Diazo Compounds

DIAZO COMPOUNDS, organic substances containing the group N₂ similar to azo compounds. Of all diazo compounds, of particular importance are those corresponding to the structural formula Ar . N = N

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X, where Ar is an aryl group, and X is an acid or hydroxyl residue. These compounds, by analogy with ammonium salts, are called diazonium salts and play a major role in the technology of the preparation of dyes and pharmaceutical preparations. Diazonium salts, upon replacement of the acid residue by a hydroxyl group, form a base

Ar . N = N

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OH, which possesses strongly expressed 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 + 2H₂O

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Cl. Practically, this operation is carried out in such a way that a solution of sodium nitrite is added to an aqueous solution of the 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 carried out with cooling, since an increase in temperature can cause decomposition or further transformation of the formed diazonium salt. One of the reasons determining 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: 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 diazo group for a cyan group is also possible: Ar . N₂ . Cl + KCN = Ar . CN + N₂ + KCl. Of even greater importance are the reactions called diazo coupling (see), in which the complex -N₂- 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 a pure form; the reaction is usually carried out in such a way that the solution obtained by the diazotization of the amine is directly used for the preparation of this or that product.

S. Medvedev.