19.27:
Diazonium Group Substitution: –OH and –H
Recall that primary arylamines, upon diazotization reaction, yield arenediazonium salts.
The arenediazonium salts are synthetically useful intermediates. When treated with appropriate reagents, they lose nitrogen, and the diazonium group can be replaced by the –H atom or the –OH group.
For instance, to directly install a –OH group on an aromatic ring, aromatic amines are first treated with sodium nitrite and sulfuric acid to generate an arenediazonium salt, which, upon warming, then undergoes hydrolysis to yield phenols.
Similarly, the conversion of aniline to substituted benzene is a classic example of using the directive effects of the –NH2 group. For example, direct halogenation of benzene to synthesize 1,3,5-tribromobenzene is not possible because halogens are o-, p– directors.
In such cases, aniline is used to exploit the –NH2 group’s activating and o-, p-directing effects. Once halogenated, the –NH2 group is removed via diazotization, followed by a reaction with H3PO2.
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.