Amines, aryl reaction with nitrous acid

A/fey/amines also react with nitrous acid, but the alkanediazonium products of these reactions are so reactive they can t be isolated. Instead, they lose nitrogen instantly to yield carbocations. The analogous loss of N2 from an arenediazonium ion to yield an aryl cation is disfavored by the instability of the cation. 

Starting from ort/w-aminoarene imines 1. 4-aryl-substituted condensed 1,2,3-triazines 2 can be obtained.7,8 Using the oximes 3 instead of imines, condensed 1,2,3-triazine 3-oxides 4 can be isolated.7,8,127-129,241 Reaction of ort/io-aminoarenecarbamidoximes 3 (R — NH2) with nitrous acid afforded condensed l,2,3-triazin-4-amine 3-oxides 4 (R = NH2).7,8,195... 

We have seen that the reaction of a primary amine with nitrous acid produces a diazonium salt. Both aryl amines and alkyl amines undergo this reaction, and both follow the same mechanism. 

When a solution of a salt of an aryl amine, aniline, for example, is treated with nitrous acid, nitrogen is not evolved if the reaction takes place at room-temperature. A substance containing nitrogen, which is called a diazonium compound, is formed. The reaction in the case of aniline hydrochloride is expressed by the equation,—... 

A related process that involves diazonium salts is the Pschorr reaction, hich also couples aryl diazo-nium compounds to other aromatic rings. This diazonium salt coupling can he done under acidic conditions, but addition of copper powder usually promotes the radical process. Aryl amines generate aryl diazonium salts upon treatment with nitrous acid.l An example is the reaction of 233 to give an aryl diazonium salt, which cyclized in the presence of copper to give thaliporphine (234) in 43% yield. Kupchan called this transformation an improved Pschorr reaction. 68... 

The most useful reaction of amines with nitrous acid is the reaction of aryl-amines to form arenediazonium salts. We consider next how these diazonium salts may be used as synthetic intermediates. 

Secondary amines—both aryl and alkyl—react with nitrous acid to yield 7V-nitrosoamines. JV-Nitrosoamines usually separate from the reaction mixture as oily yellow liquids ... 

Undoubtedly the same reaction occurs with aliphatic primary amines and nitrous acid. With aryl amines, however, the diazonium ion is stabilized by the resonance forms,... 

The preparation and reactions of diazonium salts. The reaction of aromatic amines with the nitrosonium ion (+NO), generated from nitrous acid (HN02), yields aryl diazonium salts, which can be converted (on loss of nitrogen) to a range of functional groups. 

Nitrous acid (3), which is generated from sodium nitrite and aqueous acid, undergoes dehydration under the acidic conditions to form nitrosyl cation 4. Reaction of 4 with the aryl amine eventually affords the aryl diazonium salt after tautomerization and dehydration. The addition of fluoroboric acid sets the stage for the second step leading to aryl fluoride. 

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