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Related Concept Videos

Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

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

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

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 annulenes. In...
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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.
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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

Criteria for Aromaticity and the Hückel 4n + 2 Rule

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 + 2 rule.
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...

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Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles
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Are antiaromatic rings stacked face-to-face aromatic?

Clémence Corminboeuf1, Paul von Ragué Schleyer, Philip Warner

  • 1Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, USA.

Organic Letters
|July 31, 2007
PubMed
Summary

Stacking hydrocarbon rings into superphane structures eliminates antiaromaticity, creating 3D aromatic character. This stacking method offers a new pathway to design novel aromatic systems.

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Area of Science:

  • Organic Chemistry
  • Theoretical Chemistry
  • Materials Science

Background:

  • Aromaticity is a key concept in chemistry, influencing molecular properties and reactivity.
  • Hydrocarbon rings with 4n pi electrons typically exhibit antiaromaticity, leading to instability.
  • Existing strategies like triplet and Möbius approaches aim to achieve aromaticity in such systems.

Purpose of the Study:

  • To investigate the potential of stacking 4n pi electron hydrocarbon rings into superphane structures.
  • To demonstrate the elimination of antiaromaticity and the induction of three-dimensional aromatic character.
  • To establish stacking as a viable strategy for creating novel aromatic systems.

Main Methods:

  • Computational modeling to determine optimized geometries of methano-bridged superphane series.
  • Calculation of Nuclear Independent Chemical Shift (NICS) values to assess aromaticity.
  • Analysis of bond lengths to confirm geometry equalization.

Main Results:

  • Superphane structures with stacked 4n pi electron rings exhibit bond length equalized geometries.
  • Diatropic NICS values confirm the presence of through-space three-dimensional aromatic character.
  • The study successfully demonstrates the elimination of antiaromaticity in these stacked systems.

Conclusions:

  • Stacking represents a third distinct strategy, alongside triplet and Möbius approaches, for achieving aromaticity in 4n pi electron systems.
  • Superphane structures offer a novel platform for exploring three-dimensional aromaticity.
  • This work expands the synthetic and conceptual toolkit for designing stable, aromatic hydrocarbon systems.