Balz–Schiemann reaction

The reaction is conceptually similar to the Sandmeyer reaction, which converts diazonium salts to other aryl halides (ArCl, ArBr). However, while the Sandmeyer reaction involves a copper reagent/catalyst and radical intermediates, 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.

Innovations

The traditional Balz–Schiemann reaction employs HBF₄ and involves isolation of the diazonium salt. Both aspects can be profitably modified. Other counterions have been used in place of tetrafluoroborates, such as hexafluorophosphates (PF₆⁻) and hexafluoroantimonates (SbF₆⁻) with improved yields for some substrates. The diazotization reaction can be effected with nitrosonium salts such as [NO]SbF₆ or tert-butyl nitrite without isolation of the diazonium intermediate. The reaction has also performed under continuous flow conditions, allowing the potentially hazardous aryl diazonium salt to be prepared and immediately consumed inline without accumulating or isolating it.

As a practical matter, the traditional Balz–Schiemann reaction consumes relatively expensive BF₄⁻ as a source of fluoride. An alternative methodology produces the fluoride salt of the diazonium compound. In this implementation, the diazotization is conducted with a solution of sodium nitrite in liquid hydrogen fluoride:

ArNH₂ + 2 HF + NaNO₂ → [ArN₂]F + NaF + 2 H₂O

[ArN₂]F → ArF + N₂