An Overview of the Colorful History, Synthesis, and Applications of Azo Dyes

Introduction:

The International Union of Pure and Applied Chemistry (IUPAC) defines azo compounds as the “derivatives of diazene (diimide), NH=NH, wherein both hydrogens are substituted by hydrocarbyl groups.” [8] More commonly, aryl azo compounds are referred to as azo dyes, which are a type of organic compound that contains the –N=N– functional group, or the “azo group.” Azo dyes are formed by electrophilic substitution diazonium-coupling reactions, which are a contributing factor to the trademark bright colors of azo dyes. [2] The applications, both practical and industrial, of azo dyes are numerous in today’s society. Since the founding of their synthesis, aryl azo compounds have served an important role in such industries as ink jet printing, textile production and coloring, and in laboratories as biological stains and also pH indicators. [7,9,17,18] The versatility of azo dyes led them to account for 60-70% of all synthetic dyes used to date. [18] However, as with all good things, there is a drawback to using certain azo dyes. Azo dyes synthesized from benzidine have been known to reduce to an aryl amine in the presence of certain bacteria. Exposure to an aryl amine reduced from an azo dye caused bladder cancer in those that come into contact with it. [1,5] Therefore, the production of some azo dyes has been banned, forcing scientists to find new compounds to synthesize different colored dyes from.

Structure of Azo Dyes:

Azo dyes are distinguished by the –N=N– group they contain. They are often generally represented as R—N=N—R’, where R and R’ represent aryl groups, usually of aromatic nature. [2] This creates a chromophore, which is the portion of the molecule directly responsible for the strong color of azo dyes. [12] All azo dyes must contain salt-forming groups such as hydroxyl, amino, sulfonic acid, or carboxylic groups, as well as the “azo group.” These salt-forming groups help to intensify the color of the dye due to increased conjugation. They also promote solubility in water to help the dye bind to the polar structures of common fibers such as cotton. The color of the dye itself is determined by the structure of the aromatic amine or phenol (R’) that binds to the diazonium salt. [2, 17] This coupling reaction determines the color of the dye because it changes the conjugation, or the extent of delocalization of the electrons, allowing varying wavelengths of to be absorbed. The more delocalized the electrons, the longer the lengths of wavelengths absorbed, the redder of the color of the dye. On the other side, the less delocalized the electrons are, the shorter the wavelengths absorbed, and the bluer the color. [4, 6]

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Para Red [23]
20070124232202!Trypan_blue.png
Trypan Blue [11]


Figure 1: Structures of common azo dyes, including Para Red and Trypan Blue

Azo dyes are classified to a few subcategories: monoazo, diazo, and polyazo dyes. As the name subjects, monoazo dyes are azo dyes that contain one azo group. Diazo dyes contain two azo groups, and polyazo dyes contain three or more azo groups. [16]

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Yellow
Solvent Red 14 [24]
red
Sudan Red 17 [25]

Figure 2: Structures of a monoazo and diazo (left to right)

History of Azo Compounds:

Before 1862, the chemistry of dyes was fairly unknown. The dye that was produced in the greatest quantity was a purple dye discovered by William Henry Perkins. This dye was extracted from a black solid produced by the dichromate oxidation of aniline. However, large-scale production of this dye led to a very low yield. This led to the discovery of magenta in 1856, which was quickly followed by the release of other colored dyes. [14, 17] The real break through in the beginning of azo compound research came in 1862 when a German chemist August Wilhelm von Hofmann discovered that magenta was the salt of the base rosaniline. Hofmann continued his research and in 1864, verified Edward Nicholson’s hypothesis that magenta was produced via the oxidation of commercial aniline, not pure aniline. The difference is that commercial aniline contained isomeric forms of toluidide. [14] This led to the theory that the color of a dye is determined by adding methyl, ethyl, phenyl, benzyl and other substituents to the aromatic compound. The addition of three methyl or ethyl groups to magenta led to violet. Increasing the number of these groups led to violet and blue colored dyes. [17]

