Gomberg–Bachmann reaction

Was named The Gomberg - Bachmann reaction is a name reaction in organic chemistry, after the Ukrainian- American chemist Moses Gomberg (1866 - - 1947) and the American chemist Werner Emmanuel Bachmann ( 1951, 1901 ). The reaction was developed in 1924 for the synthesis of symmetrical and unsymmetrical diaryls ( biphenyls ) from aryl diazonium salts.

Overview reaction

The Gomberg - Bachmann reaction react aryl diazonium salts in an aqueous alkaline solution with aromatics (eg, benzene ) to diaryl compounds:

The intramolecular variant of the Gomberg - Bachmann reaction is known as Pschorr reaction.

Mechanism

Using the example of benzenediazonium and benzene, the mechanism for the synthesis of biphenyl to be explained. A benzenediazonium ion 1 treated with aqueous sodium hydroxide solution, is formed in an equilibrium reaction Benzoldiazohydroxid 2, which reacts with another benzenediazonium ion to deprotonation to Benzoldiazoanhydrid 3. The proof for the existence of 3 yielded Rüchardt by crossing experiments. This anhydride is reacted to nitrogen and cleaves a Benzoldiazoanhydrid 4 radical and a phenyl radical 5 attacks the reactive phenyl radical, the benzene to form a resonance stabilized free radical ( Phenylcyclohexadienyl radical) ( 6). This reaction step is a radical substitution. The radical then reacts with 4 6 with elimination of 2 to biphenyl (7).

Due to side reactions of the Gomberg - Bachmann reaction often has a yield of less than 40%. By phase-transfer catalysts, the yields can be increased significantly.

Selectivity

According to the general rule that the selectivity decreases with increasing reactivity, the highly reactive aryl radicals are not selective enough. Therefore, it is mostly used as aromatic benzene.

Example

4- bromoaniline and benzene react by diazotization in alkaline solution to give 4- bromobiphenyl: