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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...
4.4K
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

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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...
2.1K
IR Absorption Frequency: Delocalization01:04

IR Absorption Frequency: Delocalization

1.9K
Electron delocalization refers to the distribution of electrons across multiple atoms within a molecule rather than being confined to a single atom or bond. This phenomenon is common in systems with conjugated bonds—structures where alternating single and double bonds allow π-electrons to move freely across the network. The movement of electrons stabilizes the molecule and can affect various chemical properties, including vibrational frequencies observed in IR spectroscopy.
In IR...
1.9K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

4.4K
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.
4.4K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

6.7K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
6.7K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

15.3K
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...
15.3K

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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Quantifying aromaticity with electron delocalisation measures.

Ferran Feixas1, Eduard Matito, Jordi Poater

  • 1Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus Montilivi, 17071 Girona, Spain. ematito@gmail.com miquel.sola@udg.edu.

Chemical Society Reviews
|April 11, 2015
PubMed
Summary
This summary is machine-generated.

Aromaticity, a key chemical property, is indirectly measured using various indices. This review focuses on electron delocalization measures for quantifying aromaticity in chemical analysis.

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Area of Science:

  • * Quantum Chemistry
  • * Theoretical Chemistry
  • * Organic Chemistry

Background:

  • * Aromaticity is a fundamental concept in chemistry, crucial for understanding molecular stability and reactivity.
  • * Direct experimental measurement of aromaticity is not feasible as it's not a defined physical quantity.
  • * Traditional indices of aromaticity rely on structural, energetic, or magnetic properties.

Purpose of the Study:

  • * To review and compare existing aromaticity descriptors based on electron delocalization.
  • * To evaluate the performance of electron delocalization indices against other types of aromaticity measures.
  • * To summarize applications of electron-based indices in chemical problem-solving.

Main Methods:

  • * Comprehensive literature review of aromaticity descriptors.
  • * Comparative analysis of different indices (electron delocalization vs. structural, energetic, magnetic).
  • * Case studies illustrating the application of electron-based indices.

Main Results:

  • * Electron delocalization measures have gained prominence in the last decade for quantifying aromaticity.
  • * New indices like PDI, FLU, ING, and INB have been developed.
  • * Electron delocalization indices offer valuable insights into aromaticity in complex chemical systems.

Conclusions:

  • * Electron delocalization provides a robust framework for assessing aromaticity.
  • * The reviewed indices and their applications highlight the utility of electronic measures.
  • * Further development and application of these indices are expected to advance chemical understanding.