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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.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation

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.

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.

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...

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Related Experiment Video

Updated: Apr 15, 2026

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

Halogen-bonded photoresponsive materials.

Marco Saccone¹, Gabriella Cavallo, Pierangelo Metrangolo

¹Department of Applied Physics, Aalto University, 13500, 00076, Aalto, Finland, marco.saccone@aalto.fi.

Topics in Current Chemistry

|March 28, 2015

Summary

This review explores supramolecular materials containing azobenzene, focusing on halogen bonding. These advanced materials offer unique or enhanced properties compared to traditional hydrogen-bonded systems.

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Area of Science:

Supramolecular Chemistry
Materials Science
Organic Chemistry

Background:

Azobenzene derivatives are widely used in stimuli-responsive materials.
Halogen bonding is an increasingly important non-covalent interaction for material design.
Supramolecular self-assembly offers precise control over material structure and function.

Purpose of the Study:

To review the current state of supramolecular materials incorporating azobenzene units, specifically those assembled via halogen bonding.
To highlight the advantages and unique characteristics of halogen-bonded supramolecular azobenzene materials.
To compare their performance with analogous hydrogen-bonded systems.

Main Methods:

Literature review of existing research on supramolecular azobenzene materials.
Analysis of material structures and properties based on the type of non-covalent interaction (halogen vs. hydrogen bonding).
Categorization of materials into polymeric, liquid crystalline, and crystalline species.

Main Results:

Demonstration of successful construction of azobenzene-containing supramolecular materials using halogen bonding.
Examples of polymeric, liquid crystalline, and crystalline materials exhibiting distinct properties.
Evidence of superior or unique performance of halogen-bonded materials compared to their hydrogen-bonded counterparts.

Conclusions:

Halogen bonding provides a powerful and versatile tool for designing advanced supramolecular azobenzene materials.
These materials offer distinct advantages and tunable properties for various applications.
Further exploration of halogen bonding in supramolecular chemistry is warranted.