Diazonium compounds stable

Some amines, such as the nitroanilines and the naphthylamines, give somewhat more stable diazonium compounds and may be diazotised at room temperature, when the reaction proceeds more rapidly. If the amine salt is only sparingly soluble in water, it should be suspended in the acid in a fine state of division (this is generally attained by cooling a hot solution and stirring vigorously), and it passes into solution as the soluble diazonium salt is formed. 

The conversion of an aromatic diazonium compound into the corresponding arsonic acid by treatment with sodium arsenite in the presence of a catalyst, such as copper or a copper salt, is called the Bart reaction. A modification of the reaction employs the more stable diazonium fluoborate in place of the diazonium chlorid.i. This is illustrated by the preparation of />-nitrophenylarsonic acid ... 

Primary amino groups attached to azole rings react normally with nitrous acid to give diazonium compounds via primary nitroso compounds. However, the azole series shows two special characteristics the primary nitroso compounds can be stable enough to be isolated, and diazo anhydrides are formed easily from azoles containing ring NH groups. 

Irradiation of a diazonium compound in EPA glass at 77°K and monitoring the reaction by UV revealed the presence of new absorption bands, which disappeared upon warming to room temperature. Little or no evolution of nitrogen occurred in these experiments. Hence the low-temperature intermediate has been proposed to be a rather stable triplet state of the diazonium compound. 

Amino-substituted selenophenes are in general unstable compounds, and are conveniently isolated as IV-acetyl or iV-formyl derivatives. An ortho situated electron-attracting substituent such as acetyl, formyl or nitro renders the amino compounds stable. Derivatives of this type can be diazolized and the resulting diazonium salts converted into azides (by reaction with azide ion) (75CR(C)(281)317) and selenocyanates (by reaction with selenocyanide ion) (79BSF(2)237). 3-Acetamidoselenophene can be thiocyanated or selenocyanated in the 2-position by treatment with bromine and potassium thiocyanate or potassium selenocyanate, respectively (78TL1797). 

Benzotriazinium betaines (13) and (26) are stable toward acids. They are slightly basic compounds and form salts with acids which are easily hydrolyzed. The corresponding 1-oxides (33) behave similarly, being, for instance, souble in concentrated hydrochloric acid from which they are reprecipitated on dilution with water (27JCS323). 4-Amino-l,2,3-benzotriazines (38) and 4-imino-1,2,3-benzotriazines (39) behave as masked diazonium compounds (40) and the products of acidic treatment of these compounds are best explained as reaction products of the diazonium salts (78HC(33)78>. Treatment of 1,2,3-benzotriazine... 

Commercial sodium nitrite contains 1—3% sodium nitrate, the harmful effect of which on diazonium compounds may be obviated by adding to the amine solution or the nitrite solution an amount (1-5—4 5% of the weight of nitrite used) of sodium metabisulphite slightly in excess of that necessary to reduce the nitrate. Diazonium compounds prepared in this way or with pure sodium nitrite are much more stable (J. Soc. Dyers, 41, 275.)... 

A diazonium salt is a weak electrophile and hence reacts only with highly electron rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequently, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH >pKa of phenol or naphthol typically pH 7-11), whereas aromatic amines such as N,N-dialkylamines are coupled in a slightly acid medium, typically pH 1-5. This provides optimum stability for the diazonium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

Diazonium compounds are generally stable only in aqueous solution at low temperatures. When heated, they frequently decompose by eliminating nitrogen to form the corresponding phenol. Some amines, however, can be diazotized at temperatures up to 40 °C. Metal ions also accelerate the decomposition of diazonium compounds. Therefore, diazotization is usually carried out in wooden vats or iron stirring vessels with an acid-proof lining or rubber coating. 

Diazonium compounds other than phenyldiazonium chloride are also used, but to a smaller extent. Thus, in some plants, diazotized m-xylidine is used, but this appears to have no advantage since the diazonium solution is less stable. Diazotized p-aminoacetanilide, on the other hand, is used in certain cases because it couples more vigorously and gives a very stable solution (see chromotropic acid), o- and p-Nitroaniline are used less frequently. 

Triazanaphthalene has not been prepared but several observations suggest that it would behave as an unstable diazonium compound. The neutral species of the 3-oxide was stable at pH 7.1, although it decomposed readily in acid solution. The neutral species was considered to be anhydrous because the spectrum at pH 7.1 resembled that of 4-methyl-l,2,3-triazanaphthalene 3-oxide, in which hydration would be minimized by the blocking effect of the methyl group.Decomposition in acid most probably involves addition of water across C-4 and N-3 because o-azidobenzaldehyde was formed. The 4-methyl derivative underwent similar decomposition. 

The relatively infrequent availability of stable heterocyclic diazonium compounds explains the paucity of information on this group. In diazotizing 5-amino-2,4-dichlorop3nimidine, the chlorines are displaced by hydroxy groups. A related type of activation is also indicated by the very ready decarboxylation of 5-aminopyrimidine-... 

In general, diazonium salts imdergo many reactions and, as already stated, are not stable. Their reactions, for the purpose of discussion, may be divided into two groups (a) reactions involving the replacement of the diazo group by another, such as Cl, Br, I, OH, CN (i>) reactions involving the union of other radicals with the diazonium compound. 

This procedure can be used also for other, relatively stable nitro diazonium compounds. However, when the diazonium compounds are unstable, their separation in the solid state is to be avoided. The following directions for preparation of m-chlorophenol are given by Holleman and Rinkes 558... 

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