The Schiemann reaction, also known as the Balz–Schiemann reaction, is a transformation in which an aromatic primary amine is converted into an aryl fluoride via a diazonium tetrafluoroborate intermediate under heating or photolytic conditions. The reaction was discovered in 1927 by German chemists Günther Schiemann and Günther Balz, and is named after them. It remains one of the classic methods for synthesizing aromatic fluorides.

Reagents: Tetrafluoroboric acid, sodium nitrite, hydrochloric acid
Reactants: Aromatic primary amines
Products: Aromatic fluorides
Reaction Type: Substitution reaction

Experimental Tips:

• Temperature control is critical during diazonium salt formation, as these salts are unstable and prone to decomposition.

• In addition to tetrafluoroborate, other fluorinating agents such as hexafluorophosphate (HPF₆⁻) and hexafluoroantimonate (SbF₆⁻) can be used. These reagents can influence reaction efficiency and product selectivity. Different fluorinating agents often lead to different side products.

• Performing the reaction photochemically (e.g., under UV light) can avoid side products caused by thermal decomposition and provides milder conditions, though appropriate light sources and solvents are required.

• Since nitrogen gas (N₂) is released during the reaction, proper ventilation of the reaction apparatus is essential.

Reaction Mechanism:
The Balz–Schiemann reaction likely proceeds via diazotization of an aromatic primary amine to form a diazonium ion. This ion then reacts with tetrafluoroboric acid to form the diazonium tetrafluoroborate salt, which undergoes thermal or photolytic decomposition. Upon decomposition, nitrogen gas is released, generating an aryl carbocation (Ar⁺). This carbocation subsequently captures a fluoride ion (F⁻) from the tetrafluoroborate anion (BF₄⁻), forming the aryl fluoride.

Original Reference:
Balz, Günther; Schiemann, Günther. Chemische Berichte (in German), 1927, 60 (5): 1186–1190.

Popular Citations:

• Hypervalent Iodine(III)-Catalyzed Balz–Schiemann Fluorination under Mild Conditions. Angewandte Chemie, 2018, 57(31), 9896–9900.

• Fluorodediazoniation in Ionic Liquid Solvents: New Life for the Balz–Schiemann Reaction. Journal of Fluorine Chemistry, 2001, 107(1), 31–34.