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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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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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Frost Circles for Different Conjugated Systems01:18

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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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Aromatic Hydrocarbon Cations: Structural Overview01:18

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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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Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

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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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Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is...
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Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions
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All-metal Baird aromaticity.

Dandan Chen1, Dariusz W Szczepanik2, Jun Zhu3

  • 1State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. jun.zhu@xmu.edu.cn and Institute of Computational Chemistry and Catalysis and Department of Chemistry, University of Girona, C/M. Aurèlia Capmany 69, Girona, Catalonia 17003, Spain. miquel.sola@udg.edu.

Chemical Communications (Cambridge, England)
|September 17, 2020
PubMed
Summary
This summary is machine-generated.

Baird's rule, used for organic molecules, now explains aromaticity reversal in all-metal systems. This study extends the rule to systems with both sigma- and pi-aromaticity.

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Area of Science:

  • Quantum chemistry
  • Aromaticity studies
  • Materials science

Background:

  • Baird's rule rationalizes aromaticity reversal in organic molecules' triplet states.
  • Aromaticity in all-metal systems is an emerging area of research.

Purpose of the Study:

  • To investigate the applicability of Baird's rule to all-metal systems.
  • To explore sigma- and pi-aromaticity in these systems.

Main Methods:

  • Theoretical calculations
  • Application of Baird's rule framework

Main Results:

  • Baird's rule successfully predicts aromaticity reversal in all-metal systems.
  • Demonstrated extension of the rule to systems exhibiting both sigma- and pi-aromaticity.

Conclusions:

  • Baird's rule is a versatile concept applicable beyond traditional organic chemistry.
  • Provides a new theoretical tool for understanding aromaticity in novel all-metal compounds.