Azo compounds areorganic compounds bearing thefunctional group diazenyl (R−N=N−R′, in which R and R′ can be eitheraryl oralkyl groups).
IUPAC defines azo compounds as: "Derivatives ofdiazene (diimide),HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g.PhN=NPhazobenzene or diphenyldiazene.", where Ph stands forphenyl group.[1] The more stable derivatives contain two aryl groups. TheN=N group is called anazo group (from French azote 'nitrogen', from Ancient Greek ἀ- (a-) 'not' and ζωή (zōē) 'life').
Manytextile andleather articles are dyed withazo dyes andpigments.[2]
Aryl azo compounds are usually stable, crystalline species.Azobenzene is the prototypicalaromatic azo compound. It exists mainly as thetransisomer, but upon illumination, converts to thecis isomer.
Aromatic azo compounds can be synthesized byazo coupling, which entails anelectrophilic substitution reaction where anaryl diazonium cation is attacked by another aryl ring, especially those substituted withelectron-donating groups:[3][4]
Sincediazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near 0 °C. Theoxidation ofhydrazines (R−NH−NH−R′) also gives azo compounds.[5] Azo dyes are also prepared by thecondensation ofnitroaromatics withanilines followed byreduction of the resultingazoxy intermediate:
For textile dying, a typicalnitrocoupling partner would bedisodium 4,4′-dinitrostilbene-2,2′-disulfonate. Typical aniline partners are shown below.[6]
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As a consequence ofπ-delocalization, aryl azo compounds have vivid colors, especially reds, oranges, and yellows. Therefore, they are used asdyes, and are commonly known asazo dyes, an example of which isDisperse Orange 1. Some azo compounds, e.g.,methyl orange, are used asacid-base indicators due to the different colors of their acid and salt forms. MostDVD-R/+R and someCD-R discs use blue azo dye as the recording layer. The commercial success of azo dyes motivated the development of azo compounds in general.
Aliphatic azo compounds (R and/or R′ = aliphatic) are less commonly encountered than the aryl azo compounds. A commercially important alkyl azo compound isazobisisobutyronitrile (AIBN), which is widely used as an initiator infree-radical polymerizations and other radical-induced reactions. It achieves this initiation bydecomposition, eliminating a molecule ofnitrogen gas to form two 2-cyanoprop-2-yl radicals:
For instance a mixture ofstyrene andmaleic anhydride intoluene will react if heated, forming thecopolymer upon addition of AIBN.
A simple dialkyl diazo compound is diethyldiazene,CH3CH2−N=N−CH2CH3, which can be synthesized through a variant of theRamberg–Bäcklund reaction.[7] Because of their instability, aliphatic azo compounds pose the risk ofexplosion.
AIBN is produced by convertingacetone cyanohydrin to thehydrazine derivative followed by oxidation:[8]
Many azo pigments are non-toxic, although some, such asdinitroaniline orange,ortho-nitroaniline orange, or pigment orange 1, 2, and 5 have been found to bemutagenic.[9] Likewise, several case studies have linked azo pigments withbasal cell carcinoma.[10]
Certain azo dyes can break down under reductive conditions to release any of a group of definedaromatic amines. Consumer goods which contain listed aromatic amines originating from azo dyes were prohibited from manufacture and sale inEuropean Union countries in September 2003. As only a small number of dyes contained an equally small number of amines, relatively few products were affected.[2]
