19.27 : Diazonium Group Substitution: –OH and –H

Nitrous acid, a weak acid, is prepared in situ via the reaction of sodium nitrite with a strong acid under cold conditions. This nitrous acid prepared in situ reacts with primary arylamines to form arenediazonium salts. Such reactions are known as diazotization reactions. As shown in Figure 1, the formation of arenediazonium salts begins with the decomposition of nitrous acid in an acidic solution to give nitrosonium ions.

Figure 1.

A primary arylamine attacks the nitrosonium ion to form an N-nitrosoaminium ion intermediate, which on deprotonation generates N-nitrosamine. N-nitrosamine tautomerizes to a diazohydroxide, which loses water to form the diazonium ion in acidic conditions. Diazotization reactions of arylamines are critical in the synthetic preparation of various products.

Figure 2.

Figure 2 illustrates the substitution of the diazonium group of arenediazonium salts with several functional groups like halides, hydroxyl, nitrile, etc. For instance, in the hydrolysis of arenediazonium salts, the heated arenediazonium salts react with water to give phenols. In such reactions, the hydroxyl group replaces the diazonium group. Similarly, treating arenediazonium salts with hypophosphorous acid reduces the diazonium group. Such a reaction helps remove the diazonium groups in 2,4,6-trichlorobenzenediazonium halide, where the halide can be chloride or bromide, to form 1,3,5-trichlorobenzene or 1,3,5-tribromobenzene, respectively.