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Title: Halogenation  
Author: World Heritage Encyclopedia
Language: English
Subject: Halogenated ether, Halomethane, Methane, Ethylene, Toluene
Collection: Halogens, Inorganic Reactions, Organic Reactions
Publisher: World Heritage Encyclopedia


Halogenation is a

  1. ^ Yoel Sasson "Formation of Carbon–Halogen Bonds (Cl, Br, I)" in Patai's Chemistry of Functional Groups, 2009, Wiley-VCH, Weinheim. doi:10.1002/9780470682531.pat0011
  2. ^ a b M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. doi:10.1002/14356007.a06_233.pub2
  3. ^ T. Mori, R. Rathore (1998). "X-Ray structure of bridged 2,2'-bi(adamant-2-ylidene) chloronium cation and comparison of its reactivity with a singly bonded chloroarenium cation".  
  4. ^ Illustrative procedure for chlorination of an aromatic compound: Edward R. Atkinson, Donald M. Murphy, and James E. Lufkin (1951). "dl-4,4',6,6'-Tetrachlorodiphenic Acid".  
  5. ^ Marshall, K. A. 2003. Chlorocarbons and Chlorohydrocarbons, Survey. Kirk-Othmer Encyclopedia of Chemical Technology
  6. ^ "Oxy | Oxychlorination." Oxy | Oxychlorination. N.p., n.d. Web. 10 Oct. 2012. [2]
  7. ^ Synthesis of essential drugs, Ruben Vardanyan, Victor Hruby; Elsevier 2005 ISBN 0-444-52166-6
  8. ^ Gordon W. Gribble “The diversity of naturally occurring organobromine compounds” Chemical Society Reviews, 1999, volume 28, pages 335 – 346.doi:10.1039/a900201d
  9. ^  
  10. ^ A Citizen’s Guide to Chemical Dehalogenation by the EPA


See also

Chemical dehalogenation is a treatment to remove halogens from harmful chemicals or contaminated areas by making them less toxic. There are two types of dehalogenation: glycolate dehalogenation and base-catalyzed decomposition [10]

Chemical dehalogenation

All elements aside from argon, neon, and helium form fluorides by direct reaction with phosphorus trichloride and sulfur monochloride.[9]

Inorganic chemistry

Iodine is the least reactive halogen and is reluctant to react with most organic compounds. The addition of iodine to alkenes is the basis of the analytical method called the iodine number, a measure of the degree of unsaturation for fats. The iodoform reaction involves degradation of methyl ketones.


Organobromine compounds are the most common organohalides in nature. Their formation is catalyzed by the enzyme bromoperoxidase. The oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually.[8]

Halothane synthesis

Bromination is more selective than chlorination because the reaction is less exothermic. Most commonly bromination is conducted by the addition of Br2 to alkenes. Bromination of saturated hydrocarbons and aromatic substrates is common in nature, giving rise to a host of halothane from trichloroethylene:[7]


The HCl from this cracking process is recycled by oxychlorination. The fact that the reaction is self-supplied is one of the reasons that industry uses oxychlorination instead of direct chlorination.[6]

ClCH2CH2Cl → CH2=CHCl + HCl
2 HCl + CH2=CH2 + ½ O2 → ClCH2CH2Cl + H2O

Oxychlorination is of special importance in the making of 1,2-dichloroethane, which is then converted into vinyl chloride. As can be seen in the following reaction, 1,2-dichloroethane is cracked:

½ O2 + 2 CuCl → CuOCuCl2
2 HCl + CuOCuCl2 → 2 CuCl2 + H2O

The copper(II) chloride is regenerated by sequential reactions of the cuprous chloride with oxygen and then hydrogen chloride:

CH2=CH2 + 2 CuCl2 → 2 CuCl + ClH2C-CH2Cl

The reaction is initiated by copper(II) chloride (CuCl2), which is the most common catalyst in the production of 1,2-dichloroethane. In some cases, CuCl2 is supported on silica in presence of KCl, LaCl3, or AlCl3 as cocatalysts. Aside from silica, a variety of supports have also been used including various types of alumina, diatomaceous earth, or pumice. Because this reaction is highly exothermic (238 kJ/mol), the temperature is monitored, to guard against thermal degradation of the catalyst. Since the CuCl2 molecule is the donor of the chloride atom, this catalyst has an important role in the formation of the hydrocarbon double bond of chlorination. The reaction is as follows:

