If excess diazomethane is present during the reaction, it can act as a base, abstracting a hydrogen from the diazonium-salt intermediate. The result is a neutral diazoketone, which does not react with the chloride. Instead, the byproduct, diazonium-methyl from the other diazomethane molecule, can be attacked by the chloride to produce chloromethane. The unreactive diazoketone can be re-activated and reacted by treatment with hydrogen chloride to give the normal Nierenstein product.
The Nierenstein reaction mechanism
In some cases, even limiting the amount of diazomethane gives a reaction process that stalls via the neutral diazoketone pathway, requiring the addition of HCl gas to rescue it.[3]
^Bachman, W. E.; Struve, W. S. (1942). "The Arndt-Eistert Reaction". Org. React. 1: 38. (Review)
^McPhee, W. D; Klingsberg, E. Organic Syntheses, Coll. Vol. 3, p.119 (1955); Vol. 26, p.13 (1946). (Article)
^M. Nierenstein; D. G. Wang & J. C. Warr (1924). "The Action of Diazomethane on some Aromatic Acyl Chlorides II. Synthesis of Fisetol". J. Am. Chem. Soc.46 (11): 2551–2555. doi:10.1021/ja01676a028.
^H. H. Lewis; M. Nierenstein & Enid M. Rich (1925). "The Action of Diazomethane on some Aromatic Acyl Chlorides III. The Mechanism of the Reaction". J. Am. Chem. Soc.47 (6): 1728–1732. doi:10.1021/ja01683a036.
