Nitrous acid primary arylamines

Aryl diazonium ions prepared by nitrous acid diazotization of primary arylamines are substantially more stable than alkyl diazonium ions and are of enormous synthetic value Their use m the synthesis of substituted aromatic compounds is described m the following two sections... 

Primary arylamines react with nitrous acid, HNO2, to yield stable arenediazonium +... 

Diazonium salt formation Primary arylamines react with nitrous acid (HNO2) to yield stable arenediazonium salts, Ar—N+=NX . Alkylamines also react with nitrous acid, but the alkanediazonium salts are so reactive that they cannot be isolated. 

Salts of primary aromatic amines react with nitrous acid to produce diazonium salts. The reaction is usually performed by adding a cold solution of sodium nitrite to a cold solution of the arylamine in aqueous mineral acid. The end point of the reaction is conveniently determined by the detection of excess nitrous acid with potassium iodide-starch paper. Sulfamic acid has long been used both in industry and in the laboratory to remove excess nitrous acid. It has been found to react with the more active diazo compounds. In most cases, high temperatures are avoided to prevent the formation of phenols and the decomposition of the unstable nitrous acid. An excess of mineral acid is necessary to prevent coupling between the diazonium salt and unreacted amine (cf. method 494). If the amine salt is somewhat insoluble, a fine crystalline form, which is produced by rapid crystallization from a warm aqueous solution, may be employed. ... 

Primary arylamines rea t with nitrous acid, HNOg, to yield stable arene-diazonium salts, Ar-NsNX". This diazotization reaction is compatible with the presence of a wide variety of substituents on the aromatic ring. 

We have seen that primary arylamines react with nitrous acid to form stable arene-diazonium salts (Section 16.12). Arenediazonium salts are useful to synthetic chemists because the diazonium group can be replaced by a wide variety of nucleophiles. This reaction allows a wider variety of substituted benzenes to be prepared than can be prepared solely from electrophilic aromatic substitution reactions. 

B Reactions of Primary Arylamines with Nitrous Acid... 

The most important reaction of amines with nitrous acid, by far, is the reaction of primary arylamines. We shall see why in Section 20.7. 

Primary aliphatic amines (RNH2) react with nitrous acid in the same way that primary arylamines (ArNH2) do, to form diazonium ions. But alkyldiazonium ions RN2 are much less stable than aryldiazonium ions ArN2 and readily lose nitrogen even at 0°C. Explain the difference. 

The most important reaction of amines with nitrous acid, by far, is the reaction of primary arylamines. We shall see why in Section 20.7. Primary arylamines react with nitrous add to give arenediazonium salts. Even though arenediazonium salts are unstable, they are still far more stable than aliphatic diazonium salts they do not decompose at an appreciable rate in solution when the temperature of the reaction mixture is kept below 5°C ... 

Aliphatic primary amines also give diazonium salts with nitrous acid. However, in this case the diazonium ion is not stabilized by resonance structures as in the case of arylamine, and decomposition to nitrogen and carbo-nium ions takes place. During the performance of the reaction the evolu-... 

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