In
chemistry, halogenation is a
chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...
that entails the introduction of one or more
halogens into a
compound.
Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, drugs. This kind of conversion is in fact so common that a comprehensive overview is challenging. This article mainly deals with halogenation using elemental halogens (F
2, Cl
2, Br
2, I
2). Halides are also commonly introduced using salts of the halides and halogen acids. Many specialized reagents exist for and introducing halogens into diverse substrates, e.g.
thionyl chloride.
Organic chemistry
Several pathways exist for the halogenation of organic compounds, including
free radical halogenation,
ketone halogenation,
electrophilic halogenation
In organic chemistry, an electrophilic aromatic halogenation is a type of electrophilic aromatic substitution. This organic reaction is typical of aromatic compounds and a very useful method for adding substituents to an aromatic system.
:
A few ...
, and
halogen addition reaction. The nature of the substrate determines the pathway. The facility of halogenation is influenced by the halogen.
Fluorine and
chlorine
Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...
are more
electrophilic and are more aggressive halogenating agents.
Bromine
Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simil ...
is a weaker halogenating agent than both fluorine and chlorine, while
iodine
Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
is the least reactive of them all. The facility of dehydrohalogenation follows the reverse trend: iodine is most easily removed from organic compounds, and organofluorine compounds are highly stable.
Free radical halogenation
Halogenation of saturated hydrocarbons is a substitution reaction. The reaction typically requires free radical pathways. The regiochemistry of the halogenation of alkanes is largely determined by the relative weakness of the C–H bonds. This trend is reflected by the faster reaction at tertiary and secondary positions.
Fluorinations with elemental fluorine () are particularly exothermic, so much so that highly specialised conditions and apparatus are required. The method
electrochemical fluorination generates small amounts of elemental fluorine in situ from
hydrogen fluoride. The method avoid the hazards of handling fluorine gas. Many commercially important organic compounds are fluorinated using this technology. Aside from and its electrochemically generated equivalent,
cobalt(III) fluoride is used as sources of fluorine radicals.
Free radical chlorination is used for the industrial production of some solvents:
:
CH4 + Cl2 -> CH3Cl + HCl
Naturally-occurring organobromine compounds are usually produced by free-radical pathway catalyzed by the enzyme
bromoperoxidase. The reaction requires bromide in combination with oxygen as an oxidant. The oceans are estimated to release 1–2 million tons of bromoform and 56,000 tons of bromomethane annually.
The
iodoform reaction, which involves degradation of methyl ketones, proceeds by the free-radical iodination.
Addition of halogens to alkenes and alkynes
Unsaturated compounds, especially
alkenes
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
and
alkynes, ''add'' halogens:
:
RCH=CHR' + X2 -> RCHX-CHXR'
In
oxychlorination, the combination of hydrogen chloride and oxygen serves as the equivalent of chlorine, as illustrated by this route to dichloroethane:
:
2 HCl + CH2=CH2 + 1/2 O2 -> ClCH2CH2Cl + H2O
The addition of halogens to alkenes proceeds via intermediate
halonium ions. In special cases, such intermediates have been isolated.
:
Bromination is more
selective than chlorination because the reaction is less
exothermic. Illustrative of the bromination of an alkene is the route to the anesthetic
halothane
Halothane, sold under the brand name Fluothane among others, is a general anaesthetic. It can be used to induce or maintain anaesthesia. One of its benefits is that it does not increase the production of saliva, which can be particularly useful i ...
from
trichloroethylene:
:
Iodination can be effected by the addition of iodine to alkenes. The reaction, which conveniently proceeds with the discharge of the color of , is the basis of the analytical method called the
iodine number, which is used to measure the
degree of unsaturation for
fats.
Halogenation of aromatic compounds
Aromatic compounds are subject to electrophilic halogenation:
:
RC6H5 + X2 -> HX + RC6H4X
This kind of reaction typically works well for chlorine and bromine. Often a
Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
ic catalyst is used, such as
ferric bromide.
Because
fluorine is so reactive, other methods, such as the
Balz–Schiemann reaction, must be used to prepare fluorinated aromatic compounds. Iodinations can be conducted with
hydrogen iodide in the presence of an
oxidising agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...
that generates ''in situ''.
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
Inorganic chemistry
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
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
. 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.
See also
*
Dehalogenation
*
Haloalkane
The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely ...
(Alkyl halide)
*
Halogenoarene (Aryl halide)
*
Free radical halogenation
*
Haloketone
In organic chemistry, an α-haloketone is a functional group consisting of a ketone group or more generally a carbonyl group with an α- halogen substituent. α-haloketones are alkylating agents. Prominent α-haloketones include phenacyl bromid ...
*
Electrophilic substitution
Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a compound, which is typically, but not always, aromatic. Aromatic substitution reactions are characteristic of aromatic compounds ...
References
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Halogenation reactions
Organic reactions
Inorganic reactions
Halogens