Azo compound

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General chemical formula of azo compounds Azo-group-2D-flat.png
General chemical formula of azo compounds

Azo compounds are organic compounds bearing the functional group diazenyl (R−N=N−R′, in which R and R′ can be either aryl or alkyl groups).


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." [1] The more stable derivatives contain two aryl groups. The N=N group is called an azo group (from French azote ' nitrogen ', from Ancient Greek ἀ- (a-) 'not',andζωή (zōē) 'life').

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]

Since diazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near 0 °C. The oxidation of hydrazines (R−NH−NH−R′) also gives azo compounds. [4] Azo dyes are also prepared by the condensation of nitroaromatics with anilines followed by reduction of the resulting azoxy intermediate:

For textile dying, a typical nitro coupling partner would be disodium 4,4′-dinitrostilbene-2,2′-disulfonate. Typical aniline partners are shown below. [5]

An orange azo dye 4-hydroxyphenylazobenzene.png
An orange azo dye

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 commercial success of azo dyes motivated the development of azo compounds in general.

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:

Formation of Radicals from AIBN.png

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, EtN=NEt. [6] 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: [7]

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. [8] Likewise, several case studies have linked azo pigments with basal cell carcinoma. [9]

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

Related Research Articles

In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline;

<span class="mw-page-title-main">Aniline</span> Chemical compound

Aniline is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starting material for fine chemical synthesis. Its main use is in the manufacture of precursors to polyurethane, dyes, and other industrial chemicals. Like most volatile amines, it has the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds. It is toxic to humans.

<span class="mw-page-title-main">Nitrous acid</span> Chemical compound

Nitrous acid is a weak and monoprotic acid known only in solution, in the gas phase and in the form of nitrite salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

In organic chemistry, a nitrile is any organic compound that has a −C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, including methyl cyanoacrylate, used in super glue, and nitrile rubber, a nitrile-containing polymer used in latex-free laboratory and medical gloves. Nitrile rubber is also widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons.

<span class="mw-page-title-main">Nickel(II) chloride</span> Chemical compound

Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages in cases of long-term inhalation exposure.

<span class="mw-page-title-main">Sulfonic acid</span> Organic compounds with the structure R−S(=O)2−OH

In organic chemistry, sulfonic acid refers to a member of the class of organosulfur compounds with the general formula R−S(=O)2−OH, where R is an organic alkyl or aryl group and the S(=O)2(OH) group a sulfonyl hydroxide. As a substituent, it is known as a sulfo group. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. The parent compound is the parent sulfonic acid, HS(=O)2(OH), a tautomer of sulfurous acid, S(=O)(OH)2. Salts or esters of sulfonic acids are called sulfonates.

<span class="mw-page-title-main">Azobisisobutyronitrile</span> Chemical compound

Azobisisobutyronitrile (abbreviated AIBN) is an organic compound with the formula [(CH3)2C(CN)]2N2. This white powder is soluble in alcohols and common organic solvents but is insoluble in water. It is often used as a foamer in plastics and rubber and as a radical initiator.

<span class="mw-page-title-main">Azo dye</span> Class of organic compounds used as dye

Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl and substituted aryl groups. They are a commercially important family of azo compounds, i.e. compounds containing the C-N=N-C linkage. Azo dyes are synthetic dyes and do not occur naturally. Most azo dyes contain only one azo group, but some dyes called "disazo dyes" contain two azo groups, some dyes called "trisazo dyes" contain three azo groups and are or more. Azo dyes comprise 60-70% of all dyes used in food and textile industries. Azo dyes are widely used to treat textiles, leather articles, and some foods. Chemically related derivatives of azo dyes include azo pigments, which are insoluble in water and other solvents.

<span class="mw-page-title-main">Diazonium compound</span> Diazonium salts of formula R-N≡N+

Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group [R−N+≡N]X where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide.

In organic chemistry, an azo coupling is an organic reaction between a diazonium compound and another aromatic compound that produces an azo compound. In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile and the activated arene is a nucleophile. In most cases, including the examples below, the diazonium compound is also aromatic.

