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Related Concept Videos

Radicals: Electronic Structure and Geometry01:07

Radicals: Electronic Structure and Geometry

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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is...
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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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Radical Reactivity: Electrophilic Radicals01:02

Radical Reactivity: Electrophilic Radicals

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Radicals adjacent to electron‐withdrawing groups are called electrophilic radicals. These radicals readily react with nucleophilic alkenes. For example, the malonate radical, in which the radical center is flanked by two electron‐withdrawing groups, reacts readily with butyl vinyl ether, which consists of an electron‐donating oxygen substituent. The reaction between electrophilic malonate radical and nucleophilic vinyl ether is favored because the radical has a...
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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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Radical Reactivity: Nucleophilic Radicals01:16

Radical Reactivity: Nucleophilic Radicals

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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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Radical Anti-Markovnikov Addition to Alkenes: Overview01:25

Radical Anti-Markovnikov Addition to Alkenes: Overview

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The addition of hydrogen bromide to alkenes in the presence of hydroperoxides or peroxides proceeds via an anti-Markovnikov pathway and yields alkyl bromides.
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Free Radicals in Chemical Biology: from Chemical Behavior to Biomarker Development
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Size-Matched Radical Multivalency.

Mark C Lipke1, Tao Cheng2, Yilei Wu1

  • 1Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.

Journal of the American Chemical Society
|February 8, 2017
PubMed
Summary
This summary is machine-generated.

Researchers developed a new molecular square host that selectively binds a specific diradical cyclophane guest through radical-pairing interactions. This creates a stable tetraradical complex, advancing molecular recognition in supramolecular chemistry.

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Area of Science:

  • Supramolecular Chemistry
  • Materials Science
  • Organic Chemistry

Background:

  • Persistent π-radicals, like methyl viologen radical cation (MV+•), exhibit weak radical-radical interactions.
  • These interactions are key in supramolecular chemistry, enabling the formation of stable host-guest complexes.
  • Previous work established a strong complex between MV+• and a cyclophane host [cyclobis(paraquat-p-phenylene)]2(+•) (CBPQT2(+•)).

Purpose of the Study:

  • To extend radical-pairing-based molecular recognition to a larger, square-shaped diradical host, [cyclobis(paraquat-4,4'-biphenylene)]2(+•) (MS2(+•)).
  • To evaluate the binding of isomeric diradical cyclophane guests with MS2(+•).
  • To characterize the resulting tetraradical complex and understand the factors governing its stability.

Main Methods:

  • UV-Vis-NIR spectroscopy to monitor complex formation and binding affinity.
  • Titration experiments and variable temperature spectroscopy (UV-Vis-NIR, EPR) to determine thermodynamic parameters.
  • Single-crystal X-ray diffraction and density functional theory (DFT) calculations for structural analysis.
  • Cyclic voltammetry to probe the electronic properties of the complex.

Main Results:

  • MS2(+•) selectively binds the meta-xylylene-linked diradical cyclophane (m-CBPQT2(+•)), forming a tetraradical complex [MS⊂m-CBPQT]4(+•).
  • The binding is driven by a favorable enthalpy change, though offset by an entropic penalty, resulting in an association constant (Ka = (1.12 ± 0.08) × 105 M-1) comparable to smaller systems.
  • Structural analyses reveal m-CBPQT2(+•) is ideally sized for MS2(+•), and the complex disrupts typical extended radical-pairing in crystal structures.
  • Cyclic voltammetry confirms stabilization of radical states within the complex.

Conclusions:

  • The study successfully demonstrates the extension of radical-pairing molecular recognition to a larger, square-shaped host.
  • The ideal size and electronic properties of m-CBPQT2(+•) enable strong binding within MS2(+•).
  • The findings provide insights into the design of novel supramolecular assemblies based on radical interactions.