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Concept
Aryl halide
Summary
In organic chemistry, an aryl halide (also known as haloarene) is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. The haloarene are different from haloalkanes because they exhibit many differences in methods of preparation and properties. The most important members are the aryl chlorides, but the class of compounds is so broad that there are many derivatives and applications.
Aryl chlorides are the aryl halides produced on the largest scale commercially: 150,000 tons/y in the US alone (1994). Production levels are decreasing owing to environmental concerns. Chlorobenzenes are used mainly as solvents.
Friedel-Crafts halogenation or "direct chlorination" is the main synthesis route. Lewis acids, e.g. iron(III) chloride, catalyze the reactions. The most abundantly produced aryl halide, chlorobenzene, is produced by this route:
Monochlorination of benzene is accompanied by formation of the dichlorobenzene derivatives. Arenes with electron donating groups react with halogens even in the absence of Lewis acids. For example, phenols and anilines react quickly with chlorine and bromine water to give multihalogenated products. Many detailed laboratory procedures are available. For alkylbenzene derivatives, e.g. toluene, the alkyl positions tend to be halogenated by free radical conditions, whereas ring halogenation is favored in the presence of Lewis acids. The decolouration of bromine water by electron-rich arenes is used in the bromine test.
The oxychlorination of benzene has been well investigated, motivated by the avoidance of HCl as a coproduct in the direct halogenation:
This technology is not widely used however.
The Gatterman reaction can also be used to convert diazonium salts to chlorobenzenes using using copper-based reagents. Owing to high cost of diazonium salts, this method is reserved for specialty chlorides.
The main aryl bromides produced commercially are tetrabromophthalic anhydride, decabromodiphenyl ether, and tetrabromobisphenol-A.
Official source
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