Azo compounds are compounds bearing the functional group R–N=N–R′, in which R and R′ can be either aryl or alkyl. IUPAC defines azo compounds as: "Derivatives of diazene (diimide), HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene." The more stable derivatives contain two aryl groups. The N=N group is called an azo group. The name azo comes from azote, the French name for nitrogen that is derived from the Greek ἀ- (a-, "not") + ζωή (zōē, life).
As dyes and pigments
As a consequence of п-delocalization, aryl azo compounds have vivid colors, especially reds, oranges, and yellows. Therefore, they are used as dyes, and are commonly known as azo dyes, an example of which is Disperse Orange 1. Some azo compounds, e.g., methyl orange, are used as acid-base indicators due to the different colors of their acid and salt forms. Most DVD-R/+R and some CD-R discs use blue azo dye as the recording layer. The development of azo dyes was an important step in the development of the chemical industry.
Azo pigments consist of colorless particles (typically earths or clays) 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 good lightfastness. The lightfastness depends not only on the properties of the organic azo compound, but also on the way they have been absorbed on the pigment carrier.
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 azo coupling, which entails an electrophilic substitution reaction where an aryl diazonium cation is attacked by another aryl ring, especially those substituted with electron-donating groups:
2 + Ar′H → ArN=NAr′ + H+
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. Azo dyes are also prepared by the condensation of nitroaromatics with anilines followed by reduction of the resulting azoxy intermediate:
- ArNO2 + Ar′NH2 → ArN(O)=NAr′ + H2O
- ArN(O)=NAr′ + C6H12O6 → ArN=NAr′ + C6H10O6 + H2O
Alkyl azo compounds
Aliphatic azo compounds (R and/or R′ = aliphatic) are less commonly encountered than the aryl azo compounds. A commercially important alkyl azo compound is azobisisobutyronitrile (AIBN), which is widely used as an initiator in free-radical polymerizations and other radical-induced reactions. It achieves this initiation by decomposition, eliminating a molecule of nitrogen gas to form two 2-cyanoprop-2-yl radicals:
- 2 (CH3)2C(CN)OH + N2H4 → [(CH3)2C(CN)]2N2H2 + 2 H2O
- [(CH3)2C(CN)]2N2H2 + Cl2 → [(CH3)2C(CN)]2N2 + 2 HCl
Safety and regulation
Many azo pigments are non-toxic, although some, such as dinitroaniline orange, ortho-nitroaniline orange, or pigment orange 1, 2, and 5 have been found to be mutagenic. Likewise, several case studies have linked azo pigments with basal cell carcinoma.
Azo dyes derived from benzidine are carcinogens; exposure to them has classically been associated with bladder cancer. Accordingly, the production of benzidine azo dyes was discontinued in the 1980s "in the most important western industrialized countries".
Certain azo dyes can break down under reductive conditions to release any of a group of defined aromatic amines. Consumer goods which contain listed aromatic amines originating from azo dyes were prohibited from manufacture and sale in European Union countries in September 2003. As only a small number of dyes contained an equally small number of amines, relatively few products were affected.
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