Utilisateur:Claude Rubio/Bac à sable

On désigne comme composés azoïques des molécules possédant le groupe fonctionnel R-N=N-R', où R et R' représentent un groupe aryle ou un groupe alkyle.
Les composés azoïques les plus stables sont ceux porteurs de deux groupes aryles. Ils constituent une famille chimique importante pour leur utilisation comme colorants de synthèse.

Yellow azo dye

As dyesModifier

As a consequence of п-delocalization, aryl azo compounds have vivid colours, especially reds, oranges, and yellows. Therefore, they are used as dyes, "azo dyes." Some azo compounds, eg. methyl orange, are used as acid-base indicators due to the different colours of their acid and salt forms. The development of azo dyes was an important step in the development of the chemical industry.

Organic chemistryModifier

Aryl azo compoundsModifier

Aryl azo compounds are usually stable, crystalline species. Azobenzene is the prototypical aromatic azo compound. It exists mainly as the trans isomer, but upon photolysis, converts to the cis isomer. Aromatic azo compounds can be synthesized by using an azo coupling reaction, which entails an electrophilic substitution reaction where a aryl diazonium cation attacks another aryl ring, especially those substituted with electron-releasing groups.[1] Since diazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near ice temperatures. The oxidation of hydrazines (R-NH-NH-R') also gives azo compounds.[2]

Alkyl azo compoundsModifier

Aliphatic azo compounds (R and/or R' = aliphatic) are less commonly encountered than the aryl azo compounds. One example is diethyldiazene, EtN=NEt.[3] At elevated temperatures or upon irradiation, the carbon-nitrogen (C-N) bonds in certain alkyl azo compounds cleave with the loss of nitrogen gas to generate radicals. Owing to this process, some aliphatic azo compounds are utilized as radical initiators. Representative is Azobisisobutylonitrile (AIBN) which is widely used as an initiator in polymerization. Because of their instability, especially for aliphatic ones, care should be taken with the handling of azo compounds or an explosion may occur.

ReferencesModifier

  1. H. T. Clarke and W. R. Kirner, Methyl Red, Org. Synth., coll. « vol. 1 », , p. 374
  2. March, J. “Advanced Organic Chemistry” $th Ed. J. Wiley and Sons, 1992: New York. (ISBN 0-471-60180-2).
  3. Ohme, R.; Preuschhof, H.; Heyne, H.-U., Azoethane, Org. Synth., coll. « vol. 6 », , p. 78

See alsoModifier

Modèle:Functional Groups

* Catégorie:Groupe fonctionnel Catégorie:Colorant organique