Azo compound

Yellow azo dye.

Methyl red, an azo dye, turns red in acidic solutions.

Azo compounds are compounds bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl. The N=N group is called an azo group, although the parent compound, HNNH, is called diimide. The more stable derivatives contain two aryl groups.

Aryl azo compounds have vivid colors, especially reds, oranges, and yellows. Therefore, they are used as dyes, classified as azo dyes. Examples include methyl red and Disperse Orange 1.

Etymology

The name azo comes from azote, the French name for nitrogen, which is derived from the Greek a (not) + zoe (to live).

As dyes and pigments

As noted above, aryl azo compounds are brightly colored and are useful as dyes and pigments. In chemical terms, their colors are attributed to delocalization of п (pi) electrons.

Some azo compounds, such as methyl orange and methyl red, are used as acid-base indicators due to the different colors of their acid and salt forms. The development of azo dyes was an important step in the development of the chemical industry.

Azo pigments are colorless particles (typically earths or clays), which have been colored using an azo compound. Azo pigments are important in a variety of paints including artist's paints. They have excellent coloring properties, again mainly in the yellow to red range, as well as lightfastness. The lightfastness depends not only on the properties of the organic azo compound, but also on the way they have been adsorbed on the pigment carrier. Azo pigments are advantageous because they are non-toxic.

Organic chemistry

Aryl azo compounds

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. 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.

Alkyl azo compounds

Aliphatic azo compounds (R and/or R' = aliphatic) are less commonly encountered than the aryl azo compounds. One example is diethyldiazene, EtN=NEt. 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.

Formation by azo coupling

An azo compound is formed by a reaction known as an azo coupling. It is an organic reaction between a diazonium compound and an aniline or a phenol. The reaction may be written as follows:

Mechanism: This reaction is called an electrophilic aromatic substitution. The diazonium salt acts as an electrophile, and the activated arene, a nucleophile. The reaction mechanism may be written as follows:

See also

• Diazonium compound
• Ponceau 4R
• Ponceau S

Further reading

• J. L. Hartwell and Louis F. Fieser. "Coupling of o-tolidine and Chicago acid". Org. Synth.; Coll. Vol. 2: 145. 
• H. T. Clarke and W. R. Kirner. "Methyl red". Org. Synth.; Coll. Vol. 1: 374. 

External links

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