, a New Class of Aromatic Compounds, I. Synthesis of Kekulene |journal=Chemische Berichte |date=October 1983 |volume=116 |issue=10 |pages=3487–3503 |doi=10.1002/cber.19831161021}} 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.{{cite journal |last1=Krieger |first1=Claus |last2=Diederich |first2=Francois |last3=Schweitzer |first3=Dieter |last4=Staab |first4=Heinz A. |title=Molecular Structure and Spectroscopic Properties of Kekulene |journal=Angewandte Chemie International Edition in English |date=September 1979 |volume=18 |issue=9 |pages=699–701 |doi=10.1002/anie.197906991|url=http://elib.uni-stuttgart.de/handle/11682/7566 }}{{cite journal |last1=Aihara |first1=Junichi |title=Is superaromaticity a fact or an artifact? The kekulene problem |journal=Journal of the American Chemical Society |date=January 1992 |volume=114 |issue=3 |pages=865–868 |doi=10.1021/ja00029a009}}{{cite journal |last1=Diederich |first1=François |last2=Staab |first2=Heinz A. |title=Benzenoidversus Annulenoid Aromaticity: Synthesis and Properties of Kekulene |journal=Angewandte Chemie International Edition in English |date=May 1978 |volume=17 |issue=5 |pages=372–374 |doi=10.1002/anie.197803721}}{{cite journal |last1=Jiao |first1=Haijun |last2=Schleyer |first2=Paul von Ragué |title=Is Kekulene Really Superaromatic? |journal=Angewandte Chemie International Edition in English |date=1 November 1996 |volume=35 |issue=20 |pages=2383–2386 |doi=10.1002/anie.199623831}}{{cite journal |last1=Schweitzer |first1=D. |last2=Hausser |first2=K.H. |last3=Vogler |first3=H. |last4=Diederich |first4=F. |last5=Staab |first5=H.A. |title=Electronic properties of kekulene |journal=Molecular Physics |date=11 August 2006 |volume=46 |issue=5 |pages=1141–1153 |doi=10.1080/00268978200101861|url=http://elib.uni-stuttgart.de/handle/11682/7578 }}{{cite journal |last1=Staab |first1=Heinz A. |last2=Diederich |first2=FrançOis |last3=Krieger |first3=Claus |last4=Schweitzer |first4=Dieter |title=Cycloarenes, a New Class of Aromatic Compounds, II. Molecular Structure and Spectroscopic Properties of Kekulene |journal=Chemische Berichte |date=October 1983 |volume=116 |issue=10 |pages=3504–3512 |doi=10.1002/cber.19831161022}}{{cite journal |last1=Zhou |first1=Zhongxiang |title=Are kekulene, coronene, and corannulene tetraanion superaromatic? Theoretical examination using hardness indices |journal=Journal of Physical Organic Chemistry |date=February 1995 |volume=8 |issue=2 |pages=103–107 |doi=10.1002/poc.610080209}}
File:Kekulene (kekule conjugation).png|"Kekulé" 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 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.{{cite journal |last1=Pozo |first1=Iago |last2=Majzik |first2=Zsolt |last3=Pavliček |first3=Niko |last4=Melle-Franco |first4=Manuel |last5=Guitián |first5=Enrique |last6=Peña |first6=Diego |last7=Gross |first7=Leo |last8=Pérez |first8=Dolores |title=Revisiting Kekulene: Synthesis and Single-Molecule Imaging |journal=Journal of the American Chemical Society |date=17 September 2019 |volume=141 |issue=39 |pages=15488–15493 |doi=10.1021/jacs.9b07926|pmid=31525873 |pmc=6786662 |doi-access=free }} 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.