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Related Concept Videos

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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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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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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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?  
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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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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Dynamic Four-Membered Rings with 2π Aromaticity.

Y Kalyanamurthy Sahana1, Tiya Garg1, Vanshika Jain1

  • 1Department of Chemistry, Shiv Nadar Institution of Eminence, Greater Noida, Delhi-NCR, Gautam Buddha Nagar, Uttar Pradesh 201314, India.

The Journal of Physical Chemistry. A
|October 9, 2025
PubMed
Summary
This summary is machine-generated.

Structural dynamism in aromatic systems is now achievable with novel 2π aromatic compounds. These four-membered rings (4MRs) exhibit dynamic behavior influenced by aromaticity in transition states, confirmed by simulations.

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Area of Science:

  • Computational chemistry
  • Aromaticity studies
  • Materials science

Background:

  • Achieving structural dynamism in aromatic systems presents significant challenges.
  • Previous research has not fully explored the dynamic potential of 2π aromatic compounds.

Purpose of the Study:

  • To investigate the structural preferences and dynamic behavior of four-membered rings (4MRs) isoelectronic to cyclobutadiene dication (CBD).
  • To explore the impact of isoelectronic replacements and substituents on the dynamism of these 4MRs.

Main Methods:

  • Density functional theory (DFT) calculations with CCSD(T) and DLPNO-CCSD(T) energy corrections.
  • Investigation of various substituents and their effect on structural dynamism.
  • Molecular dynamics simulations at room temperature.

Main Results:

  • Several 4MRs with the molecular formula A₂X₂ (A=B, Al, Ga; X=Si, Ge) were identified.
  • Si₂Al₂ exhibited the lowest transition barrier for structural changes, with transition state aromaticity significantly influencing dynamism.
  • Molecular dynamics confirmed the structural dynamism of these compounds.

Conclusions:

  • 2π aromatic compounds can achieve significant structural dynamism.
  • The aromaticity of transition states is a critical factor in controlling dynamic behavior.
  • Si₂X₂ compounds favor cyclic isomers, while Ge₂X₂ compounds show comparable stability between cyclic and open structures.