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Related Concept Videos

Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

3.4K
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
3.4K
Structure of Benzene: Molecular Orbital Model01:18

Structure of Benzene: Molecular Orbital Model

11.2K
According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).
11.2K
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

3.3K
Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous...
3.3K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

3.4K
The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
3.4K
Structure of Benzene: Kekulé Model01:07

Structure of Benzene: Kekulé Model

11.1K
In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions—formula of benzene is C6H6, all the hydrogens of benzene are equivalent, and each carbon must have four bonds due to its tetravalency.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.
11.1K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

12.2K
Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?  
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as Hückel’s rule or the 4n +...
12.2K

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Related Experiment Video

Updated: Nov 30, 2025

Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

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Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization.

Anja Haags1,2,3, Alexander Reichmann4, Qitang Fan5

  • 1Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany.

ACS Nano
|November 13, 2020
PubMed
Summary

This study investigates kekulene

Keywords:
ARPESDFTSTMaromaticitykekulenephotoemission

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Area of Science:

  • Physical Chemistry
  • Materials Science
  • Organic Chemistry

Background:

  • Aromaticity in polycyclic aromatic hydrocarbons is a fundamental concept.
  • Kekulene's aromaticity has been debated, with competing models.
  • Understanding frontier orbital electronic structure is key to assessing aromaticity.

Purpose of the Study:

  • To investigate the electronic structure of kekulene's frontier orbitals.
  • To clarify the nature of π-conjugation in kekulene.
  • To determine if kekulene follows the Clar model or a superaromatic model.

Main Methods:

  • Synthesis of a novel precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene.
  • High-purity kekulene preparation on a Cu(111) surface.
  • Angle-resolved photoemission spectroscopy and photoemission tomography.
  • Density functional theory calculations.

Main Results:

  • High-purity kekulene was successfully synthesized and characterized.
  • The highest occupied molecular orbital structure was determined.
  • Experimental results align with the Clar model for π-conjugation.

Conclusions:

  • Kekulene's π-conjugation is best described by the Clar model.
  • Superaromaticity is not the preferred model for kekulene.
  • Photoemission tomography provides insights into the electronic consequences of aromaticity.