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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.

Hydrogen Bonds

Hydrogen Bonds

A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another polar molecule, such as water (H2O), hydrogen fluoride (HF), or ammonia (NH3). The huge electronegativity difference between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for an N atom), combined with the very small size of an H atom...

Valence Bond Theory

Valence Bond Theory

Overview of Valence Bond Theory

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...

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.

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Related Experiment Video

Updated: Jun 23, 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

Halogen Bonding in Perfluoroalkyl Adsorption.

Lei Lu¹, Chongzheng Na¹

¹Department of Civil, Environmental and Construction Engineering, Texas Tech University, Lubbock, Texas 79409, United States.

|June 24, 2024

Summary

This summary is machine-generated.

This study reveals halogen bonding as the key mechanism for removing per- and polyfluoroalkyl substances (PFAS), like PFOS and PFOA, from water. This finding aids in designing better materials for PFAS water purification.

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Published on: April 18, 2019

Area of Science:

Environmental Chemistry
Materials Science
Water Treatment

Background:

Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants.
Adsorption is a key technology for PFAS removal from water.
A molecular-level mechanistic understanding for adsorbent selection is lacking.

Purpose of the Study:

To identify the primary molecular mechanism driving perfluoroalkyl substance (PFAS) adsorption.
To establish guidelines for the rational design and selection of PFAS adsorbents.
To explore the role of electrostatic interactions and noncovalent bonding in PFAS adsorption.

Main Methods:

A materiomic approach comparing electrostatic polarities of various materials.
Novel interpretation of the Freundlich isotherm to elucidate adsorption mechanisms.
Literature review of carbon, polymer, and mineral-based adsorbents for PFAS removal.

Main Results:

Halogen bonding identified as the dominant mechanism for perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) adsorption.
Materials with high π electron density, lone electron pairs, and negative charges show favorable adsorption.
Adsorption is consistent with Lewis acid-base interactions involving fluorine's σ-hole.

Conclusions:

Halogen bonding is a critical, previously unappreciated, noncovalent interaction in PFAS adsorption.
Understanding this mechanism facilitates the development of effective PFAS removal materials.
This research provides a molecular basis for selecting and designing superior PFAS adsorbents.