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Azo compounds are bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups).
IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted by hydrocarbyl groups, e.g. azobenzene or diphenyldiazene.", where Ph stands for phenyl group. The more stable derivatives contain two aryl groups. The group is called an azo group (, ).
Many textile and leather articles are dyed with and .
Aromatic azo compounds can be synthesized by azo coupling, which entails an electrophilic substitution reaction where an diazonium salt is attacked by another aryl ring, especially those substituted with electron-donating groups:
Since are often unstable near room temperature, the azo coupling reactions are typically conducted near 0 °C. The oxidation of hydrazines () also gives azo compounds. Azo dyes are also prepared by the condensation of Nitro compound with Aniline followed by redox of the resulting azoxy intermediate:
As a consequence of Pi bond-delocalization, aryl azo compounds have vivid colors, especially reds, oranges, and yellows. Therefore, they are used as , and are commonly known as , an example of which is Disperse Orange 1. Some azo compounds, e.g., methyl orange, are used as pH indicator 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 commercial success of azo dyes motivated the development of azo compounds in general.
A simple dialkyl diazo compound is diethyldiazene, , which can be synthesized through a variant of the Ramberg–Bäcklund reaction. Because of their instability, aliphatic azo compounds pose the risk of explosion.
AIBN is produced by converting acetone cyanohydrin to the hydrazine derivative followed by oxidation:Jean-Pierre Schirmann, Paul Bourdauducq: "Hydrazine" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. .
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 illumination, converts to the cis isomer.
(2025). 9780387954684, Springer. ISBN 9780387954684
For textile dying, a typical Nitro compound coupling partner would be disodium 4,4′-dinitrostilbene-2,2′-disulfonate. Typical aniline partners are shown below.Klaus Hunger, Peter Mischke, Wolfgang Rieper, Roderich Raue, Klaus Kunde, Aloys Engel: "Azo Dyes" in Ullmann’s Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim..
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:
For instance a mixture of styrene and maleic anhydride in toluene will react if heated, forming the copolymer upon addition of AIBN.
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 . Likewise, several case studies have linked azo pigments with basal cell carcinoma.
European regulation
Certain azo dyes can break down under reductive conditions to release any of a group of defined . 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. European Ban on Certain Azo Dyes , Dr. A. Püntener and Dr. C. Page, Quality and Environment, TFL
See also
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