Water with nitrous acid

The reaction is usually carried out in water with nitrous acid generated firom silver or alkali metal nitrites, although in much recent work acetic acid has been used as the solvent. The rate-determining step in the overall reaction is apparently the formation of a nitroso-amine (equation 145) . Isomerization to the tautomer 50 yields a... 

FIGURE 22 5 The diazo mum ion generated by treatment of a primary al kylamine with nitrous acid loses nitrogen to give a car bocation The isolated prod ucts are derived from the carbocation and include in this example alkenes (by loss of a proton) and an al cohol (nucleophilic capture by water)... 

Dilution of a solution of diazonium salts in sulfuric acid with water affords triazene derivatives (95IZV1315,95RCB1269). Treatment of a bridged bis(amino) furazans 200 with nitrous acid in acetic or hydrochloric acid gives macrocyclic triazene 201 (Scheme 132) (95MI4). 

TABLE 1. Values of the third-order rate constant for the acid catalysed nitrosation of thiols with nitrous acid in water at 25 °C... 

The most broadly useful intermediates for nucleophilic aromatic substitution are the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.75 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Section 10.1), aryl diazonium are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.76 The steps in forming a dizonium ion are addition of the nitrosonium ion, +NO, to the amino group, followed by elimination of water. 

Since the neutralization reaction of an amine with nitrous acid may be presumed to be instantaneous while the nitrosation of an amine proceeds at a slower rate, further investigations would be of interest to elucidate the problem of whether nitrite salt formation is a necessary preliminary step to the formation of iV-nitrosoamines or whether the covalent product forms independently. In the latter case, there would be competitive reactions of significantly different reaction rates and mechanisms. The yield of A-nitrosoamines may be influenced by the extent to which the more water-soluble nitrite salts may be present in the course of a preparation. 

Tertiary amines do not react with nitrous acid, acetyl chloride, benzoyl chloride, benzenesulfonyl chloride, but react with alkyl halides to form quaternary ammonium halides, which are converted by silver hydroxide to quaternary ammonium hydroxides. Quaternary ammonium hydroxides upon heating yield (1) tertiary amine pins alcohol (or, for higher members, olefin plus water). Tertiary amines may also be formed (2) by alkylation of secondary amines, e.g., by dimethyl sulfate, (3) from amino acids by living organisms, e g, decomposition of fish in the case of trimethylamine. 

Group II contains tannins of mixed or uncertain nature, the solutions giving a precipitate with bromine water and a bluish-black or purple-black coloration with ferric alum. It Is divided into tivo sub-groups, (a) giving no reaction or, at most, a browning with nitrous acid, (b) giving with nitrous acid a red coloration which tends slowly to purple and afterwards to blue or green. 

Group III comprises tanning materials containing tannins derived from pyrogallol, and are not precipitated by bromine water and give a bluish-black coloration with ferric alum. These also are subdivided into (a) those which give the above characteristic reaction with nitrous acid, and (6) those which give no reaction or only a brown colour. 

---