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

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

in which a primary

aromatic amine In organic chemistry, an aromatic amine is an organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be a ...

is transformed to an

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

fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...

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. The parent, compou ...

tetrafluoroborate intermediate. This reaction is a traditional route to

fluorobenzene Fluorobenzene is an aryl fluoride and the simplest of the fluorobenzenes, with the formula C6H5F, often abbreviated Phenyl group, PhF. A colorless liquid, it is a precursor to many fluorophenyl compounds. Preparation PhF was first reported in 18 ...

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

The reaction is conceptually similar to the Sandmeyer reaction, which converts diazonium salts to other

aryl halide In organic chemistry, an aryl halide (also known as a haloarene) is an aromatic compound in which one or more hydrogen atoms directly bonded to an aromatic ring are replaced by a halide ion (such as fluorine F''−'', chlorine Cl−1,−3,−5, br ...

s (ArCl, ArBr). However, while the Sandmeyer reaction involves a

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

reagent/catalyst and

radical Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...

intermediates, the

thermal decomposition Thermal decomposition, or thermolysis, is a chemical decomposition of a substance caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic ...

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 perc ...

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 Boron trifluoride is the inorganic compound with the formula . This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bonding The g ...

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

counterion 160px, cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion. In chemistry, a counterion (sometimes written as "counter ...

s have been used in place of tetrafluoroborates, such as

hexafluorophosphate Hexafluorophosphate is an fluoroanion, anion with chemical formula of . It is an Octahedral molecular geometry, octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the Hexafluorosilicic acid, h ...

s (PF₆⁻) and hexafluoroantimonates (SbF₆⁻) with improved yields for some substrates. The diazotization reaction can be affected with

nitrosonium The nitrosonium ion is , in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. It can be viewed as nitric oxide with one electron removed. This ion is usually o ...

salts such as ObF₆ without isolation of the diazonium intermediate. 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 Sodium nitrite is an inorganic compound with the chemical formula . It is a white to slightly yellowish crystalline powder that is very soluble in water and is hygroscopic. From an industrial perspective, it is the most important nitrite sa ...

in liquid

hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

: : :

History

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

Examples

4-Fluorotoluene is made in ~89% yield by Balz–Schiemann reaction on

p-toluidine There are three isomers of toluidine, which are organic compounds discovered and named by James Sheridan Muspratt and August Wilhelm von Hofmann in 1845. These isomers are ''o''-toluidine, ''m''-toluidine, and ''p''-toluidine, with the prefixed l ...

. This is then used as a precursor for 4-fluorobenzaldehyde,

Additional literature

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References

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