Halex process

In chemistry, the Halex process is used to convert aromatic chlorides to the corresponding aromatic fluorides. The process entails Halide exchange, hence the name.^[1] The reaction conditions call for hot (150-250 °C) solution of the aryl chloride and anhydrous potassium fluoride. Typical solvents are dimethylsulfoxide, dimethylformamide, and sulfolane. Potassium chloride is generated in the process. The reaction is mainly applied to nitro-substituted aryl chlorides. Sometimes more soluble fluorides, such as caesium fluoride and TBAF are used.^[2]

The following reactions are practiced commercially in this manner:^[3]

2-nitrochlorobenzene → 2-fluoronitrobenzene

4-nitrochlorobenzene → 1-fluoro-4-nitrobenzene

1,2-dichloronitrobenzene → 1-chloro-2-fluoro-5-nitrobenzene

1,4-dichloronitrobenzene → 1-chloro-4-fluoro-3-nitrobenzene

1-chloro-2,4-dinitrobenzene → 1-fluoro-2,4-dinitrobenzene

5-chloro-2-nitrobenzotrifluoride → 5-fluoro-2-nitrobenzotrifluoride

1,3-dichloro-4-nitrobenzene → 1,3-difluoro-4-nitrobenzene

2,6-dichlorobenzonitrile → 2,6-difluorobenzonitrile

The nitro groups in the above compounds can be reduced to give the corresponding aniline. For example, selective hydrogenation of 4-fluoronitrobenzene gives 4-fluoroaniline. Thus, the Halex method provides access to a host of fluoroanilines.

For producing aryl fluorides, the Halex process is complementary to the Balz-Schiemann reaction and its variants.