Balz–Schiemann reaction
# Balz–Schiemann reaction

The Balz–Schiemann reaction (also called the Schiemann reaction) is a chemical reaction in which a primary aromatic amine is transformed to an aryl fluoride via a diazonium tetrafluoroborate intermediate. This reaction is a traditional route to fluorobenzene and some related derivatives, including 4-fluorobenzoic acid.

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 $BF_4^−$ to give the fluoroarene (ArF), along with boron trifluoride as the byproduct. Also Nitrogen is the by product.

## Innovations

The traditional Balz–Schiemann reaction employs $HBF_4$ 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 $(PF_6^−)$ and hexafluoroantimonates $(SbF_6^−)$ with improved yields for some substrates. The diazotization reaction can be effected with nitrosonium salts such as $[NO]SbF_6$ without isolation of the diazonium intermediate.

As a practical matter, the traditional Balz–Schiemann reaction consumes relatively expensive $BF_4^-$ 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:

    $ArNH_2$ + 2 HF + $NaNO_2$ → $[ArN_2]F$ + NaF + 2 $H_2O$
    $[ArN_2]F$ → ArF + $N_2$

## History

The reaction is named after the German chemists Günther Schiemann [de] and Günther Balz.

## Additional literature

* Roe A (1949). "Preparation of Aromatic Fluorine Compounds from Diazonium Fluoborates". Org. React. 5: 193. doi:10.1002/0471264180.or005.04. ISBN 0471264180.
* Becker H. G. O., Israel G. (1978). "Ionenpaareffekte bei der Photolyse und Thermolyse von Aryldiazonium-tetrafluoroboraten". J. Prakt. Chem. 321 (4): 579–586. doi:10.1002/prac.19793210410.
