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Related Concept Videos

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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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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Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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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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Radicals: Electronic Structure and Geometry01:07

Radicals: Electronic Structure and Geometry

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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is...
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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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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
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A flat carborane with multiple aromaticity beyond Wade-Mingos' rules.

Wei Lu1, Dinh Cao Huan Do1, Rei Kinjo2

  • 1Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore, 637371, Singapore.

Nature Communications
|July 8, 2020
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Summary

Researchers synthesized a flat carborane cluster with 16 skeletal electrons, challenging established polyhedral skeletal electron pair theory (PSEPT). This novel structure exhibits quadruple aromaticity, demonstrating a new class of aromatic compounds.

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Area of Science:

  • Inorganic Chemistry
  • Computational Chemistry
  • Materials Science

Background:

  • Polyhedral skeletal electron pair theory (PSEPT), also known as Wade and Mingos rules, correlates cluster skeletal structure with electron pairs.
  • Recent computational studies suggest deviations from PSEPT with increased skeletal electrons, leading to flatter structures, but experimental evidence is limited.

Purpose of the Study:

  • To experimentally synthesize and characterize a carborane cluster with a flat structure that challenges existing theories.
  • To investigate the electronic properties and aromaticity of the novel flat carborane structure.

Main Methods:

  • Synthesis of a C2B4R4 carborane (compound 2) with a unique flat, ribbon-like structure.
  • Spectroscopic and computational analyses to determine the skeletal electron count and electronic properties.

Main Results:

  • The synthesized C2B4 core features 16 skeletal electrons in its singlet ground state.
  • This structure violates both Hückel's rule ([4n+2]) and Baird's rule for aromaticity.
  • Delocalization of skeletal electrons results in two independent π- and two independent σ-aromatic ring currents, indicating quadruple aromaticity.

Conclusions:

  • The experimental synthesis of a flat carborane cluster provides the first empirical evidence for structures beyond PSEPT predictions.
  • The quadruple aromaticity of this novel cluster opens new avenues for understanding electronic delocalization and designing advanced materials.