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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...
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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).
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Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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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...
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Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

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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 +...
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Structure of Benzene: Kekulé Model01:07

Structure of Benzene: Kekulé Model

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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.
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π Molecular Orbitals of 1,3-Butadiene01:24

π Molecular Orbitals of 1,3-Butadiene

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Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the number...
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Related Experiment Video

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Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
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A Sterically Congested Nitrogenated Benzodipentaphene with a Double π-Expanded Helicene Structure.

Fengkun Chen1, Wenting Gu2, Akinori Saeki2

  • 1POLYMAT, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018 Donostia-San Sebastian, Spain.

Organic Letters
|April 16, 2020
PubMed
Summary

Researchers synthesized novel nitrogenated benzodipentaphenes. One compound features a unique, distorted aromatic backbone forming a double π-expanded helicene structure.

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Area of Science:

  • Organic Chemistry
  • Materials Science
  • Supramolecular Chemistry

Background:

  • Nitrogenated polycyclic aromatic hydrocarbons (NPAHs) are crucial in organic electronics.
  • Benzodipentaphenes offer a unique fused aromatic system for exploring novel electronic properties.
  • Steric congestion can significantly influence molecular geometry and electronic behavior.

Purpose of the Study:

  • To synthesize and characterize novel, sterically hindered nitrogenated benzodipentaphenes.
  • To investigate the impact of steric congestion on the molecular architecture of these systems.
  • To explore the formation of unusual structural motifs, such as helicenes, within these frameworks.

Main Methods:

  • Multi-step organic synthesis involving cyclization and functionalization reactions.
  • Single-crystal X-ray diffraction for detailed structural analysis.
  • Spectroscopic techniques (NMR, UV-Vis, Fluorescence) for characterization.

Main Results:

  • Successful synthesis of three sterically congested nitrogenated benzodipentaphenes.
  • One derivative exhibited a highly distorted aromatic core.
  • An unprecedented double π-expanded helicene structure was identified in one of the synthesized molecules.

Conclusions:

  • Steric hindrance can drive the formation of complex, non-planar architectures in polycyclic aromatic systems.
  • The discovered double π-expanded helicene represents a novel structural motif with potential applications in advanced materials.
  • This work expands the scope of accessible complex PAHs and their unique structural possibilities.