Reactivity

The most important reaction of N-unsubstituted anilines is nitrosation, leading to diazonium salts.

Click here to recall the nitrosation of amines.

Diazonium salts are an important entry to aromatic hydrocarbons bearing a great variety of organic functions.

Arenediazonium salts, formed from anilines with NaNO₂/HCl, are converted into phenols (H₂O), iodides (HI), chlorides and bromides (CuX), nitriles (CuCN), fluorides (HBF₄), arenes (H₃PO₂) and azo compounds (with another arene)

Formation of the nitrosyl cation: the nitrite anion is protonated twice by HCl and loses water

Nitrite anion is basic enough to react twice with a mineral acid like hydrochloric acid. The double protonation allows the elimination of a water molecule rendering the nitrosyl cation, strongly electrophilic and extremely reactive. This cation is formed in situ, i.e., in the presence of the aniline.

Diazotization of an aniline, first step: the nitrogen attacks the nitrosyl cation to form the N-nitrosoammonium salt, which loses a proton to give the carcinogenic N-nitrosoamine

The nitrogen of anilines, nucleophilic enough, can easily react with the nitrosyl cation. This reaction leads to a cascade of intermediates which finally produce the diazonium salt.

In a first step, the amine lone electron pair bonds to the nitrosyl cation's electron-deficient centre which is the nitrogen. A proton is lost rendering a N-nitrosoamine, hazardous product due to its carcinogenic properties.

Diazotization, second step: the N-nitrosoamine tautomerizes and is protonated on oxygen

The N-nitrosoamine bears numerous lone electron pairs being thus basic. Do not forget that the nitrosyl cation was formed "in situ" with a strong acid. This medium is able to protonate the final N-nitrosoamine that ends up losing a water molecule.

Diazotization, last step: water is lost and the diazonium salt is formed, which on heating loses N₂ to give an aryl cation

The loss of water leads to the diazonium salt, that must be carefully stored because it tends to lose nitrogen explosively with ease. However, if the loss of nitrogen is controlled, the N₂ group can be replaced by almost any other organic function (see above).

The reaction resembles a nucleophilic aromatic substitution (S_NAr). Click here to go to the S_NAr.