CH2=CH2 + 2 HCl + ½ O2 → ClCH2CH2Cl + H2O

Oxychlorination is the process of chlorinating hydrocarbons, using a mixture of hydrogen chloride (HCl) and oxygen (O2). This process is attractive industrially, because hydrogen chloride is less expensive than chlorine.[5] The most common substrate for this reaction is ethylene:


Chlorination is generally highly exothermic. Both saturated and unsaturated compounds react directly with chlorine, the former usually requiring UV light to initiate homolysis of chlorine. Chlorination is conducted on a large scale industrially; major processes include routes to 1,2-dichloroethane (a precursor to PVC), as well as various chlorinated ethanes, as solvents. Competitive with direct chlorination (use of Cl2) is oxychlorination, which uses hydrogen chloride in combination with oxygen.[2]


Organic compounds, saturated and unsaturated alike, react readily, usually explosively, with fluorine. This process requires highly specialized conditions. In practice, organic compounds are fluorinated electrochemically. Reactions occur at an anode using hydrogen fluoride as the source of fluorine. The method is called electrochemical fluorination. Aside from F2 and its electrochemically generated equivalent, a variety of fluorinating reagents are known such as xenon difluoride and cobalt(III) fluoride.


Halogenation of organic compounds: classification by halogen

In the Hell–Volhard–Zelinsky halogenation, carboxylic acids are alpha-halogenated.

The Sandmeyer reaction is used to give aryl halides from diazonium salts, which are obtained from anilines.

RCO2Ag + Br2 → RBr + CO2 + AgBr

In the Hunsdiecker reaction, from carboxylic acids are converted to the chain-shortened halide. The carboxylic acid is first converted to its silver salt, which is then oxidized with halogen:

Other halogenation methods

The facility of halogenation is influenced by the halogen. Fluorine and chlorine are more electrophilic and are more aggressive halogenating agents. Bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is least reactive of them all. The facility of hydrogenolysis follows the reverse trend: iodine is most easily removed from organic compounds and organofluorine compounds are highly stable.

RC6H5 + X2 → HX + RC6H4X

Aromatic compounds are subject to electrophilic halogenation:[4]

Halogenation of aromatic compounds

Structure of a bromonium ion

The addition of halogens to alkenes proceeds via intermediate halonium ions. In special cases, such intermediates have been isolated.[3]


Unsaturated compounds, especially alkenes and alkynes, add halogens:

Addition of halogens to alkenes and alkynes

CH4 + Cl2 → CH3Cl + HCl

Saturated hydrocarbons typically do not add halogens but undergo free radical halogenation, involving substitution of hydrogen atoms by halogen. The regiochemistry of the halogenation of alkanes is usually determined by the relative weakness of the available C-H bonds. The preference for reaction at tertiary and secondary positions results from greater stability of the corresponding free radicals and the transition state leading to them. Free radical halogenation is used for the industrial production of chlorinated methanes:[2]

Free radical halogenation

There are several processes for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen addition reaction. The determining factors are the functional groups.

Halogenation of organic compounds: Kinds of reactions


  • Halogenation of organic compounds: Kinds of reactions 1
    • Free radical halogenation 1.1
    • Addition of halogens to alkenes and alkynes 1.2
    • Halogenation of aromatic compounds 1.3
    • Other halogenation methods 1.4
  • Halogenation of organic compounds: classification by halogen 2
    • Fluorination 2.1
    • Chlorination 2.2
      • Oxychlorination 2.2.1
    • Bromination 2.3
    • Iodination 2.4
  • Inorganic chemistry 3
  • Chemical dehalogenation 4
  • See also 5
  • References 6

of halogenation depends on the structural features and functional groups of the organic substrate, as well as on the specific halogen. Inorganic compounds such as metals also undergo halogenation. stoichiometry The pathway and [1]

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