The Aromatic Diazonium Ion as a Dibasic Acid

In the historical introduction to this book (Sec. 1.1) it was mentioned that the discoverer of diazo compounds, Peter Griess, realized quite early (1864 a) that these species could react with alkali hydroxides. Thirty years later Schraube and Schmidt (1894) found that the primary product from the addition of a hydroxide ion to a diazo compound can isomerize to form a secondary product. In this section we will discuss the equilibria of the first acid-base process of aromatic diazonium ions. In the following section additional acid-base reactions will be treated in connection with the isomerism of addition products of hydroxide ions to diazonium ions. 

Diazo Chemistry I Aromatic and Heteroaromatic Compounds. By Heinrich Zollinger Copyright 1994 VCH Verlagsgesellschaft mbH ISBN 3-527-29213-6 

5 Acid-Base and Isomerization Reactions of Diazo Compounds in Water 

Wittwer and Zollinger (1954) determined the neutralization curves of aqueous solutions of diazonium salts under standard conditions of ionic strength, etc., and found that the acidity depended on the degree of neutralization in a manner different to that expected for a dibasic acid. The curve obtained did not exhibit two steps with an intermediate region of a few pH units in which the monobasic acid is stable, as is the case, for instance, with oxalic acid (Fig. 5-1). On the contrary, there was only one step, but it extended over two equivalents of base per diazonium ion. 

With the usual type of dibasic acid the equilibrium constant for the second step is always smaller than that for the first (Kx K2)y but the diazonium ion represents another kind of acid in which the second constant is greater than the first (K2 Kf), Schwarzenbach (1943) was the first to discover analogous abnormal acid-base equilibria and he explained under what circumstances the phenomenon can occur (for a historical account of Schwarzenbach s work see Zollinger, 1992). 

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