Halex process

{{Short description|Used to convert aromatic chlorides to the corresponding aromatic fluorides.}}

In chemistry, the Halex process is used to convert aromatic chlorides to the corresponding aromatic fluorides. The process entails Halide exchange, hence the name.{{cite journal|journal=Synthesis|year=2010|pages=1804–1821|doi=10.1055/s-0029-1218742|title=

C–F Bond Formation for the Synthesis of Aryl Fluorides|first1=Takeru|last1=Furuya|first2=Johannes E. M. N.|last2=Klein|first3=Tobias|last3=Ritter|pmc=2953275}} 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.[https://reagents.acsgcipr.org/reagent-guides/s-sub-n-sub-ar-solvents-and-reagents/list-of-reagents/halex-reaction/#:~:text=The%20Halex%20reaction%20is%20a,to%20make%20fluorinated%20fine%20chemicals. ACS Green Chemistry Halex Reaction]

The following reactions are practiced commercially in this manner:{{Ullmann|first1=Günter |last1=Siegemund|first2=Werner|last2=Schwertfeger|first3=Andrew|last3=Feiring|first4=Bruce|last4=Smart|first5=Fred|last5=Behr|first6=Herward|last6=Vogel|first7=Blaine |last7=McKusick|title=Fluorine Compounds, Organic|year=2002|doi=10.1002/14356007.a11_349}}.

: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.