Aryl diazonium salt

Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group R−N⁺
₂X⁻
where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halogen. Diazonium salts, especially those where R is an aryl group, are important intermediates in the organic synthesis of azo dyes.

The process of forming diazonium compounds is called "diazotation", "diazoniation", or "diazotization". The reaction was first reported by Peter Griess in 1858, who subsequently discovered several reactions of this new class of compounds. Most commonly, diazonium salts are prepared by treatment of aromatic amines with nitrous acid and additional acid. Usually the nitrous acid is generated in situ (in the same flask) from sodium nitrite and the excess mineral acid:

Aqueous solutions of diazonium salts are unstable above 5 °C. One can isolate diazonium compounds as tetrafluoroborate salts, which are stable at room temperature. It is often preferred that the diazonium salt remain in solutions, but they do tend to supersaturate. Operators have been injured or even killed by an unexpected crystallization of the salt followed by its detonation.

Usually, diazonium compounds are not isolated. Instead they are used in situ. This approach is illustrated in the preparation of an arylsulfonyl compound:

The most widely practiced reaction of diazonium salts is azo coupling. In this process, the diazonium compound is attacked by, i.e., coupled to, electron-rich substrates. When the coupling partners are arenes such as anilines and phenols, the process is an example of electrophilic aromatic substitution:

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