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Related Concept Videos

Electrophilic Aromatic Substitution: Chlorination and Bromination of Benzene01:15

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...
Halogenation of Alkenes02:46

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.
Reactions at the Benzylic Position: Halogenation01:11

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.
Radical Substitution: Halogenation of Alkanes and Alkyl Substituents01:27

Radical Substitution: Halogenation of Alkanes and Alkyl Substituents

In the presence of heat or light, alkanes react with molecular halogens to form alkyl halides by a substitution reaction called radical halogenation. This reaction has three steps: initiation, propagation, and termination, as seen in the radical chlorination of methane to produce methyl chloride.
In the initiation step of the reaction, the chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. Propagation occurs in two...
Formation of Halohydrin from Alkenes02:41

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.
Radical Substitution: Allylic Chlorination01:31

Radical Substitution: Allylic Chlorination

Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic...

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Related Experiment Video

Updated: May 18, 2026

Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides
07:50

Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides

Published on: May 26, 2019

Enzymatic chlorination and bromination.

Karl-Heinz van Pée1

  • 1Allgemeine Biochemie, TU Dresden, Dresden, Germany. karl-heinz.vanpee@chemie.tu-dresden.de

Methods in Enzymology
|October 5, 2012
PubMed
Summary

Enzymes called halogenases are crucial for creating halometabolites. Recent research details flavin-dependent and nonheme iron halogenases, advancing our understanding of halogenation mechanisms.

Area of Science:

  • Biochemistry
  • Enzymology
  • Metabolic Engineering

Background:

  • Halometabolite biosynthesis knowledge has grown significantly in 15 years.
  • Haloperoxidases were historically considered the primary halogenating enzymes, but direct links to biosynthesis remain unproven.
  • FADH(2)-dependent halogenases, identified in 1997, incorporate chloride and bromide into activated compounds via two-component systems.

Purpose of the Study:

  • To describe the detection, purification, characterization, and reaction mechanisms of key halogenating enzymes.
  • To elucidate the mechanisms of flavin-dependent and nonheme iron, α-ketoglutarate-, and O(2)-dependent halogenases.
  • To advance the understanding of halometabolite biosynthesis pathways.

Main Methods:

  • Enzyme purification and characterization.

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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species
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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

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Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides
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The Portable Chemical Sterilizer (PCS), D-FENS, and D-FEND ALL: Novel Chlorine Dioxide Decontamination Technologies for the Military
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A Two-Step Protocol for Umpolung Functionalization of Ketones Via Enolonium Species

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  • Structural elucidation of FADH(2)-dependent halogenases.
  • Investigation of reaction mechanisms, including substrate radical formation and hypohalous acid intermediates.

    • Main Results:

    • FADH(2)-dependent halogenases utilize a flavin reductase to provide FADH(2), facilitating electrophilic attack.
    • Nonheme iron, α-ketoglutarate-, and O(2)-dependent halogenases halogenate unactivated carbon atoms through substrate radical mechanisms.
    • These two enzyme classes, along with fluorinases, represent the major known types of halogenating enzymes.

    Conclusions:

    • Flavin-dependent and nonheme iron halogenases are the primary enzymes responsible for halometabolite biosynthesis.
    • Understanding their distinct reaction mechanisms provides critical insights into biochemical pathways.
    • Further research may uncover additional types of halogenating enzymes.