Electrophilic substitution with arenediazonium salts

Sec. 16.1, 16.2) from anenes by electrophilic aromatic substitution with halogen (Sec. 24.8) from arenediazonium salts by reaction with cuprous halides (Sandmeyer... 

Diazo coupling Electrophilic substitution with arenediazonium salts, giving uzo dyes. 

Couplings between arenediazonium cations and phenols take place most rapidly in slightly alkaline solution. Under these conditions an appreciable amount of the phenol is present as a phenoxide ion, ArO , and phenoxide ions are even more reactive toward electrophilic substitution than are phenols themselves. (Why ) If the solution is too alkaline (pH > 10), however, the arenediazonium salt itself reacts with hydroxide ion to form a relatively unreactive diazohydroxide or diazotate ion ... 

Electrophilic Substitution with Arenediazonium Salts CHAPTER 22... 

ELECTROPHILIC SUBSTITUTION WITH ARENEDIAZONIUM SALTS DIAZO COUPLING... 

Diazonium Salts as Electrophiles Diazo Coupling Arenediazonium ions act as weak electrophiles in electrophilic aromatic substitutions. The products have the structure Ar—N=N—Ar, containing the —N=N— azo linkage. For this reason, the products are called azo compounds, and the reaction is called diazo coupling. Because they are weak electrophiles, diazonium salts react only with strongly activated rings (such as derivatives of aniline and phenol). 

Aniline and substituted-anilines react with nitrous acid to form arenediazonium salts a diazonium group can be displaced by a nucleophile. Arenediazonium ions can be used as electrophiles with highly activated benzene rings to form azo compounds that can exist in cis and trans forms. Secondary amines react with nitrous acid to form nitrosamines. 

We have seen that primary arylamines react with nitrous acid to form stable arene-diazonium salts (Section 16.12). Arenediazonium salts are useful to synthetic chemists because the diazonium group can be replaced by a wide variety of nucleophiles. This reaction allows a wider variety of substituted benzenes to be prepared than can be prepared solely from electrophilic aromatic substitution reactions. 

Aprotic nitrosation of 1,3-diaryl-triazenes RC6H4NHN2C6H4R (R = H, 4-Me, 4-OMe, 4-Cl, 4-NO2, 2-Me, 2-Cl, or 3-Cl) with Me(CH2)40NO yields the A-nitroso-compound, which decomposes to arenediazonium salts and radicals and yields biphenyls, which are also formed together with (183), by electrophilic substitution of sulphonyl-triazenes (184) with benzene and AICI3. 

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