Azo dye

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Chemical structure of Solvent Yellow 7, an orange colored azo dye. 4-hydroxyphenylazobenzene.png
Chemical structure of Solvent Yellow 7, an orange colored azo dye.

Azo dyes are organic compounds bearing the functional group RN=NR′, 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. [1] Azo dyes are synthetic dyes and do not occur naturally. [2] [3] Most azo dyes contain only one azo group but there are some that contain two or three azo groups, called "diazo dyes" and "triazo dyes" respectively. Azo dyes comprise 60-70% of all dyes used in food and textile industries. [3] 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. [4] [5]



Many kinds of azo dyes are known, and several classification systems exist. Some classes include disperse dyes, metal-complex dyes, reactive dyes, and substantive dyes. Also called direct dyes, substantive dyes are employed for cellulose-based textiles, which includes cotton. The dyes bind to the textile by non-electrostatic forces. In another classification, azo dyes can be classified according to the number of azo groups.

Trypan blue is an example of a direct dye, used for cotton. TrypanBlueSalt.png
Trypan blue is an example of a direct dye, used for cotton.

Physical properties, structure, and bonding

As a consequence of π-delocalization, aryl azo compounds have vivid colors, especially reds, oranges, and yellows. An example is Disperse Orange 1. Some azo compounds, e.g., methyl orange, are used as acid-base indicators. Most DVD-R/+R and some CD-R discs use blue azo dye as the recording layer.

Many phenolic diazo dyes participate in tautomeric equilibria shown here in simplified form (Ar = aryl). DiazonaphtholEquil.png
Many phenolic diazo dyes participate in tautomeric equilibria shown here in simplified form (Ar = aryl).

Azo dyes are solids. Most are salts, the colored component being the anion usually, although some cationic azo dyes are known. The anionic character of most dyes arises from the presence of 1-3 sulfonic acid groups, which are fully ionized at the pH of the dyed article:

RSO3H → RSO3 + H+

Most proteins are cationic, thus dyeing of leather and wool corresponds to an ion exchange reaction. The anionic dye adheres to these articles through electrostatic forces. Cationic azo dyes typically contain quaternary ammonium centers.


Most azo dyes are prepared by azo coupling, which entails an electrophilic substitution reaction of an aryl diazonium cation with another compound, the coupling partner. Generally, coupling partners are other aromatic compounds with electron-donating groups: [7]

+ Ar′H → ArN=NAr′ + H+

In practice, acetoacetic amide are widely used as coupling partners:

+ Ar′NHC(O)CH2C(O)Me → ArN=NCH(C(O)Me)(C(O)NHAr′) + H+

Azo dyes are also prepared by the condensation of nitrated aromatic compounds 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

For textile dying, a typical nitro coupling partner would be disodium 4,4′-dinitrostilbene-2,2′-disulfonate. Typical aniline partners are shown below. Since anilines are prepared from nitro compounds, some azo dyes are produced by partial reduction of aromatic nitro compounds. [5]

Many azo dyes are produced by reactions from pre-existing azo compounds. Typical reactions include metal complexation and acylation.

Azo pigments

Azo pigments are similar in chemical structure to azo dyes, but they lack solubilizing groups. Because they are practically insoluble in all solvents, they are not readily purified, and thus require highly purified precursors.

Synthesis of C.I. Pigment Yellow 12, an azo pigment (also classified as a diarylide pigment). PigmentYellow12corrected.png
Synthesis of C.I. Pigment Yellow 12, an azo pigment (also classified as a diarylide pigment).

Azo pigments are important in a variety of plastics, rubbers, and paints (including artist's paints). They have excellent coloring properties, 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.

Azo pigments amongst food pigments are the oldest and also the most widely used. They were discovered by Peter Griess in 1858. [8]


In order for dyes to be useful, they must possess a high degree of chemical and photolytic stability. As a result of this stability, photolysis is not considered to be a degradation pathway for azo dyes. In order to prolong the lifetime of products dyed with azo dyes, it is essential to ensure stability against microbial attack, and tests have shown that azo dyes biodegrade negligibly in short term tests under aerobic conditions. Under anaerobic conditions, however, discoloration may be observed as a consequence of biodegradation. [9]

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 are mutagenic and carcinogenic. [10] [11]

Azo dyes derived from benzidine are carcinogens; exposure to them has classically been associated with bladder cancer. [12] Accordingly, the production of benzidine azo dyes was discontinued in the 1980s in many western countries. [5]

European regulation

Certain azo dyes degrade under reductive conditions to release any of a group of defined aromatic amines. Since September 2003, the European Union has banned the manufacture or sale of consumer goods which contain the listed amines. Since only a small number of dyes produced those amines, relatively few products were actually affected. [4]

See also

Related Research Articles

<span class="mw-page-title-main">Amine</span> Chemical compounds and groups containing nitrogen with a lone pair (:N)

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. Inorganic derivatives of ammonia are also called amines, such as monochloramine.