Meanwhile, Dr. Peter Griess discovered that when nitrous acid is reacted with aniline or another benzene derivative containing the –NH2 functional group, an unstable compound results. He called this compound a diazo compound. Dr. Greiss also found that when these diazo compounds were reacted with aromatic amines or phenols, a coupling reaction proceeded. The product of this diazo coupling reaction were some of the first azo dyes every produced, include Para Red and Aniline (Butter) Yellow. [17] Although there were azo dyes for sale, it was not until 1876 when the structure of the azo dyes were publicized that azo dyes became a major topic of research. Once it was commonly understood that azo compounds could be synthesized from a wide variety of derivatives of aromatic compounds the world saw a rapid increase in the number of dyes being produced. However, the dyes being produced were only able to dye silk and wool directly. It was not until 1884 when German chemist Wilhelm Böttinger discovered that using a cold, dilute solution of nitrous acid allowed the dye to bond to cotton and linens. [17] This greatly increased the use of azo dyes in households and industry. Today, azo dyes represent between 60-70% of all synthetic dyes used as commercial coloring agents.[18]

Synthesis of Aryl Azo Compounds:

The most widely used synthesis, both in large (industry) and small (laboratory) scale settings of azo dyes is a two-step process. The first step synthesizes a diazonium salt, which is accomplished by reacting a primary aromatic amine with the nitrosonium ion, NO+. [10] The nitrosonium ion was previous produced via a reaction of sodium nitrite with hydrochloric acid. [2,4]

Formation_of_nitrosonium_ion.gif
Figure 3: Formation of a nitrosonium ion

The nitrosonium ion is used because N2+ is a better leaving group than NH2. Immediately after the diazonium salt is formed, the diazonium coupling portion of the reaction occurs. Azo couplings are organic reactions that couple a diazonium ion, which is usually aromatic, with a phenol or an aromatic amine. Azo coupling reactions are electrophilic substitution reactions, in which the diazonium salt is the electrophile and aromatic compound acts as the nucleophile. [4] Most diazonium salts decompose rapidly in solution and are unstable at room temperatures, therefore azo coupling reactions are completed quickly and at a near freezing temperatures, somewhere between 3oC-5oC. The coupling reaction must be carried out at a neutral pH. The free amine is converted to the unreactive ammonium salt at a low pH and at a high pH, the diazonium ion is converted to an unreactive diazoate ion. It is only at these intermediate pH’s that there is an adequate concentration of both the diazonium salt and phenol or aromatic amine to yield a coupling rate that will be fast enough to complete the reaction before the diazonium salt decomposes. [16] An example of an azo dye that is synthesized under the above conditions is methyl orange. To produce methyl orange, sulfanilic acid is dissolved in sodium carbonate and water. Adding a solution of sodium nitrite and hydrochloric acid at near freezing temperatures forms the diazonium salt. Reacting the diazonium salt with dimethylaniline (aromatic amine) produces the azo dye (methyl orange).

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Figure 5: Synthesis of Methyl Orange

The method of diazotiation is completely dependant on how basic and soluable the amine being coupled is. Primary aromatic amines can be directly diazotized but amines with sulfonic or carboxylic acid groups must be indirectly diazotized by dissolving the amine in a dilute acid or water. Amines that are weakly basic must be diazotized by dissolving them in a concentrated strong acid solution. Organic solvents (water insoluble amines as described above with the synthesis of methyl orange) must first be dissolved in an organic solvent before they can diazotized.[17]

Due to the facts mentioned above, azo coupling reactions are usually carried out under very strict conditions. However, recent efforts have been made to synthesis azo dyes under mild conditions via a reaction of phenols and amines. These experiments proved that under mild conditions, the reaction of a phenol derivative with a corresponding aniline derivative produced azo compounds in a substantial yield. However, adding substituents to the phenol compounds has a significant effect on the azo coupling reaction. Under these mild conditions, the yield of the reaction is significantly lower when compared to the same reaction under strict conditions. Therefore, producing azo dyes under mild conditions is not advantageous, especially in a large-scale approach.[22]

Dangers of Azo Dyes:

