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Related Concept Videos

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.
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.
Base-Promoted α-Halogenation of Aldehydes and Ketones00:51

Base-Promoted α-Halogenation of Aldehydes and Ketones

α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base.  The reaction begins with the abstraction of  α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction at the stage of...
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...
Radical Halogenation: Thermodynamics01:34

Radical Halogenation: Thermodynamics

The thermodynamic favorability of a reaction is determined by the change in Gibbs free energy (ΔG). ΔG has two components- enthalpy (ΔH) and entropy (ΔS). The entropy component is negligible for alkane halogenation because the number of reactants and product molecules are equal. In this case, the ΔG is governed only by the enthalpy component. The most crucial factor that determines ΔH is the strength of the bonds. ΔH can be determined by comparing the energy between bonds broken and bonds...

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Production and Measurement of Organic Particulate Matter in a Flow Tube Reactor
13:29

Production and Measurement of Organic Particulate Matter in a Flow Tube Reactor

Published on: December 15, 2018

Halogen production from aqueous tropospheric particles.

H Herrmann1, Z Majdik, B Ervens

  • 1Institut für Troposphärenforschung, Permoserstrasse 15, 04318 Leipzig, Germany. hermann@tropos.de

Chemosphere
|May 10, 2003
PubMed
Summary

This study reveals that aqueous phase chemistry significantly influences halogen activation in marine and urban clouds. Different chemical pathways in diluted droplets and sea-salt aerosols control halogen release, with radical reactions crucial in dilute conditions and non-radical reactions in concentrated aerosols.

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13:29

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Published on: December 15, 2018

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09:46

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Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions
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Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

Published on: October 5, 2019

Area of Science:

  • Atmospheric Chemistry
  • Environmental Science
  • Chemical Kinetics

Background:

  • Aqueous phase chemistry plays a critical role in atmospheric halogen activation.
  • Marine and urban clouds, along with marine aerosols, are key environments for these processes.
  • Understanding the distinct chemical pathways in diluted cloud droplets versus concentrated sea-salt aerosols is essential.

Purpose of the Study:

  • To investigate the role of aqueous phase chemistry in halogen activation for marine and urban clouds and marine aerosols.
  • To differentiate chemical pathways governing halogen activation in cloud droplets and sea-salt particles.
  • To develop and apply a comprehensive halogen module for simulating these multiphase processes.

Main Methods:

  • Utilized box model studies to simulate halogen activation.
  • Developed a halogen module incorporating gas and aqueous phase processes.
  • Coupled the halogen module with the Regional Atmospheric Chemistry Mechanism (RACM) and Chemical Aqueous Phase Radical Mechanism (CAPRAM 2.4).
  • Employed the resistance model for phase exchange calculations.

Main Results:

  • Radical chemistry (OH, NO(3)) dominates halogen activation in diluted cloud droplets, leading to pH-independent halogen release.
  • Non-radical reactions at high ionic strengths and low pH control chemistry in concentrated sea-salt aerosols.
  • Bromine atom formation is primarily OH-initiated (diurnal peak at noon), while chlorine atom levels are linked to NO(3) radical chemistry (smaller day-night variation).
  • Sea-salt aerosols act as a major source of halogen molecules (e.g., Cl(2), BrCl, Br(2)) for subsequent gas-phase photolysis.

Conclusions:

  • Aqueous phase chemistry dictates halogen activation mechanisms, differing significantly between diluted cloud droplets and concentrated sea-salt aerosols.
  • The developed multiphase model accurately simulates halogen activation, highlighting the importance of both radical and non-radical pathways.
  • Sea-salt aerosols are identified as a primary source of reactive halogens in the troposphere.