[1][2][3] This reaction is a traditional route to fluorobenzene and some related derivatives,[4] including 4-fluorobenzoic acid.
[6] However, while the Sandmeyer reaction involves a copper reagent/catalyst and radical intermediates,[7] the thermal decomposition of the diazonium tetrafluoroborate proceeds without a promoter and is believed to generate highly unstable aryl cations (Ar+), which abstract F− from BF4− to give the fluoroarene (ArF), along with boron trifluoride and nitrogen as the byproducts.
Other counterions have been used in place of tetrafluoroborates, such as hexafluorophosphates (PF6−) and hexafluoroantimonates (SbF6−) with improved yields for some substrates.
[8][9] The diazotization reaction can be effected with nitrosonium salts such as [NO]SbF6 without isolation of the diazonium intermediate.
[2] As a practical matter, the traditional Balz–Schiemann reaction consumes relatively expensive BF4− as a source of fluoride.