Diazonium Salts

One of the most useful reactions of primary amines is that with nitrous acid, HNO2. With a primary aliphatic amine, nitrogen gas is eliminated rapidly and quantitatively, and if the gas is collected and its volume measured under standard conditions of temperature and pressure, the reaction serves as a valuable method for the analysis of amines (the Van Slyke method). If the primary amine is aromatic, and the reaction is conducted at ice-bath temperatures, a diazonium salt is formed (Fig. 11-7). These salts are highly reactive and explode when dry, but their great reactivity makes them ideal for the synthesis of other compounds. We have already seen that aryl halides are totally unreactive toward displacement reactions. Diazonium salts play the same role in aromatic chemistry that halides do in aliphatic chemistry, for the N2+ group can be replaced by a great many other groups as shown in Fig. 11-8. The reactions are not simple displacements, however, since some of the better displacing bases, such as cyanide, chloride, and bromide, do not work unless they are present as their cuprous salts. Those reactions run with cuprous salts are known as Sandmeyer reactions. The mechanism is complex, and involves free radicals.

Figure 11-7. Treatment of a primary amine with nitrous acid gives a diazonium salt. Such salts of alkylamines are unstable and immediately decompose with the evolution of nitrogen. Aromatic diazonium salts are stable at 0° but eliminate N2 at room temperature.

Figure 11-8. Aromatic diazonium salts are readily transformed into a variety of substituted aromatic compounds.

The importance of diazonium displacements is even greater than may appear at first glance because diazonium salts are obtained from amines, which in turn are best formed by reduction of a nitro group. By way of a diazonium salt, then, a meta directing nitro group may be transformed into an ortho-para directing hydroxyl or halo group. Suppose, for example, that one wishes to prepare m-chlorobromobenzene. Since both chlorine and bromine are o-p directors, neither chlorination of bromobenzene nor bromination of chlorobenzene would give the desired isomer. But bromination of nitrobenzene to give m-bromonitrobenzene, followed by conversion of the nitro group to chlorine by way of the Sandmeyer reaction, would give the desired compound (Fig. 11-9). Although the path seems a long one, yields are high and some of the intermediates do not have to be isolated.

Figure 11-9. The Sandmeyer reaction is especially useful in preparing benzene derivatives with ortho-para directing groups meta to one another (or meta-directing groups ortho or para to one another).

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