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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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Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
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ortho–para-Directing Deactivators: Halogens01:24

ortho–para-Directing Deactivators: Halogens

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Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate...
6.5K
Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN101:14

Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1

2.6K
Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo,...
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Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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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.
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Directing Effect of Substituents: ortho–para-Directing Groups01:14

Directing Effect of Substituents: ortho–para-Directing Groups

8.1K
Ortho–para directors are substituent groups attached to the benzene ring and direct the addition of an electrophile to the positions ortho or para to the substituent. All electron-donating groups are considered ortho–para directors. They donate electrons to the ring and make the ring more electron-rich. The ring is therefore susceptible to the addition of electrophiles. Substituents such as amino, hydroxy, or alkoxy, containing lone pairs on the atom adjacent to the ring, donate...
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Fluoreno[2,1-a]fluorene: an ortho-naphthoquinodimethane-based system with partial diradical character.

Allison S Hacker1, Mauricio Pavano1, James E Wood1

  • 1Department of Chemistry and Biochemistry, California Polytechnic State University, 1 Grand Avenue, San Luis Obispo, CA 93407, USA. dfrantz@calpoly.edu.

Chemical Communications (Cambridge, England)
|November 9, 2019
PubMed
Summary

Researchers synthesized and studied Fluoreno[2,1-a]fluorene, a novel ortho-naphthoquinodimethane. This molecule shows an intermediate diradical character, unlike related para- and meta-quinodimethanes.

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Area of Science:

  • Organic Chemistry
  • Materials Science
  • Computational Chemistry

Background:

  • Para-quinodimethanes (2,6-substitution) are closed-shell species.
  • Meta-quinodimethanes (2,7-substitution) exhibit high diradical character.
  • The electronic properties of ortho-naphthoquinodimethanes remain less explored.

Purpose of the Study:

  • To synthesize and characterize a novel 1,5-substituted ortho-naphthoquinodimethane system, Fluoreno[2,1-a]fluorene.
  • To investigate its electronic and magnetic properties.
  • To compare its diradical character with related para- and meta-quinodimethanes.

Main Methods:

  • Spectroscopy: UV-Vis-NIR, 1H-NMR, Raman spectroscopy.
  • Magnetic measurements: SQUID magnetometry.
  • Electrochemical analysis: Spectroelectrochemistry.
  • Theoretical calculations: Quantum chemistry.

Main Results:

  • Successful synthesis and characterization of Fluoreno[2,1-a]fluorene.
  • Demonstration of an intermediate degree of diradical character in the 1,5-substituted ortho-naphthoquinodimethane system.
  • Distinct electronic behavior compared to 2,6-para- and 2,7-meta-quinodimethanes.

Conclusions:

  • The 1,5-substitution pattern in ortho-naphthoquinodimethanes leads to unique electronic properties.
  • Fluoreno[2,1-a]fluorene represents a new class of organic molecules with tunable diradical character.
  • This study expands the understanding of structure-property relationships in quinodimethane systems.