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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.
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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.
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In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
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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?  
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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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A Zwitterionic, 10 π Aromatic Hemisphere.

Nema Hafezi1, Wondimagegn T Shewa1, James C Fettinger1

  • 1Department of Chemistry, University of California Davis, 1 Shields Avenue, Davis, CA, 95616, USA.

Angewandte Chemie (International Ed. in English)
|September 26, 2017
PubMed
Summary
This summary is machine-generated.

Researchers created a stable, aromatic [9]annulene using a novel anionic 10 π aromaticity concept. This involved an alkyl ammonium bridge within a cyclononatetraenide ring, demonstrating strong aromatic properties.

Keywords:
ab initio calculationsannulenesaromaticityhemispherical moleculeszwitterions

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Area of Science:

  • Organic Chemistry
  • Aromaticity Studies
  • Supramolecular Chemistry

Background:

  • Anionic aromaticity is crucial for understanding electronic structures.
  • Cyclononatetraenide systems offer unique possibilities for charge embedding.
  • Alkyl ammonium bridges present a novel approach to stabilizing charged organic species.

Purpose of the Study:

  • To introduce a new concept of anionic 10 π aromaticity.
  • To synthesize and characterize a stable zwitterionic [9]annulene.
  • To investigate the structural and electronic properties of the novel aromatic system.

Main Methods:

  • Methylation of azatriquinacene to form a quaternary ammonium salt.
  • Oxidation to the tetraene followed by deprotonation to yield the zwitterion.
  • Analysis of C-C bond equalization and nucleus-independent chemical shift (NICS) values.

Main Results:

  • Successful synthesis of a stable zwitterionic [9]annulene exhibiting strong aromaticity.
  • Demonstrated significant C-C bond equalization, indicative of aromatic character.
  • Observed NICS value lower than benzene, confirming enhanced aromaticity.
  • Solid-state structure revealed an eclipsed stacking motif with charge compensation.

Conclusions:

  • The study presents a novel method for achieving anionic 10 π aromaticity.
  • The synthesized zwitterionic [9]annulene is a stable system with pronounced aromaticity.
  • The findings expand the understanding of aromaticity in non-benzenoid systems.