Despite the extreme popularity and functionality of azo dyes, some of them are proven carcinogens. Azo dyes derived from benzidine have been discovered as carcinogens that lead to the development of bladder cancer. [1, 4] These dyes have been exposed to humans in many ways. For example, benzidine based azo dyes interact with a person’s skin if that person sweats while wearing azo dye based clothing or comes into contact azo dye inks and papers. Babies and small children have also been known to ingest benzidine based azo dyes by sucking or chewing on clothing, blankets, or toys that contain the dyes. The use of azo dye ink has led to exposure via inhalation if artists use the benzidine based bye to airbrush a piece of artwork or to paint a building. Various studies have shown that bacteria present in the human skin contains the necessary azo reductases that convert an azo dye to an aryl amine, which has been proven to be carcinogenic. If the azo dye is slightly absorbed into a person’s skin, this reduction process can also occur in a person’s liver. [19] Between the years of 1972-1974, 150,000 Americans were reported to be exposed to benzidine based azo dyes in the work place. This led to the United States Government taking regulatory actions against benzidine derived azo dyes. Despite this initiative, approximately 3.3 million pounds of benzidine based azo dyes were either manufactured or imported into the United States in 1978. [5] Of the roughly 2,000 azo dyes discovered at that point, the 1987 Color Index listed roughly 447 azo dyes that were derived from benzidine, 3,3-dichlorobenzidine, 3,3-dimethylbenzidine, 3,3-dimethoxybenzidine, or 2-napthylamine. Such dyes included Congo Red, Direct Green 1, and Direct Blue 2. [1] As a result of the dangers of azo dyes synthesized from benzidine and its derivatives, production of such dyes was finally put to a halt during the 1980s.[1, 4, 5]

Azo dye wastewater usually has a low environmental impact. However, azo dyes in the wastewater will degrade easily to aromatic amines when introduced to bacteria. As this degradation occurs, an aryl amine derivative produced is toxic and acts as a carcinogen for humans and animals. Therefore, azo dyes should be removed from wastewater by reducing the dye, followed by oxidating the dyes’ aromatic amines substituents.[1, 20]

Applications of Azo Dyes:

Azo dyes are the most widely used dye in today’s industry, accounting for between 60-70% of the market. [18] This is directly attributed to the many advantages of azo dyes, including the wide color ranges available, their ability to bind to a wide variety of fabrics, the cheap production cost, and the fact that the environmental impact is low because water is the primary solvent used in all synthesis reactants. In the modern world, the impact of azo dyes can be seen everywhere. [2, 17] They have been applied as acid/base indicators, biological stains, textile dyes, plastic/polymer colorants, and in high-tech industries. [2, 16] For example, blue azo dyes are often used to produce the recording layer of most DVD and CD disks on the market today. [2] However, due to research carcinogenic concerns, azo dyes are much less commonly used as food colorants in products such as mouthwash and cosmetics. [16]

One of the most popular uses of azo dyes is in the laboratory as pH indicators or biological stains. Three common azo dyes used as pH indicators in many laboratories are Methyl Orange, Para Red, and Methyl Red. Methyl Orange is often used in titrations of acids. At a pH below 3.1, the indicator is red. At a pH above 4.4, Methyl Orange changes to a bright orange. [15] Para Red experiences the same color change as Methyl Orange, except the color change for Para Red occurs between pH 1.0-2.0. [23] Methyl Red is another good pH indicator to use in the titration of acids. Below a pH of 4.4, Methyl Red is red in color. Above a pH of 6.2, it is a bright yellow. [13] The biological stain seen in almost every cell biology lab is trypan blue, which is a diazo dye. [20] Trypan blue is commonly used to stain dead cells in order to count them. [11] This application ranges from looking at onion cells in a high school microscopy unit to counting cells in order to grow a cell culture with a certain concentration of cells.

