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Radical Substitution: Allylic Bromination01:27

Radical Substitution: Allylic Bromination

5.2K
In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide...
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Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene01:13

Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene

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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.
6.3K
Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene01:15

Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene

8.5K
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...
8.5K
α-Bromination of Carboxylic Acids: Hell–Volhard–Zelinski Reaction01:15

α-Bromination of Carboxylic Acids: Hell–Volhard–Zelinski Reaction

3.1K
The method to achieve α-brominated carboxylic acids using a mixture of phosphorus tribromide and bromine is known as the Hell–Volhard–Zelinski reaction. The reaction is catalyzed by phosphorus tribromide, which can be used directly or produced in situ from red phosphorus and bromine. The mechanism comprises PBr3 catalyzed conversion of acid to acid bromide and hydrogen bromide. The acid bromide enolizes to its enol form in the presence of HBr. The nucleophilic enol attacks the...
3.1K
Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene01:14

Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene

2.6K
Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.
2.6K
Halogenation of Alkenes02:46

Halogenation of Alkenes

16.2K
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.
16.2K

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Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of PhosphorusI
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On the potential intermediacy of PhIBr2 as a brominating agent.

Tania1, Andrew Molino1, Lachlan Sharp-Bucknall1

  • 1Department of Biochemistry and Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne 3086, Australia. j.dutton@latrobe.edu.au.

Organic & Biomolecular Chemistry
|October 18, 2022
PubMed
Summary

This study challenges the existence of PhIBr2 in aryl brominations. Spectroscopic and computational evidence suggests that elemental bromine (Br2) is the active brominating agent, not the proposed intermediate.

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Area of Science:

  • Organic Chemistry
  • Synthetic Chemistry

Background:

  • Hypervalent iodine compounds are versatile reagents in organic synthesis.
  • PhIBr2 has been historically proposed but rarely observed due to instability.
  • A recent study suggested its in situ generation for aryl brominations.

Purpose of the Study:

  • To investigate and validate the proposed in situ synthesis of PhIBr2.
  • To determine the active species responsible for bromination in the described reaction.
  • To provide spectroscopic and computational evidence regarding the reaction mechanism.

Main Methods:

  • Replication of published aryl bromination reactions.
  • Spectroscopic analysis (e.g., NMR, IR) of reaction mixtures.
  • Computational chemistry calculations to model potential intermediates.

Main Results:

  • The reaction produced iodobenzene (PhI) and elemental bromine (Br2).
  • Spectroscopic and computational data did not support the formation of PhIBr2.
  • Elemental bromine (Br2) was identified as the species responsible for aryl bromination.

Conclusions:

  • The proposed in situ generation of PhIBr2 is not supported by experimental evidence.
  • Elemental bromine (Br2) is the active brominating agent in this reaction.
  • The role of PhIBr2 as an intermediate in these aryl brominations is unlikely.