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Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

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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,...
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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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π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

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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...
1.3K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

3.1K
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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Frost Circles for Different Conjugated Systems01:18

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3.0K
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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Structure of Benzene: Molecular Orbital Model01:18

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10.2K
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).
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A Planar Five-Membered Aromatic Ring Stabilized by Only Two π-Electrons.

Oleksandr Kysliak1, Simon H F Schreiner1,2, Niklas Grabicki3

  • 1Institute of Inorganic and Analytical Chemistry (IAAC), Friedrich Schiller University Jena, Humboldtstraße 8, 07743, Jena, Germany.

Angewandte Chemie (International Ed. in English)
|May 20, 2022
PubMed
Summary

Researchers synthesized a novel dipotassium cyclopentagallene, a five-membered aromatic ring with only two π-electrons. This discovery expands the understanding of aromatic stabilization beyond previously known minimum electron counts.

Keywords:
AromaticityGalliumInorganic Ring SystemsNICS

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Area of Science:

  • Inorganic Chemistry
  • Materials Science
  • Theoretical Chemistry

Background:

  • Aromaticity provides significant stabilization to cyclic chemical structures through π-electron delocalization.
  • Stable aromatic rings typically possess an equal number of π-electrons and ring atoms (e.g., benzene).
  • Aromatic systems with a minimal two π-electrons are rare, usually confined to small rings or metal complexes.

Purpose of the Study:

  • To report the synthesis of a novel five-membered aromatic ring with a minimal two π-electron system.
  • To investigate the structural and electronic properties of this unique cyclopentagallene derivative.
  • To challenge and expand the established criteria for aromatic stabilization.

Main Methods:

  • Facile synthesis of dipotassium cyclopentagallene.
  • Single-crystal X-ray diffraction to determine the molecular structure.
  • Computational modeling and spectroscopic analysis to confirm aromaticity.

Main Results:

  • Successful synthesis of a planar five-membered gallium ring (Ga5).
  • Observation of nearly equal gallium-gallium bond lengths, indicative of delocalization.
  • Confirmation of aromatic character through combined experimental and computational data.

Conclusions:

  • Dipotassium cyclopentagallene represents a rare example of a stable two π-electron aromatic system in a five-membered ring.
  • The findings demonstrate that aromatic stabilization is achievable with fewer π-electrons than previously thought.
  • This work broadens the scope of known aromatic compounds and the principles governing their stability.