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Related Concept Videos

Structure of Benzene: Kekulé Model01:07

Structure of Benzene: Kekulé Model

10.3K
In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions—formula of benzene is C6H6, all the hydrogens of benzene are equivalent, and each carbon must have four bonds due to its tetravalency.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.
10.3K
Structure of Benzene: Molecular Orbital Model01:18

Structure of Benzene: Molecular Orbital Model

10.3K
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).
10.3K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

11.6K
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...
11.6K
NMR Spectroscopy of Benzene Derivatives01:34

NMR Spectroscopy of Benzene Derivatives

9.1K
Simple unsubstituted benzene has six aromatic protons, all chemically equivalent. Therefore, benzene exhibits only a singlet peak at δ 7.3 ppm in the 1H NMR spectrum. The observed shift is far downfield because the aromatic ring current strongly deshields the protons. Any substitution on the benzene ring makes the aromatic protons nonequivalent, and the protons split each other. The peak is, therefore, no longer a singlet and the splitting pattern and their associated coupling...
9.1K
Chair Conformation of Cyclohexane02:02

Chair Conformation of Cyclohexane

16.1K
The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this...
16.1K
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.
Removing one hydrogen from the intervening CH2 group...
3.1K

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Application of Elemental Lanthanides in the Selective C-F Activation of Trifluoromethylated Benzofulvenes Providing Access to Various Difluoroalkenes
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A Conical Intersection Influences the Ground State Rearrangement of Fulvene to Benzene.

Barry K Carpenter1, G Barney Ellison2, Mark R Nimlos3

  • 1School of Chemistry, Cardiff University, Main Building, Park PL, Cardiff CF10 3AT, U.K.

The Journal of Physical Chemistry. A
|February 22, 2022
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Summary

The unimolecular rearrangement of fulvene to benzene, crucial for soot formation, has a newly discovered lower energy pathway. This finding makes the unimolecular mechanism competitive with the bimolecular pathway during combustion.

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Area of Science:

  • Chemical Dynamics
  • Combustion Chemistry
  • Theoretical Chemistry

Background:

  • Fulvene rearrangement to benzene is key in soot formation.
  • Two mechanisms exist: unimolecular and bimolecular.
  • Previous calculations showed a high barrier for the unimolecular path.

Purpose of the Study:

  • Reinvestigate the unimolecular fulvene to benzene rearrangement computationally.
  • Determine the accurate energy barrier for the unimolecular pathway.
  • Assess the competitiveness of unimolecular vs. bimolecular mechanisms.

Main Methods:

  • Advanced computational electronic structure theory.
  • Identification of transition states and reaction pathways.
  • Analysis of conical intersections between electronic states.

Main Results:

  • A second transition state was identified, lowering the unimolecular barrier by ~10 kcal/mol.
  • The new barrier aligns with experimental values, resolving previous discrepancies.
  • Conical intersections explain the existence of two transition states.

Conclusions:

  • The unimolecular mechanism is now computationally competitive with the bimolecular pathway.
  • Experimental data on anisole pyrolysis supports competitive mechanisms.
  • Similar conical intersections are observed in related isomerizations.