The most popular use of azo dyes is in the textile industry. Typically, azo dyes have been used to dye common fabrics such as silk, cotton, wool, etc, and this has been done with great success. [14, 16, 17] Recently, the textile industry has begun using fibers synthesized from poly(lactic acid) or PLA. These fibers are being used to create such consumer goods as carpets, bed linens, shirts, underwear, and other clothing. However, PLA fibers dye very differently than usual poly(ethyleneterephthalate) or PET. [3, 18] Therefore, scientists have been forced to change the method of which the azo dyes are applied. The first and more successful way being utilized involved the synthesis of a yellow azo-anthraquinone dye. It was then determined that the ideal conditions to apply the dye were at 90oC, at a pH of 5.0, and for a total of 60 minutes. Under these conditions, the azo dye had a very similar affinity (wash fastness) as it would towards a fiber such as silk or cotton. [3] The second process, which does not lead to as effective of a wash fastness, involves dyeing the PLA fibers in supercriteral carbon dioxide at temperatures above 50oC but below 90oC instead of in water. The temperature must be kept below 90oC in an effort to not cause structural damage to the PLA fibers. [21]

Azo compounds are also used in the manufacturing industry to test for flaws or cracks in the surfaces of castings, forgings, and wells. Specifically, the azo dye CI Red 164 is used in non-destrcutive testing of many metals. This azo compound is commonly used because of the bright red color and the ease of which it will point out the flaws. It is also a very simple and relatively cheap method of testing. The dye is applied to the surface of the object to be tested via a spray can, airbrush, or paintbrush. Excess dye is then washed away and the surface is developed. This in turn reveals the crack. However, CI Red 164 is thought to be a carcinogen so workers are strongly urged to cover all skin and wear a breathing apparatus in an effort to minimize the chances of exposure to the dye. In the meantime, two non azo dye based alternatives are currently available, but it is also unknown whether these are any safer. [26]

Another example of one of the many uses of azo dyes is in paper and ink. Many colored ink jet printer cartridges contain polyazo dyes. Azo compounds are very desirable for this application because they provide sharp and bright colors that dry quickly, without drying over the thin-tipped injection nozzle of the printer. [7,9] They also have a good water fastness and light fastness, allowing them to last for long periods of time without decomposing. Azo dyes are also decently soluble in the ink, making them an ideal colorant for ink jet printing. [9] Two common polyazo dyes used in printer ink are CI Direct Yellow 28 and CI Direct Yellow 29. [7] Polyazo dyes used for this application often contain substituents that are derivatives of carboxylic acid, sulfonic acid, and phosphoric acid. These groups are very attractive and therefore preferable for an ink jet chemist to use because they not fiber reactive groups. A fiber reactive group is one that will react with the hydroxy group of cellulose, which is present in most papers, or with the amino group present in most natural fibers. The covalent linkage that forms between these fiber mediums and the die is not advantageous towards the brightness or longevity of the dye so it is avoided at all costs.[7,9]

Conclusion

Since the founding of azo dyes, over 2000 have been synthesized and are used regularly in the production of many products to brighten up daily life. Azo dyes are synthesized via a diazo coupling reaction, which involves the coupling of a diazonium salt, formed by reacting a primary aromatic amine with a nitrosonium ion, to a phenol or an aromatic amine. This synthesis must be carried out under very specific conditions, which has led to scientists researching methods of synthesis that are able to be carried out under much more mild conditions, such as at a temperature of 60oC instead of 3-5oC. However, the yield from these reactions is not substantial enough so moving forward, chemists are continuing to research methods in which they can improve the yield. Despite the many uses of azo compounds, such as in laboratories, in the production of textiles, and as colorants for inks and papers, azo dyes synthesized from benzidine and the corresponding waste water are proven carcinogens when reduced to an aryl amine. The aryl amine has been proven to cause bladder cancer in those people exposed to it, hence why the production of such compounds is now banned. Therefore, scientists have been forced to find different ways to synthesize these previously used colors. As time progresses, chemists will continue to focus their research on methods of disposal of azo dye waste water in a way that does not reduce the azo compound to a carcinogen. Despite the large amount of research already done and the quantity of azo dyes known, chemists are still researching and developing new colors for the use in all of the applications previously listed.

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