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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]
Fluorobenzene is an aryl fluoride and the simplest of the fluorobenzenes, with the formula C 6 H 5 F, often abbreviated PhF. A colorless liquid, it is a precursor to many fluorophenyl compounds. A colorless liquid, it is a precursor to many fluorophenyl compounds.
The mechanism of electrophilic fluorination remains controversial. At issue is whether the reaction proceeds via an S N 2 or single-electron transfer (SET) process. In support of the S N 2 mechanism, aryl Grignard reagents and aryllithiums give similar yields of fluorobenzene in combination with N-fluoro-o-benzenedisulfonimide (NFOBS), even though the tendencies of these reagents to ...
For example, 2,4-dinitrochlorobenzene reacts in basic solution to give a phenol. Unlike in most other substitution reactions, fluoride is the best leaving group, and iodide the worst. [ 16 ] A 2018 paper indicates that this situation may actually be rather common, occurring in systems that were previously assumed to proceed via S N Ar mechanisms.
An individual reaction can be described as: R 3 C–H + HF → R 3 C–F + H 2. In the course of a typical synthesis, this reaction occurs once for each C–H bond in the precursor. The cell potential is maintained near 5–6 V. The anode is nickel-plated.
A classic example is the reaction of salicylic acid with a mixture of nitric and sulfuric acid to form picric acid. The nitration of the 2 position involves the loss of CO 2 as the leaving group. Desulfonation in which a sulfonyl group is substituted by a proton is a common example. See also Hayashi rearrangement.
Difluorobenzenes can be prepared by the Balz-Schiemann reaction, which entails conversion of diazonium tetrafluoroborate salts to their fluorides. The synthesis of 1,2-difluorobenzene starts with 2-fluoroaniline: [3] C 6 H 4 F(NH 2) + HNO 2 + HBF 4 → [C 6 H 4 F(N 2)]BF 4 + 2 H 2 O [C 6 H 4 F(N 2)]BF 4 → C 6 H 4 F 2 + N 2 + BF 3
4-Fluorobromobenzene is synthesized via bromination of fluorobenzene in the presence of a Lewis acid catalyst such as iron(III) bromide or aluminium tribromide. [2]4-Bromofluorobenzene is regarded by the Toxic Substances Control Act as a high production volume chemical, that is, a chemical that 1 million pounds (about 500 tonnes) per year is either produced in or imported to the United States.