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Halogenation of Alkenes02:46

Halogenation of Alkenes

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Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
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Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene01:13

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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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Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene01:15

Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene

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Chlorination and bromination are important classes of electrophilic aromatic substitutions, where benzene reacts with chlorine or bromine in the presence of a Lewis acid catalyst to give halogenated substitution products. A Lewis acid such as aluminium chloride or ferric chloride catalyzes the chlorination, and ferric bromide catalyzes the bromination reactions. During the bromination of alkenes, bromine polarizes and becomes electrophilic. However, in the bromination of benzene, the bromine...
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Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

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Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
9.9K
Reactions at the Benzylic Position: Halogenation01:11

Reactions at the Benzylic Position: Halogenation

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Benzylic halogenation takes place under conditions that favor radical reactions such as heat, light, or a free radical initiator like peroxide.
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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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Oxidation-induced σ-aromaticity in halogenated cycloalkanes.

Slavko Radenković1, Slađana Đorđević1

  • 1University of Kragujevac, Faculty of Science, P. O. Box 60, 34000 Kragujevac, Serbia. slavkoradenkovic@kg.ac.rs.

Physical Chemistry Chemical Physics : PCCP
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Summary

Neutral halogenated cycloalkanes can become σ-aromatic dicationic species via oxidation. These novel compounds exhibit significant σ-electron delocalization in halogen rings, resembling known aromatic systems.

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Area of Science:

  • * Computational chemistry
  • * Organic chemistry
  • * Quantum chemistry

Background:

  • * Investigating novel aromatic systems is crucial for understanding chemical bonding.
  • * Cyclic halogenated compounds offer unique structural and electronic properties.
  • * Exploring σ-aromaticity in non-traditional ring systems expands the definition of aromaticity.

Purpose of the Study:

  • * To investigate the potential for σ-aromaticity in dicationic halogenated cycloalkanes.
  • * To explore the conversion of neutral cycloalkanes into σ-aromatic dicationic species.
  • * To compare the electronic properties of these systems with known aromatic compounds.

Main Methods:

  • * Magnetically Induced Current Density (MICD) calculations.
  • * Electron Density of Delocalized Bonds (EDDB) analysis.
  • * Aromatic Stabilization Energy (ASE) calculations.

Main Results:

  • * Significant σ-electron delocalization was observed in the halogen atom rings of the studied dications.
  • * All applied aromaticity indices consistently supported the presence of σ-aromaticity.
  • * The electronic delocalization closely mirrors that of the C6I62+ system.

Conclusions:

  • * Nonaromatic, neutral halogenated cycloalkanes can be oxidized to form σ-aromatic dicationic species.
  • * The observed σ-aromaticity does not always adhere to Hückel's rule, suggesting complex electronic interactions.
  • * Results can be explained by selection rules governing molecular orbital transitions.