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.^[1][2][3] This reaction is a traditional route to fluorobenzene and some related derivatives,^[4] including 4-fluorobenzoic acid.^[5]

Balz-Schiemann reaction
Named after	Günther Balz Günther Schiemann
Reaction type	Substitution reaction
Identifiers
Organic Chemistry Portal	balz-schiemann-reaction
RSC ontology ID	RXNO:0000127

The reaction is conceptually similar to the Sandmeyer reaction, which converts diazonium salts to other aryl halides (ArCl, ArBr).^[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.

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.^[8][9] The diazotization reaction can be affected 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. 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:^[10]

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

[ArN₂]F → ArF + N₂

History

The reaction is named after the German chemists Günther Schiemann [de] and Günther Balz.^[1]

Examples

4-Fluorotoluene is made in ~89% yield by Balz–Schiemann reaction on p-toluidine.^[11] This is then used as a precursor for 4-fluorobenzaldehyde,^[12]

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. {{cite journal}}: ISBN / Date incompatibility (help)
• 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.