19.24:
1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Mechanism
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.
19.24:
1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Mechanism
Nitrous acid is a relatively weak and unstable acid prepared in situ by the reaction of sodium nitrite and cold, dilute hydrochloric acid. In an acidic solution, the nitrous acid undergoes protonation when it loses water to form a nitrosonium ion—an electrophile. Nitrous acid reacts with primary amines to give diazonium salts. The reaction is called diazotization of primary amines.
Figure 1. The mechanism of the diazotization reaction of primary amines.
As illustrated in Figure 1, in the mechanism of the reaction, a primary amine attacks as the nucleophile on the nitrosonium ion. The subsequent deprotonation yields an N-nitrosamine. Next, the N-nitrosamine readily undergoes keto-enol tautomerism to give a diazotic acid. Protonation of the diazotic acid followed by water loss results in the formation of diazonium ions. The aliphatic diazonium ions formed are extremely unstable and readily decompose to form a carbocation by releasing molecular nitrogen even at 0 °C, which is a good and stable leaving group.