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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...

Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene

Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene

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.

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.

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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Electrophilic halogenations of propargyl alcohols: paths to α-haloenones, β-haloenones and mixed β,β-dihaloenones.

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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

Halogen bond-induced electrophilic aromatic halogenations.

Wanutcha Lorpaiboon¹, Pakorn Bovonsombat¹

¹Mahidol University International College, Mahidol University, Salaya, Nakorn Pathom 73170, Thailand. pakorn.bov@mahidol.ac.th.

Organic & Biomolecular Chemistry

|August 4, 2021

Summary

Lewis base catalysts enable a new paradigm in aromatic halogenation by forming halogen bonds. This review details the evolution from traditional acid activation to nucleophilic catalysis for halonium ion generation.

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Area of Science:

Organic Chemistry
Catalysis
Synthetic Methodology

Background:

Halogen bonds are increasingly utilized in organic synthesis, particularly for aromatic halogenation.
N-haloimides are common halonium ion sources, traditionally activated by Brønsted or Lewis acids.
A recent shift involves activation by nucleophilic Lewis base catalysts.

Purpose of the Study:

To review the evolution of activation modes for N-haloimides in aromatic halogenation.
To discuss the mechanistic evidence for halogen bond formation in nucleophilic catalysis.
To highlight the paradigm shift brought by Lewis base catalysis.

Main Methods:

Literature review of aromatic halogenation reactions.
Analysis of mechanistic studies on N-haloimide activation.
Discussion of catalyst motifs involved in halogen bond formation.

Main Results:

Two primary modes of N-haloimide activation exist: electrophilic and nucleophilic.
Lewis base catalysts, featuring motifs like sulfides and amines, form halogen bonds with N-haloimides.
This halogen bond formation is crucial for generating active halonium ions.

Conclusions:

Nucleophilic Lewis base catalysis represents a significant advancement in aromatic halogenation.
Understanding halogen bond formation is key to developing novel catalytic systems.
The review provides insights into the mechanistic underpinnings of this transformation.