Knoevenagel diazotization method

What is Knoevenagel diazotization method?

The process of generating an anhydrous diazonium salt through the treatment of a primary amine with amyl nitrite in an acidified absolute alcohol was first discovered by Knoevenagel in 1890. This method is commonly referred to as the Knoevenagel diazotization method.

Knoevenagel diazotization method - general reaction scheme
Knoevenagel diazotization method
  • R = alkyl
  • X = Cl, Br, I, etc.

In this process, a crystalline precipitate of the diazonium salt is formed through the generation of nitrous acid in situ, which subsequently diazotizes the amines.

For instance, the bisulfates of various diazonium ions such as p-nitrobenzenediazonium, p-methoxybenzenediazonium, p-methylbenzenediazonium, and benzenediazonium ions can be produced in an ethanol-sulfuric acid solution with amyl nitrite, resulting in the formation of colorless platelets upon standing or upon the addition of a small amount of ether.

However, the bisulfate of m-nitrobenzenediazonium cannot be obtained using these conditions and must be prepared in an acetic acid/sulfuric acid mixture with butyl nitrite.

It is important to note that the reaction of aromatic amine with amyl nitrite in aromatic solvents yields a moderate yield of biaryls, and the yield of biaryls is not affected by changes in reaction temperature, nitrite concentration, and the order of addition of reagents, suggesting the presence of aryl radicals.

References

Knoevenagel, E. (1890), Zur Darstellung trockner Diazosalze. [For the preparation of dry diazo salts] Ber. Dtsch. Chem. Ges., 23: 2994-2998. https://doi.org/10.1002/cber.189002302224

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