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Radical Reactivity: Overview01:11

Radical Reactivity: Overview

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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Radical Reactivity: Intramolecular vs Intermolecular01:33

Radical Reactivity: Intramolecular vs Intermolecular

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Radical reactions can occur either intermolecularly or intramolecularly. In an intermolecular radical reaction, a nucleophilic radical adds to an electrophilic alkene or vice versa. In such reactions, the radical and generally the alkene, which is also called the radical trap, are two different molecules. Additionally, for such intermolecular reactions to occur, the radical trap must be active, present in an excess concentration, and the radical starting material must have a weak...
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Nucleophilic Aromatic Substitution: Elimination–Addition01:11

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5.0K
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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Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)01:30

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Nucleophilic substitution in aromatic compounds is feasible in substrates bearing strong electron-withdrawing substituents positioned ortho or para to the leaving group. The reaction proceeds via two steps: the addition of the nucleophile and the elimination of the leaving group.
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π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

1.8K
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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Edge Migration of Aromatic Rings by Radical Reactions─Kinetics and Directionality.

Alexander M Mebel1, Michael Frenklach2

  • 1Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, United States.

The Journal of Physical Chemistry. A
|November 12, 2025
PubMed
Summary
This summary is machine-generated.

Polyaromatic radical migrations are facile, but their direction depends on molecular geometry. This study challenges the isolated pentagon rule by showing fused five-membered rings can form, not split.

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Area of Science:

  • Theoretical Chemistry
  • Physical Chemistry
  • Computational Chemistry

Background:

  • Polyaromatic hydrocarbons (PAHs) are crucial in various fields, including astrophysics and materials science.
  • Understanding the reactivity and rearrangement mechanisms of PAHs is essential for predicting their behavior and properties.
  • The isolated pentagon rule (IPR) has long governed the stability and formation of PAH structures.

Purpose of the Study:

  • To theoretically investigate the migration reactions of polyaromatic radicals involving five- and seven-ring systems.
  • To determine the energetics and rate constants governing these migration processes.
  • To elucidate the factors controlling the directionality of radical migrations in PAHs and assess their implications for the IPR.

Main Methods:

  • Quantum chemical calculations were employed to compute reaction energetics and rate constants.
  • Harmonic oscillator model of aromaticity (HOMA) analysis was used to assess strain-induced changes in molecular geometry.
  • Theoretical examination of five- and seven-ring migration reactions in polyaromatic radicals.

Main Results:

  • Computed energetics and rate constants confirm the facile nature of polyaromatic radical migrations.
  • The direction of migration was found to be non-identical for homologous reactions, influenced by strain-induced geometric changes.
  • Analysis revealed that fused five-membered rings can form, rather than split, challenging the conventional understanding of PAH stability.

Conclusions:

  • Polyaromatic radical migrations are energetically favorable processes.
  • Molecular geometry and strain play a critical role in determining the regioselectivity of these migrations.
  • The findings suggest a revision of the isolated pentagon rule, as fused five-membered rings can be thermodynamically accessible.