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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.

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.

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.

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...

Regioselectivity of Electrophilic Additions-Peroxide Effect

Regioselectivity of Electrophilic Additions-Peroxide Effect

In the presence of organic peroxides, the addition of hydrogen bromide to an alkene yields the isomer that is not predicted by Markovnikov’s rule. For example, the addition of hydrogen bromide to 2-methylpropene in the presence of peroxides gives 1-bromo-2-methylpropane. This addition reaction proceeds via a free radical mechanism, which reverses the regioselectivity. The free radical reaction mechanism involves three stages: initiation, propagation, and termination.

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Halogen-mediated electrochemical organic synthesis.

Hai-Tao Tang¹, Jun-Song Jia¹, Ying-Ming Pan¹

¹State Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry and Pharmaceutical Sciences of Guangxi Normal University, Guilin, 541004, People's Republic of China. panym@mailbox.gxnu.edu.cn.

Organic & Biomolecular Chemistry

|July 9, 2020

Summary

Halide anions undergo anodic oxidation to form reactive species like halogens or radicals. These species then react with various substrates, yielding halogenated products through well-defined reaction mechanisms.

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Area of Science:

Electrochemistry
Organic Chemistry
Reaction Mechanisms

Background:

Halide anions are common in chemical reactions.
Anodic oxidation is a key electrochemical process.
Halogenation reactions are important in synthesis.

Purpose of the Study:

To review the mechanisms of anodic oxidation of halide anions.
To elucidate the formation of active halogenating species.
To understand subsequent reactions with organic substrates.

Main Methods:

Literature review of electrochemical and organic reaction studies.
Analysis of reaction pathways and intermediates.
Compilation of known halogenation reactions.

Main Results:

Anodic oxidation generates elemental halogens, halogen cations, or radicals.
These active species react with olefins, ketones, and amines.
Diverse halogenated products are formed depending on the substrate and conditions.

Conclusions:

The anodic oxidation of halides provides a versatile route to halogenated compounds.
Understanding these mechanisms is crucial for synthetic applications.
This review consolidates knowledge on halide electrochemistry and halogenation.