<span class="mw-page-title-main">Aniline</span> Organic compound (C₆H₅NH₂); simplest aromatic amine

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.

In organic chemistry, an aromatic amine is an organic compound consisting of an aromatic ring attached to an amine. It is a broad class of compounds that encompasses anilines, but also many more complex aromatic rings and many amine substituents beyond NH2. Such compounds occur widely.

<span class="mw-page-title-main">Azo compound</span> Organic compounds with a diazenyl group (–N=N–)

Azo compounds are organic compounds bearing the functional group diazenyl.

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

Sudan I is an organic compound, typically classified as an azo dye. It is an intensely orange-red solid that is added to colourise waxes, oils, petrol, solvents, and polishes. Sudan I has also been adopted for colouring various foodstuffs, especially curry powder and chili powder, although the use of Sudan I in foods is now banned in many countries, because Sudan I, Sudan III, and Sudan IV have been classified as category 3 carcinogens by the International Agency for Research on Cancer. Sudan I is still used in some orange-coloured smoke formulations and as a colouring for cotton refuse used in chemistry experiments.

<span class="mw-page-title-main">Acid dye</span> Dye applied to low pH textile

An acid dye is a dye that is typically applied to a textile at low pH. They are mainly used to dye wool, not cotton fabrics. Some acid dyes are used as food colorants, and some can also be used to stain organelles in the medical field.

<span class="mw-page-title-main">Diazonium compound</span> Group of organonitrogen compounds

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 carbon act as a nucleophile. In most cases, including the examples below, the diazonium compound is also aromatic.

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

Benzidine (trivial name), also called 1,1'-biphenyl-4,4'-diamine (systematic name), is an organic compound with the formula (C6H4NH2)2. It is an aromatic amine. It is a component of a test for cyanide. Related derivatives are used in the production of dyes. Benzidine has been linked to bladder and pancreatic cancer.

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

Aniline Yellow is a yellow azo dye and an aromatic amine. It is a derivative of azobenzene. It has the appearance of an orange powder. Aniline Yellow was the first azo dye. it was first produced in 1861 by C. Mene. The second azo dye was Bismarck Brown in 1863. Aniline Yellow was commercialized in 1864 as the first commercial azo dye, a year after aniline black. It is manufactured from aniline.

4-Aminobiphenyl (4-ABP) is an organic compound with the formula C6H5C6H4NH2. It is an amine derivative of biphenyl. It is a colorless solid, although aged samples can appear colored. 4-Aminobiphenyl was commonly used in the past as a rubber antioxidant and an intermediate for dyes. Exposure to this aryl-amine can happen through contact with chemical dyes and from inhalation of cigarette smoke. Researches showed that 4-aminobiphenyl is responsible for bladder cancer in humans and dogs by damaging DNA. Due to its carcinogenic effects, commercial production of 4-aminobiphenyl ceased in the United States in the 1950s.

<span class="mw-page-title-main">3-Nitroaniline</span> Chemical compound

3-Nitroaniline is an organic compound with the formula H2NC6H4NO2. A yellow solid, it is a derivative of aniline, carrying a nitro functional group in position 3. It is an isomer of 2-nitroaniline and 4-nitroaniline. It is used as a precursor to dyes.

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.

The Béchamp reduction is a chemical reaction that converts aromatic nitro compounds to their corresponding anilines using iron as the reductant:

<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.

<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.

Zinin reaction or Zinin reduction involves reduction of nitro aromatic compounds to the amines using sodium sulfide. It is used to convert nitrobenzenes to anilines. The reaction selectively reduces nitro groups in the presence of other easily reduced functional groups are present in the molecule.

<i>N</i>-Ethyl-<i>N</i>-(2-chloroethyl)aniline Chemical compound

N-Ethyl-N-(2-chloroethyl)aniline is the organic compound with the formula C6H5N(Et)(CH2CH2Cl) (Et = ethyl). It is a low-melting colorless solid that is an alkylating agent. The compound is a precursor to several cationic azo dyes via reaction of the chloroethyl group with tertiary amines or pyridine followed by azo coupling. Examples of derived dyes include C. I. Basic Red 18, Maxilon Red 2GL, and Yoracryl Red 2G.

Basic Red 18 is a cationic azo dye used for coloring textiles. The chromophore is the cation, which contains many functional groups, but most prominently the quaternary ammonium center.

<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.


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