Diazo Compounds

DIAZO COMPOUNDS, organic substances containing, like azo compounds, the N₂ group. Of all diazo compounds, those corresponding to the structural formula Ar.N=N-X, where Ar is an aryl group and X is an acid or hydroxyl residue, are of particular importance.

These compounds, by analogy with ammonium salts, are called diazonium salts and play a major role in the technology of preparing dyes and pharmaceutical preparations. When the acid residue is replaced by a hydroxyl group, diazonium salts form a base Ar.N=N-OH, which possesses sharply defined alkaline properties.

The preparation of diazo compounds is called diazotization and has not only laboratory but also great technical significance. Diazotization occurs upon the action of nitrous acid on an aromatic amine: Ar.NH₂ + HNO₂ + HCl = Ar.N=N-Cl + 2H₂O.

Practically, this operation is performed in such a way that a solution of a nitrous acid salt (sodium nitrite) is added to an aqueous solution of an amine salt containing an excess of acid; the completeness of the reaction is judged by the turning blue of starch-iodide paper.

Diazotization should be conducted under cooling, as an increase in temperature may cause decomposition or further transformation of the formed diazonium salt. One of the reasons for the wide application of diazo compounds in organic syntheses is their property of exchanging their diazo group for other groups or atoms. Of these exchange decompositions, the following deserve special attention. 1. Upon heating aqueous solutions of diazonium salts, the N₂ group is replaced by a hydroxyl group: Ar.N₂.Cl + H₂O = Ar.OH + N₂ + HCl. 2. Upon heating with alcohol, the N₂ group is replaced by an alcohol residue: Ar.N₂.Cl + HO.C₂H₅ = Ar.O.C₂H₅ + N₂ + HCl. Under certain conditions, heating with alcohol leads to the formation of a hydrocarbon (or its derivatives): Ar.N₂.Cl + HO.C₂H₅ = ArH + N₂ + HCl + C₂H₄O (aldehyde).

3. Upon the interaction of a diazonium chloride salt with a solution of cuprous chloride in concentrated hydrochloric acid, a chloro-compound is obtained: Ar.N₂.Cl = ArCl + N₂. Under the influence of cuprous salts, the exchange of the N₂ group for a cyano group is also possible: Ar.N₂.Cl + KCN = Ar.CN + N₂ + KCl. Even greater significance is held by reactions called diazo-coupling (see), in which the -N₂- complex is preserved, forming corresponding diazo-, amino-, and azo-compounds with aromatic amines or phenols. To perform all these operations, there is no need to isolate the diazonium salt in pure form; the reaction is usually conducted in such a way that the solution obtained by diazotization of the amine is used directly for the preparation of this or that product.

S. Medvedev.