Halogenoarene

In organic chemistry, a halogenoarene, haloarene, or aryl halide, is an organic compound in which a halogen atom is bonded to a carbon atom which is part of an aromatic ring. The haloarene are studied separately from haloalkanes because they exhibit many differences in methods of preparation and properties.

Preparation

There are two main preparatory routes for aryl halides.^[1] The first is the Friedel-Crafts halogenation. A Lewis acid such as iron(III) chloride or aluminium chloride are used.

The second main route is the Sandmeyer reaction. Anilines (aryl amines) are reduced to diazonium salts using sodium nitrite. The diazo compounds readiliy undergo nucleophilic attack by copper(I) chloride or halide to give a haloarene. Nitrogen gas is the leaving group, which makes this reaction very favorable. The similar Schiemann reaction uses the tetrafluoroborate anion as the fluoride donor; copper(I) fluoride 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.^[2]

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..^[2]

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.

Reaction between benzene and halogen to form an halogenobenzene

Reactions

A characteristic reaction of the halogenoarenes is nucleophilic aromatic substitution. 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.

References

1. ^ Boyd, Robert W.; Morrison, Robert (1992). Organic chemistry. Englewood Cliffs, N.J: Prentice Hall. pp. 947. ISBN 0-13-643669-2. 
2. ^ ^{a b} Boyd, Robert W.; Morrison, Robert (1992). Organic chemistry. Englewood Cliffs, N.J: Prentice Hall. pp. 565–570. ISBN 0-13-643669-2.