Azo coupling

Uses of the reaction

Aromatic azo compounds tend to be brightly colored due to their extended conjugated systems. Many are useful dyes (see azo dye). Important azo dyes include methyl red and pigment red 170.

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.

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 can also be used to form or to modify proteins such as tRNA synthetase.

Examples of azo C-coupling reactions

Illustrative is the reaction of phenol with benzenediazonium chloride to give a Solvent Yellow 7, a yellow-orange azo compound. The reaction is faster at high pH.{{cite journal |doi=10.15227/orgsyn.003.0007 |title=1,4-Aminonaphthol Hydrochloride |journal=Organic Syntheses |date=1923 |volume=3 |page=7|first1=J. B.|last1=Conant|first2=R. E.|last2=Lutz|first3=B. B.|last3=Corson

:[[Image:Azo-coupling-A-mechanism-2D-skeletal.png|800px]]

Naphthols are used as coupling agents. One example is the synthesis of the dye organol brown (4) from aniline (1) and 1-naphthol (3): :[[File:Synthesis_of_Organol_Brown_N.svg|left|alt=Synthesis of organol brown from aniline and 1-naphthol]]

Similarly, β-naphthol couples with phenyldiazonium electrophile to produce an intense orange-red dye.

Besides activated aromatic coupling agents, other nucleophilic carbons also couple with diazonium salts as illustrated by the synthesis of azo pigments. :[[File:FormationPigYellow13.svg|600px|Synthesis of C.I. [[Pigment Yellow 13]], a diarylide pigment.]]

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. This chemical reaction is called azo N-coupling, or the synthesis of azoamines.

:[[File:Azo N coupling.jpg|500px|Azo N-coupling under basic condition|left]]

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.

References

References

1. {{March6th
2. {{Ullmann. (2005)
3. "Imaging Technology, 2. Copying and Nonimpact Printing Processes".
4. (2017-08-30). "A Chemoselective Rapid Azo-Coupling Reaction (CRACR) for Unclickable Bioconjugation". Journal of the American Chemical Society.
5. J. L. Hartwell and Louis F. Fieser. (1936). "Coupling of o-tolidine and Chicago acid".
6. H. T. Clarke and W. R. Kirner. (1922). "Methyl red".
7. K. Hunger. W. Herbst "Pigments, Organic" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim, 2012. {{doi. 10.1002/14356007.a20_371
8. Khazaei. (2012). "azo amine coupling giving triazenes, and triazene's decomposition giving diazonium salt". Synlett.
9. Wiley Subscription Services. (2013). "Synthesis, characterization, and application of a triazene-base polymer". Journal of Applied Polymer Science.
10. Serge Ratton, Bernard Botannet. (1981). "Preparation of aromatic azoamines by diazotization/coupling/rearrangement of aromatic amines". US Patent 4275003A.