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Halogens

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. Astatine is a highly unstable radioactive element, so currently, most of its properties are unknown due to its short half-life. Tennessine is a synthetic element also predicted to be in this group.

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.

ortho–para-Directing Deactivators: Halogens

ortho–para-Directing Deactivators: Halogens

Halogens are ortho–para directors. They are more electronegative than carbon. Therefore, as ring substituents, they can withdraw electrons through the inductive effect and deactivate the aromatic ring towards electrophilic substitution. Halogens also have an electron-donating resonance effect on the ring, which influences the orientation of the incoming electrophile. If an electrophile attacks at the ortho or the para position, the halogen donates electrons and stabilizes the intermediate...

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...

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...

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Related Experiment Video

Updated: Dec 16, 2025

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

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

Published on: March 24, 2018

Halogenases: structures and functions.

Hannes Ludewig¹, Samuel Molyneux¹, Scarlet Ferrinho¹

¹School of Chemistry, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST, UK.

Current Opinion in Structural Biology

|July 4, 2020

Summary

This summary is machine-generated.

Over 5000 halogenated natural products are known, often from marine sources. Enzymatic halogenation offers precise molecular modification, making halogenases valuable tools for advanced chemical synthesis and discovery.

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Area of Science:

Biochemistry
Organic Chemistry
Natural Products Chemistry

Background:

Over 5000 halogenated natural products have been identified, with many originating from marine environments.
Halogen incorporation significantly alters molecular bioavailability and bioactivity.
Enzymatic halogenation enables late-stage functionalization via site-selective halogenation and cross-coupling reactions.

Purpose of the Study:

To review the diverse classes of discovered halogenases.
To examine halogenases from structural and mechanistic viewpoints.
To reflect on recent advancements in halogenase discovery and application.

Main Methods:

Literature review of halogenase classes.
Analysis of structural and mechanistic data.
Synthesis of recent findings in halogenase research.

Main Results:

Categorization of various halogenase classes.
Insights into the structural and mechanistic diversity of halogenases.
Highlighting of recent progress in the field of halogenase discovery.

Conclusions:

Halogenases are increasingly recognized as essential tools in synthetic chemistry.
Understanding halogenase structure and mechanism is key to their broader application.
Continued discovery and characterization of halogenases will drive innovation in natural product synthesis and drug discovery.