Diazotic acid

Thus the 2,4-dinitrophenyldiazonium cation will couple with PhOMe and the 2,4,6-compound with even the hydrocarbon 2,4,6-trimethylbenzene (mesitylene). Diazonium cations exist in acid and slightly alkaline solution (in more strongly alkaline solution they are converted first into diazotic acids, PhN=N—OH, and then into diazotate anions, PhN=N—O ) and coupling reactions are therefore carried out under these conditions, the optimum pH depending on the species being attacked. With phenols this is at a slightly alkaline pH as it is PhO , and not PhOH, that undergoes attack by... 

However, if the pH is too high, coupling is inhibited because the diazonium salt is transformed into ArN=N—0°, which is the nonelectrophilic conjugate base of a diazotic acid (Table 23-4). 

An especially elegant way of carrying out this replacement is to use hypophos-phorous acid as the diazotizing acid. The amine is dissolved in hypophosphorous acid, and sodium nitrite is added the diazonium salt is reduced as fast as it is formed. 

When potassium anti-diazotate is acidified it is converted to the unstable diazotate acid which immediately undergoes transformation to the corresponding nitrosamine, (10). 

Step 3 Keto-enol tautomerism. The N-nitrosamine undergoes keto-enol tautomerism (Section 16.9) to give a diazotic acid, which is so named because it has two (di-) nitrogen (-azot-) atoms within its structure. 

The iV-nitrosoaniline equilibrates with a molecule called a diazotic acid, which loses water in acid to generate the diazonium ion (Fig. 14.53).These transformations are probably bewilderingly complex right now, even though they are made up of very simple reactions in sequence. It certainly takes some practice to get them right. However, many reaction mechanisms involve similar sequences of protonations, deprotonations, and additions and losses of water molecules. Now is the time to start practicing them. We wll encoimter many more when we discuss carbonyl chemistry in Chapters 16 through 19. 

FIGURE 14.53 Loss of water from the protonated diazotic acid gives the diazonium ion, benzenediazonium chloride. 

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