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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.

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.

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.

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...

Base-Promoted α-Halogenation of Aldehydes and Ketones

Base-Promoted α-Halogenation of Aldehydes and Ketones

α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base. The reaction begins with the abstraction of α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction...

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.

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Updated: Jun 6, 2025

Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes

Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes

Published on: September 12, 2018

Hypervalent iodine-mediated intramolecular alkene halocyclisation.

Charu Bansal¹, Oliver Ruggles¹, Albert C Rowett¹

¹University of Bristol, School of Chemistry, Bristol, BS8 1TS, UK.

Beilstein Journal of Organic Chemistry

|December 3, 2024

Summary

Hypervalent iodine (HVI) reagents offer a green and versatile approach to synthesizing complex molecules. This review details HVI-mediated halocyclisation reactions for creating diverse halogenated cyclic compounds.

Keywords:

cyclisation halogenation heterocycles hypervalent iodine oxidation

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Synthesis of Hypervalent Iodonium Alkynyl Triflates for the Application of Generating Cyanocarbenes

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A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

Published on: February 16, 2020

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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes

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Published on: September 12, 2018

Synthesis of Hypervalent Iodonium Alkynyl Triflates for the Application of Generating Cyanocarbenes

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Published on: September 8, 2013

A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

A Microwave-Assisted Direct Heteroarylation of Ketones Using Transition Metal Catalysis

Published on: February 16, 2020

Area of Science:

Organic Chemistry
Synthetic Chemistry
Green Chemistry

Background:

Hypervalent iodine (HVI) reagents are increasingly utilized in organic synthesis.
These reagents act as potent oxidants and electrophiles.
HVI reagents are favored for their low cost, non-toxicity, and environmental friendliness.

Purpose of the Study:

To review the literature on intramolecular HVI-mediated halocyclisation reactions.
To highlight the synthesis of halogenated cyclic compounds using HVI reagents.
To categorize examples based on halogens and nucleophiles.

Main Methods:

Literature survey of HVI-mediated halocyclisation reactions.
Organization of synthetic examples by halogen type (e.g., iodine, bromine, chlorine).
Classification of reactions based on the internal nucleophile (C, O, N, S).

Main Results:

Demonstrated versatility of HVI reagents in forming C-X bonds (X=halogen).
Successful synthesis of various halogenated cyclic structures via intramolecular reactions.
Compilation of numerous literature examples showcasing different reaction pathways.

Conclusions:

HVI reagents are effective tools for synthesizing halogenated cyclic compounds.
Intramolecular halocyclisation offers a powerful strategy for molecular construction.
The reviewed methods provide valuable insights for synthetic chemists seeking green methodologies.