Halogenation is a chemical reaction that involves the reaction of a compound with a halogen and results in the halogen being added to the compound. Organic compounds undergo halogenation much more readily than inorganic compounds. Dehalogenation is the reverse of halogenation and results in the removal of a halogen from a molecule. The pathway and stoichiometry 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.
- 1 Halogenation of organic compounds: Kinds of reactions
- 2 Halogenation of organic compounds: classification by halogen
- 3 Inorganic chemistry
- 4 Chemical dehalogenation
- 5 See also
- 6 References
Halogenation of organic compounds: Kinds of reactions
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.
Free radical halogenation
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:
- CH4 + Cl2 → CH3Cl + HCl
Addition of halogens to alkenes and alkynes
- RCH=CHR' + X2 → RCHX-CHXR'
Halogenation of aromatic compounds
Aromatic compounds are subject to electrophilic halogenation:
- RC6H5 + X2 → HX + RC6H4X
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.
Other halogenation methods
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:
- RCO2Ag + Br2 → RBr + CO2 + AgBr
In the Hell–Volhard–Zelinsky halogenation, carboxylic acids are alpha-halogenated.
Halogenation of organic compounds: classification by halogen
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.
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.
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. The most common substrate for this reaction is ethylene:
- CH2=CH2 + 2 HCl + ½ O2 → ClCH2CH2Cl + H2O
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 CuCl2 → 2 CuCl + ClH2C-CH2Cl
The copper(II) chloride is regenerated by sequential reactions of the cuprous chloride with oxygen and then hydrogen chloride:
- ½ O2 + 2 CuCl → CuOCuCl2
- 2 HCl + CuOCuCl2 → 2 CuCl2 + 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:
- ClCH2CH2Cl → CH2=CHCl + HCl
- 2 HCl + CH2=CH2 + ½ O2 → ClCH2CH2Cl + H2O
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.
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 organobromine compounds. The usual catalyst is the bromoperoxidase which utilizes bromide in combination with oxygen as an oxidant. An example of bromination can be found in the organic synthesis of the anesthetic halothane from trichloroethylene:
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.
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.
All elements aside from argon, neon, and helium form fluorides by direct reaction with fluorine. Chlorine is slightly more selective, but still reacts with most metals and heavier nonmetals. Following the usual trend, bromine is less reactive and iodine least of all. Of the many reactions possible, illustrative is the formation of gold(III) chloride by the chlorination of gold. The chlorination of metals is usually not very important industrially since the chlorides are more easily made from the oxides and the hydrogen halide. Where chlorination of inorganic compounds is practiced on a relatively large scale is for the production of phosphorus trichloride and sulfur monochloride.
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 
|Wikiquote has quotations related to: Halogenation|
- Haloalkane (Alkyl halide)
- Halogenoarene (Aryl halide)
- Free radical halogenation
- Electrophilic substitution
- 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
- M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. doi:10.1002/14356007.a06_233.pub2
- 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". Chem. Commun. (8): 927–928. doi:10.1039/a709063c.
- 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". Org. Synth.; Coll. Vol. 4, p. 872
- Marshall, K. A. 2003. Chlorocarbons and Chlorohydrocarbons, Survey. Kirk-Othmer Encyclopedia of Chemical Technology
- "Oxy | Oxychlorination." Oxy | Oxychlorination. N.p., n.d. Web. 10 Oct. 2012. 
- Synthesis of essential drugs, Ruben Vardanyan, Victor Hruby; Elsevier 2005 ISBN 0-444-52166-6
- Gordon W. Gribble “The diversity of naturally occurring organobromine compounds” Chemical Society Reviews, 1999, volume 28, pages 335 – 346.doi:10.1039/a900201d
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
- A Citizen’s Guide to Chemical Dehalogenation by the EPA