In organic chemistry, an azo coupling is an reaction between a diazonium compound (R−N≡N+) and another aromatic compound that produces an azo compound (R−N=N−R’). In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile, and the activated carbon (usually from an arene, which is called coupling agent), serves as a nucleophile. Classical coupling agents are phenols and naphhthols.[1] Usually the diazonium reagent attacks at the para position of the coupling agent. When the para position is occupied, coupling occurs at a ortho position, albeit at a slower rate.
Azo printing exploits this reaction as well. In this case, the diazonium ion is degraded by light, leaving a latent image in undegraded diazonium salt which is made to react with a phenol, producing a colored image: the blueprint.[3]
Prontosil, the first sulfa drug, was once produced by azo coupling. The azo compound is a prodrug that is activated in-vivo to produce the sufanilamide.
The reaction is also used in the Pauly reaction test to detect tyrosine or histidine residues in proteins.
Additionally, through the azo coupling reaction between the aromatic diazonium ion and aromatic amino acid residues, this reaction also be used to form or to modify proteins such as tRNA synthetase.[4]
Besides activated aromatic coupling agents, other nucleophilic carbons could also be coupled with diazonium salt:
Examples of azo N-coupling reactions
In alkaline media, diazonium salt can react with most primary and secondary amines, which exist as a free base to produce triazene.[8] This chemical reaction is called azo N-coupling,[9] or the synthesis of azoamines.[10]
The dye called aniline yellow is produced by the reaction of aniline and a diazonium salt. In this case the C- and N-coupling compete.[2]
This article uses material from the Wikipedia article Diazonium_coupling, and is written by contributors.
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