In organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

, a halogenoarene, haloarene, or aryl halide, is an organic compound in which a halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...

 atom is bonded to a carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom which is part of an aromatic ring. The haloarene are studied separately from haloalkane
The haloalkanes are a group of chemical compounds derived from alkanes containing one or more halogens. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially and, consequently, are known under many chemical and...

s because they exhibit many differences in methods of preparation and properties.


There are two main preparatory routes for aryl halides. The first is the Friedel-Crafts halogenation. A Lewis acid such as iron(III) chloride
Iron(III) chloride
Iron chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red...

 or aluminium chloride
Aluminium chloride
Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...

 are used.

The second main route is the Sandmeyer reaction
Sandmeyer reaction
The Sandmeyer reaction is a chemical reaction used to synthesize aryl halides from aryl diazonium salts. It is named after the Swiss chemist Traugott Sandmeyer....

. Aniline
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...

s (aryl amines) are reduced to diazonium salts using sodium nitrite
Sodium nitrite
Sodium nitrite is the inorganic compound with the chemical formula NaNO2. It is a white to slight yellowish crystalline powder that is very soluble in water and is hygroscopic...

. The diazo compounds readiliy undergo nucleophilic attack by copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

(I) chloride or halide to give a haloarene. Nitrogen gas is the leaving group, which makes this reaction very favorable. The similar Schiemann reaction
Schiemann reaction
The Schiemann reaction is a chemical reaction in which anilines are transformed to aryl fluorides via diazonium fluoroborates...

 uses the tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...

 anion as the fluoride donor; copper(I) fluoride
Copper(I) fluoride
Copper fluoride is an inorganic compound with the chemical formula CuF. Its existence is uncertain. It was reported in 1933 to have a sphalerite-type crystal structure. Modern textbooks state that CuF is not known, since fluorine is so electronegative that it will always oxidise copper to its +2...

 does not yield the desired product.

Direct halogenation of arenes are possible. In the presence of light or at high temperature, the alkyl positions tend to be halogenated first in the free radical halogenation. The benzylic carbon (Ph-C) tends to undergo free-radical halogenation first, due to the stabilization of the benzyl free radical by the aromatic ring. To halogenate the ring, Lewis acids are required, and light should be excluded to avoid the competing reaction.

Direct halogenation of an aromatic compound is easy, and doesn't require light or high temperature. The reagents used are X2/FeX3 where X is Cl or Br. Fluorine must be added via the diazonium salt path, then treated with HBF4 and heated..

Without Lewis acid catalysts, arenes with electron donating groups such as phenols and anilines may react with halogens. For example, phenols and anilines react quickly with bromine water to give multiply brominated benzene rings, which precipitate. The bromine water is decolorized as well. This is one possible reaction in the bromine test
Bromine test
The bromine test is a qualitative test for the presence of unsaturated C=C bonds and phenols.In this laboratory method, an unknown sample is treated with a small amount of elemental bromine — either as an aqueous solution, or as a solution in dichloromethane or carbon tetrachloride...



A characteristic reaction of the halogenoarenes is nucleophilic aromatic substitution
Nucleophilic aromatic substitution
right|300px|Aromatic nucleophilic substitutionA nucleophilic aromatic substitution is a substitution reaction in organic chemistry in which the nucleophile displaces a good leaving group, such as a halide, on an aromatic ring...

. This reaction can happen by two different methods: bimolecular displacement mechanism or elimination-addition mechanism. However, haloarenes rarely undergo nucleophilic substitution, except in high-energy industrial conditions.

Aryl halides react with magnesium metals in a free-radical reaction to give Grignard reagents, useful "Ar"reagents.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.