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.
References
edit- ^ Furuya, Takeru; Klein, Johannes E. M. N.; Ritter, Tobias (2010). "C–F Bond Formation for the Synthesis of Aryl Fluorides". Synthesis: 1804–1821. doi:10.1055/s-0029-1218742. PMC 2953275.
- ^ ACS Green Chemistry Halex Reaction
- ^ Siegemund, Günter; Schwertfeger, Werner; Feiring, Andrew; Smart, Bruce; Behr, Fred; Vogel, Herward; McKusick, Blaine (2002). "Fluorine Compounds, Organic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_349. ISBN 978-3527306732..