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Structure of Benzene: Molecular Orbital Model01:18

Structure of Benzene: Molecular Orbital Model

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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).
12.1K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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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...
3.6K
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

3.5K
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...
3.5K
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

5.3K
Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom,...
5.3K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

12.8K
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 +...
12.8K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

3.6K
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.
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Bis(azuleno)pentalenes with Six Consecutive Odd-Membered Rings.

Qing Jiang1,2, Tianzuo Wang3, Yi Han2

  • 1College of Chemistry and Bioengineering, Hunan University of Science and Engineering, Yongzhou, 425100, China.

Angewandte Chemie (International Ed. in English)
|October 1, 2025
PubMed
Summary

We synthesized novel non-alternant polycyclic hydrocarbons (PHs) with fused odd-membered rings. These unique molecules exhibit promising semiconducting properties for organic electronics.

Keywords:
AromaticityAzuleneHole mobilityNon‐alternant Non‐benzenoid polycyclic hydrocarbonsPentalene

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Area of Science:

  • Organic Chemistry
  • Materials Science
  • Supramolecular Chemistry

Background:

  • Non-alternant, non-benzenoid polycyclic hydrocarbons (PHs) offer distinct properties compared to benzenoid systems.
  • Synthesizing PHs exclusively from odd-membered carbocycles presents a significant challenge.

Purpose of the Study:

  • To report the first synthesis of diazuleno[a,e]pentalene derivatives.
  • To investigate the electronic structure and properties of these novel PHs.

Main Methods:

  • Palladium-catalyzed homoannulation of o-alkynylazulenyliodide.
  • X-ray crystallography, NMR spectroscopy, and theoretical calculations (ACID, NICS).
  • Fabrication and characterization of organic field-effect transistors (OFETs).

Main Results:

  • Successful synthesis of three diazuleno[a,e]pentalene derivatives (AP-1, AP-2, AP-3) with a 7-5-5-5-5-7 topology.
  • Electronic structure revealed two aromatic azulene units fused to an antiaromatic pentalene core.
  • AP-2 exhibited p-type semiconducting behavior with a hole mobility of 0.72 cm² V⁻¹ s⁻¹.

Conclusions:

  • The synthesized compounds possess a unique electronic structure with a small HOMO-LUMO gap and amphoteric redox behavior.
  • Reversible protonation/deprotonation observed in AP-1 and AP-2.
  • Demonstrated potential of non-alternant PHs in organic electronics.