The Balz–Schiemann reaction (also called the Schiemann reaction) is a

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

in which a primary

aromatic amine In organic chemistry, an aromatic amine is an organic compound consisting of an aromatic ring attached to an amine. It is a broad class of compounds that encompasses aniline Aniline is an organic compound with the formula C6 H5 NH2. Consi ...

is transformed to an

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromaticity, aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar ...

fluoride via a

diazonium Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. General properti ...

tetrafluoroborate intermediate. This reaction is a traditional route to

fluorobenzene Fluorobenzene is the chemical compound with the formula C6H5F, often abbreviated PhF. A colorless liquid, it is a precursor to many fluorophenyl compounds. Preparation PhF was first reported in 1886 by O. Wallach at the University of Bonn, who ...

and some related derivatives, including 4-fluorobenzoic acid. : Balz-Schiemann Reaction

The reaction is conceptually similar to the

Sandmeyer reaction The Sandmeyer reaction is a chemical reaction used to synthesize aryl halides from aryl diazonium salts using copper salts as reagents or catalysts. It is an example of a radical-nucleophilic aromatic substitution. The Sandmeyer reaction provide ...

, which converts diazonium salts to other

aryl halide In organic chemistry, an aryl halide (also known as haloarene) is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. The haloarene are different from haloalkanes because they exhi ...

s (ArCl, ArBr). However, while the Sandmeyer reaction involves a copper reagent/catalyst and radical intermediates, the thermal decomposition of the diazonium

tetrafluoroborate Tetrafluoroborate is the anion . This tetrahedral species is isoelectronic with tetrafluoroberyllate (), tetrafluoromethane (CF4), and tetrafluoroammonium () and is valence isoelectronic with many stable and important species including the perchlo ...

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 as the byproduct.

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

hexafluorophosphate Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this an ...

s (PF₆⁻) and

hexafluoroantimonate Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...

s (SbF₆⁻) with improved yields for some substrates. The diazotization reaction can be effected with nitrosonium salts such as ObF₆ without isolation of the diazonium intermediate.

History

The reaction is named after the German chemists and Günther Balz.

Additional literature

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References

{{DEFAULTSORT:Schiemann Reaction Halogenation reactions Substitution reactions Name reactions