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Halogenation of Alkenes

Halogenation of Alkenes

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.

Formation of Halohydrin from Alkenes

Formation of Halohydrin from Alkenes

An alkene, such as propene, reacts with bromine in the presence of water to yield a halohydrin. Halohydrins contain a halogen and a hydroxyl group attached to adjacent carbons. When the halogen is bromine, it is called a bromohydrin, while a chlorohydrin has chlorine as the halogen.

Alkyl Halides

Alkyl Halides

Structural Properties
Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon atom in an alkyl halide is bonded to the halogen atom, which is sp3-hybridized and exhibits a tetrahedral shape.
Unlike alkyl halides, compounds in which a halogen atom is bonded to an sp2 -hybridized carbon atom of a carbon-carbon double bond (C=C) are called vinyl halides. Whereas aryl...

Electrophilic Addition to Alkynes: Halogenation

Electrophilic Addition to Alkynes: Halogenation

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.

Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene

Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene

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...

Reactions at the Benzylic Position: Halogenation

Reactions at the Benzylic Position: Halogenation

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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From Molecules to Materials: Engineering New Ionic Liquid Crystals Through Halogen Bonding

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Published on: March 24, 2018

Stabilisation of Bromenium Ions in Macrocyclic Halogen Bond Complexes.

Andrew Docker¹, Heike Kuhn², Paul D Beer²

¹Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K.

Angewandte Chemie (International Ed. in English)

|October 1, 2024

Summary

Researchers developed macrocyclic ligands to stabilize highly reactive bromenium cations (Br+). This breakthrough enables the isolation of bench-stable bromenium nitrate and reveals new insights into halogen cation chemistry.

Keywords:

Bromonium Halenium Ions Halogen Bonding Halogen(I) Complexes Macrocyclic Effect

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Published on: May 26, 2019

Area of Science:

Inorganic Chemistry
Supramolecular Chemistry
Halogen Chemistry

Background:

Halenium ions (X+) are reactive intermediates in halogenation reactions.
Stabilizing these species is challenging, often achieved with bis-pyridine (Py) complexes.
Existing methods struggle with the inherent reactivity of halenium ions.

Purpose of the Study:

To investigate the macrocyclic stabilisation effect on halenium species.
To synthesize and characterize novel macrocyclic complexes of bromenium cations.
To explore the coordination behavior and reactivity of stabilized halenium ions.

Main Methods:

Synthesis of bis-pyridine macrocyclic ligands.
Complexation of bromenium cations with macrocyclic ligands.
Isolation and characterization of bromenium complexes, including X-ray crystallography.
Ligand exchange reactions to study solution-phase stabilization.

Main Results:

Demonstrated the first macrocyclic stabilisation effect for halenium species.
Successfully isolated a bench-stable bromenium cation (Br+) complex.
Provided structural insights by comparing Br(I) complexes with Ag(I) and Au(I) analogues.
Reported the first ligand exchange reactions for Br(I) complexes, showing solution-phase macrocycle effects.

Conclusions:

Macrocyclic ligands effectively stabilize bromenium cations via endotopic complexation.
This work provides a new strategy for handling reactive halogen cations.
The findings offer a deeper understanding of halenium ion coordination and reactivity.