<span class="mw-page-title-main">Nitroso</span> Class of functional groups with a –N=O group attached

In organic chemistry, nitroso refers to a functional group in which the nitric oxide group is attached to an organic moiety. As such, various nitroso groups can be categorized as C-nitroso compounds, S-nitroso compounds, N-nitroso compounds, and O-nitroso compounds.

<span class="mw-page-title-main">Hydrazines</span>

Hydrazines (R2N−NR2) are a class of chemical compounds with two nitrogen atoms linked via a covalent bond and which carry from one up to four alkyl or aryl substituents. Hydrazines can be considered as derivatives of the inorganic hydrazine (H2N−NH2), in which one or more hydrogen atoms have been replaced by hydrocarbon groups.

<span class="mw-page-title-main">Organocopper compound</span> Compound with carbon to copper bonds

Organocopper compounds is the chemistry of organometallic compounds containing a carbon to copper chemical bond. Organocopper chemistry is the study of organocopper compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry.

The reduction of nitro compounds are chemical reactions of wide interest in organic chemistry. The conversion can be effected by many reagents. The nitro group was one of the first functional groups to be reduced. Alkyl and aryl nitro compounds behave differently. Most useful is the reduction of aryl nitro compounds.

<span class="mw-page-title-main">Tolidine</span> Chemical compound

2-Tolidine (orthotolidine, o-tolidine; not to be confused with o-toluidine) is an organic compound with the chemical formula (C6H4(CH3)NH2)2. Several isomers are known; the 3-tolidine derivative is also important commercially. It is a colorless compound although commercial samples are often colored. It is slightly soluble in water. It forms salts with acids, such as the hydrochloride, which is commercially available.

Azobenzene reductase also known as azoreductase (EC is an enzyme that catalyzes the chemical reaction:

<span class="mw-page-title-main">Diketene</span> Organic compound with formula (CH2CO)2

Diketene is an organic compound with the molecular formula C4H4O2, and which is sometimes written as (CH2CO)2. It is formed by dimerization of ketene, H2C=C=O. Diketene is a member of the oxetane family. It is used as a reagent in organic chemistry. It is a colorless liquid.

<span class="mw-page-title-main">Tripotassium phosphate</span> Chemical compound

Tripotassium phosphate, also called tribasic potassium phosphate is a water-soluble salt with the chemical formula K3PO4.(H2O)x (x = 0, 3, 7, 9). Tripotassium phosphate is basic.

Zinin reaction or Zinin reduction was discovered by a Russian organic chemist Nikolay Zinin. This reaction involves conversion of nitro aromatic compounds like nitrobenzene to amines by reduction with sodium sulfides. The reaction exhibits excellent selectivity for the nitro group and is useful in cases where other easily reduced functional groups are present in the molecule. Moreover, dinitrobenzenes can often be reduced selectively to the nitroaniline.

<i>o</i>-Dianisidine Chemical compound

o-Dianisidine is an organic compound with the formula [(CH3O)(H2N)C6H3]2. A colorless or white solid, it is a bifunctional compound derived via the benzidine rearrangement from o-anisidine.


  1. IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006) " azo compounds ". doi : 10.1351/goldbook.A00560
  2. 1 2 European Ban on Certain Azo Dyes Archived 2012-08-13 at the Wayback Machine , Dr. A. Püntener and Dr. C. Page, Quality and Environment, TFL
  3. H. T. Clarke; W. R. Kirner (1941). "Methyl Red". Organic Syntheses .; Collective Volume, vol. 1, p. 374
  4. March, J. (1992). Advanced Organic Chemistry (5th ed.). New York: J. Wiley and Sons. ISBN   978-0-471-60180-7.
  5. 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. doi : 10.1002/14356007.a03_245.
  6. Ohme, R.; Preuschhof, H.; Heyne, H.-U. (1988). "Azoethane". Organic Syntheses .; Collective Volume, vol. 6, p. 78
  7. Jean-Pierre Schirmann, Paul Bourdauducq: "Hydrazine" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2002. doi : 10.1002/14356007.a13_177.
  8. 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.
  9. 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.