Azo compound
Template:Short description Template:Redirect-distinguish2 Azo compounds are organic compounds bearing the functional group diazenyl (Page Module:Chem2/styles.css has no content.R−N=N−R′, in which N is nitrogen and R and R′ can be either aryl or alkyl groups).
IUPAC defines azo compounds as: "Derivatives of diazene (diimide), Page Module:Chem2/styles.css has no content.HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. Page Module:Chem2/styles.css has no content.PhN=NPh azobenzene or diphenyldiazene.", where Ph stands for phenyl group.[1] The more stable derivatives contain two aryl groups. The Page Module:Chem2/styles.css has no content.N=N group is called an azo group (Script error: No such module "params"., Script error: No such module "params".).
Many textile and leather articles are dyed with azo dyes and pigments.[2]
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.
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:[3][4]
- Page Module:Chem2/styles.css has no content.ArN+2 + Ar'H → ArN=NAr' + H+
Since diazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near 0 °C.[5]
The oxidation of hydrazines (Page Module:Chem2/styles.css has no content.R−NH−NH−R′) also gives azo compounds.[5] In the opposite direction, condensation of nitroaromatics with anilines gives an azoxy intermediate reducible to the diazo:[6]
- Page Module:Chem2/styles.css has no content.ArNO2 + Ar'NH2 → ArN(O)=NAr' + H2O
- Page Module:Chem2/styles.css has no content.ArN(O)=NAr' + C6H12O6 → ArN=NAr' + C6H10O6 + H2O
As a consequence of π-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 commercial success of azo dyes motivated the development of azo compounds in general.
Alkyl azo compounds
Script error: No such module "Labelled list hatnote". 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.
A simple dialkyl diazo compound is diethyldiazene, Page Module:Chem2/styles.css has no content.CH3CH2−N=N−CH2CH3, which can be synthesized through a variant of the Ramberg–Bäcklund reaction.[7] 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:[8]
- Page Module:Chem2/styles.css has no content.2 (CH3)2C(CN)OH + N2H4 → [(CH3)2C(CN)]2N2H2 + 2 H2O
- Page Module:Chem2/styles.css has no content.[(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.[9] Likewise, several case studies have linked azo pigments with basal cell carcinoma.[10]
European regulation
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.[2]
See also
- Azo coupling
- Disodium 4,4′-dinitrostilbene-2,2′-disulfonate – common coupling partner in azo dye synthesis
References
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- ^ IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "azo compounds". Script error: No such module "CS1 identifiers".
- ^ a b European Ban on Certain Azo Dyes Script error: No such module "webarchive"., Dr. A. Püntener and Dr. C. Page, Quality and Environment, TFL
- ^ Page Module:Citation/CS1/styles.css has no content.Grossman, Robert B. (2003). The Art of Writing Reasonable Organic Reaction Mechanisms (2nd ed.). New York: Springer. pp. 130–131. ISBN 0-387-95468-6.
- ^ Page Module:Citation/CS1/styles.css has no content.H. T. Clarke; W. R. Kirner (1941). "Methyl Red". Organic Syntheses; Page Module:Citation/CS1/styles.css has no content.Collected Volumes, vol. 1, p. 374.
- ^ a b Page Module:Citation/CS1/styles.css has no content.March, J. (1992). Advanced Organic Chemistry (5th ed.). New York: J. Wiley and Sons. ISBN 978-0-471-60180-7.
- ^ 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.Script error: No such module "CS1 identifiers"..
- ^ Page Module:Citation/CS1/styles.css has no content.Ohme, R.; Preuschhof, H.; Heyne, H.-U. (1988). "Azoethane". Organic Syntheses; Page Module:Citation/CS1/styles.css has no content.Collected Volumes, vol. 6, p. 78.
- ^ Jean-Pierre Schirmann, Paul Bourdauducq: "Hydrazine" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. Script error: No such module "CS1 identifiers"..
- ^ Page Module:Citation/CS1/styles.css has no content.Tucson University. "Health & Safety in the Arts, A Searchable Database of Health & Safety Information for Artists". Tucson University Studies. Archived from the original on 2009-05-10.
- ^ Page Module:Citation/CS1/styles.css has no content.Eva Engel; Heidi Ulrich; Rudolf Vasold; Burkhard König; Michael Landthaler; Rudolf Süttinger; Wolfgang Bäumler (2008). "Azo Pigments and a Basal Cell Carcinoma at the Thumb". Dermatology. 216 (1): 76–80. doi:10.1159/000109363. PMID 18032904. S2CID 34959909.
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