Nitrosation

A very important reaction of amines takes place with nitrous acid. Such process is called nitrosation.

Formation of the nitrosyl cation: sodium nitrite with HCl gives nitrous acid, which is protonated and loses water

Nitrous acid decomposes in acidic medium, releasing the nitrosyl cation, a very strong electrophile.

The nitrosyl cation is attacked by the amine leading to an N-nitrosoammonium ion, that may evolve in a different fashion depending whether the amine is primary, secondary or tertiary.

Tertiary amines attack the nitrosyl cation and form N-nitrosoammonium salts

PRIMARY AMINES

Nitrosation of a primary amine: the N-nitrosoamine tautomerizes, loses water and forms the diazonium salt, which loses N₂ to give a carbocation

The two hydrogens on the nitrogen of the primary amine allow the elimination of a water molecule. A very reactive diazonium salt is thus formed, which easily loses a nitrogen molecule, yielding a carbocation that will react with any present nucleophile.

The aliphatic diazonium salts are extremely unstable and quickly decompose. The so-formed carbocation uncontrollably reacts with any present nucleophile (for instance water, chloride, etc.) leading to a mixture of alcohols, alkyl halides and even alkenes by elimination.

AROMATIC PRIMARY AMINES (ANILINES)

The most important reaction of non-substituted anilines is their transformation into diazonium salts, relatively stable intermediates on the way to numerous aromatic derivatives.

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)

SECONDARY AMINES

Nitrosation of a secondary amine: the N-nitrosoammonium ion loses a proton to give the N-nitrosoamine

The N-nitrosoamines are very dangerous because they are proven potent carcinogens. Nitrites (E-249 and E-250) are preserving agents for meat and its common derivatives like sausages, etc., that avoid the proliferation of microorganisms producing botulinic toxins, lethal for human beings. The N-nitrosoamines might be produced by an overcooking of nitrite-containing food.

TERTIARY AMINES

Tertiary amines do not bear any hydrogen on their nitrogen and the N-nitrosoammonium salt cannot evolve. However they easily decompose.

The reaction does not have any synthetic usefulness.