19.24 : 1° アミンからジアゾニウム塩またはアリールジアゾニウム塩へ: NaNO_2 メカニズムによるジアゾ化

During a diazotization reaction, primary amines react with nitrosonium ions—generated in situ via nitrosating agents—to yield diazonium salts through a series of steps.

The mechanism begins with a nucleophilic attack of the 1° amine on the electrophilic nitrosonium ion to form an unstable N-nitrosoaminium ion—with a new N–N bond—followed by deprotonation to form an N-nitrosamine.

The N-nitrosamine, in turn, tautomerizes to a diazohydroxide bearing a N=N.

Next, protonation of the –OH group results in the loss of a water molecule—an excellent leaving group—to generate the diazonium ion.

Primary aliphatic diazonium ions, being highly unstable, rapidly decompose to give carbocations by releasing molecular nitrogen—a stable leaving group.

Since the carbocation formed undergoes nucleophilic substitution, elimination, or rearrangement reactions to provide a mixture of products; diazotization of primary aliphatic amines is generally not useful.

In contrast, primary aryldiazonium ions are relatively stable intermediates. Diazotization of primary aromatic amines and treatment with an appropriate reagent results in the replacement of an –NH₂ group to generate substituted aromatic compounds.