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Radical Autoxidation01:20

Radical Autoxidation

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The oxidation of an organic compound in the presence of air or oxygen is called autoxidation. For example, cumene reacts with oxygen to form hydroperoxide. Autoxidation involves initiation, propagation, and termination steps. Many organic compounds are susceptible to autoxidation—especially ethers in the presence of oxygen, which form hydroperoxides. Even though this reaction is slow, old ether bottles contain small amounts of peroxide, which leads to laboratory explosions during ether...
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Radical Reactivity: Steric Effects01:10

Radical Reactivity: Steric Effects

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The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
Along with electronic...
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Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)01:30

Nucleophilic Aromatic Substitution: Addition–Elimination (SNAr)

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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.
The reaction begins with an attack of the nucleophile on the carbon that holds the leaving group. This results in the delocalization of the π electrons over the ring carbons. The resonance interaction between...
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Radical Formation: Elimination00:51

Radical Formation: Elimination

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Another method of radical formation is the elimination process. It is the opposite of the addition route and is driven by the instability of the radical. For example, as depicted in Figure 1, dibenzoyl peroxide yields a pair of unstable radicals upon homolysis. Given its instability, this radical spontaneously undergoes elimination via a C–C bond cleavage to form a relatively more stable phenyl radical. The mechanism involves cleavage of the bond between the α and β positions...
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Nucleophilic Aromatic Substitution: Elimination–Addition01:11

Nucleophilic Aromatic Substitution: Elimination–Addition

4.2K
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...
4.2K
Radical Reactivity: Nucleophilic Radicals01:16

Radical Reactivity: Nucleophilic Radicals

2.2K
Radicals adjacent to electron-donating groups are called nucleophilic radicals. These radicals readily react with electrophilic alkenes. The SOMO–LUMO interactions are the driving force for the reaction, where the high-energy SOMO of the electron-rich, nucleophilic radicals interacts with the low-energy LUMO of the electron-deficient, electrophilic alkenes. Such SOMO–LUMO interactions are the basis of reactive radical traps, affecting the selectivity in radical reactions. For...
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Free Radicals in Chemical Biology: from Chemical Behavior to Biomarker Development
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Anti-Aromaticity Relief as an Approach to Stabilize Free Radicals.

Rui Zhang1, Arkady Ellern1, Arthur H Winter1

  • 1Department of Chemistry, Iowa State University, 1605 Gilman Hall, Ames, IA, 50010, USA.

Angewandte Chemie (International Ed. in English)
|September 16, 2021
PubMed
Summary

Researchers developed a novel electronic strategy to stabilize free radicals by attaching antiaromatic groups. This method enhances radical stability by relieving antiaromaticity, offering a new approach for radical stabilization.

Keywords:
anti-aromaticityfree radicalsstereoelectronic effectszwitterions

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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
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Area of Science:

  • Organic Chemistry
  • Physical Chemistry

Background:

  • Free radicals are highly reactive species crucial in many chemical processes.
  • Stabilizing free radicals is essential for controlling reactions and developing new materials.
  • Current stabilization methods often involve steric hindrance or electronic donation, with limited success for highly reactive radicals.

Purpose of the Study:

  • To introduce and investigate a new electronic strategy for stabilizing free radicals.
  • To explore the role of antiaromatic substituents in radical stabilization.
  • To understand the mechanism of antiaromaticity relief in stabilizing radical species.

Main Methods:

  • Conjugation of formally antiaromatic substituents to the free radical core.
  • Utilizing X-ray crystallography to determine molecular structures.
  • Employing Variable Temperature Electron Paramagnetic Resonance (VT-EPR) and Variable Temperature Ultraviolet-Visible (VT-UV/Vis) spectroscopy.
  • Performing computational analysis to elucidate electronic structures and mechanisms.

Main Results:

  • The strategy successfully stabilizes free radicals by utilizing antiaromatic substituents as electron sinks.
  • Configuration mixing with a low-energy zwitterionic state provides antiaromaticity relief to the substituent.
  • Stabilization is effective only when the antiaromatic substituent is conformationally constrained to be planar, enabling state mixing.
  • Increased antiaromaticity of the substituent correlated with enhanced radical stability.

Conclusions:

  • A novel stereoelectronic approach for stabilizing free radicals has been demonstrated.
  • The electronic stabilization of radicals via antiaromaticity relief is feasible under specific conformational constraints.
  • This work provides a new paradigm for designing and synthesizing stable radical species.