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.
As a consequence of п-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.
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:
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:
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.
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.
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.
Azo dyes derived from benzidine are carcinogens; exposure to them has classically been associated with bladder cancer. Accordingly, the production of benzidine azo dyes was discontinued in the 1980s in many western countries.
Certain azo dyes degrade 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.
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Tartrazine is an artificial azo dye commonly used in human food and pharmaceutical products. The present study was conducted to evaluate the toxic effect of tartrazine on the learning and memory functions in mice and rats. Animals were administered different doses of tartrazine for a period of 30 d and were evaluated by open-field test, step-through test, and Morris water maze test, respectively. Furthermore, the biomarkers of the oxidative stress and pathohistology were also measured to explore the possible mechanisms involved. The results indicated that tartrazine extract significantly enhanced active behavioral response to the open field, increased the escape latency in Morris water maze test and decreased the retention latency in step-through tests. The decline in the activities of catalase, glutathione peroxidase (GSH-Px), and superoxide dismutase (SOD) as well as a rise in the level of malonaldehyde (MDA) were observed in the brain of tartrazine-treated rats, and these changes were associated with the brain from oxidative damage. The dose levels of tartrazine in the present study produced a few adverse effects in learning and memory functions in animals. The mechanisms might be attributed to promoting lipid peroxidation products and reactive oxygen species, inhibiting endogenous antioxidant defense enzymes and the brain tissue damage.
Practical application: Tartrazine is an artificial azo dye commonly used in human food and pharmaceutical products. Since the last assessment carried out by the Joint FAO/WHO Expert Committee on Food Additives in 1964, many new studies have been conducted. However, there is a little information about the effects on learning and memory performance. The present study was conducted to evaluate the toxic effect of tartrazine on the learning and memory functions in animals and its possible mechanism involved. Based on our results, we believe that more extensive assessment of food additives in current use is warranted.
Azo dyes are synthetic colours that contain an azo group, -N=N-, as part of the structure. Azo groups do not occur naturally. Most azo dyes contain only one azo group, but some contain two (disazo), three (trisazo) or more.
Azo dyes account for approximately 60-70% of all dyes used in food and textile manufacture. In theory, azo dyes can supply a complete rainbow of colours, but yellow/red dyes are more common as blue/brown dyes.
The different, mainly aromatic, side groups around the azo bond help to stabilise the N=N group by making it part of an extended delocalised system. This also has the effect of making many azo compounds coloured, as delocalised or conjugated systems often absorb visible frequencies of light. Aromatic azo compounds (R = R' = aromatic) are usually stable and tend to produce strong vivid colours.
The general formula for making an azo dye requires two organic compounds- a coupling component and a diazo component. Since these can be altered considerably, an enormous range of possible dyes are available, especially as the starting molecules are readily available and cheap. Furthermore, the simplicity of the reactions mean that the process can be scaled up or down very easily. Energy requirements for the reaction are low, since most of the chemistry occurs at or below room temperature. The environmental impact is reduced by the fact that all reactions are carried out in water, which is easy and cheap to obtain, clean and dispose of. All these factors make azo dyes very cheap to produce.
Azo dyes are much more stable than most of the natural food dyes. Azo dyes are stable in the whole pH range of foods, are heat stable and do not fade when exposed to light or oxygen. This makes azo dyes applicable in nearly all foods. The only disadvantage is that azo dyes are not soluble in oil or fat. Only when azo dyes are coupled to a fat soluble molecule, or when they are dispersed as very fine particles, oils can be coloured.
The acute toxicity of azo dyes, as defined by the EU criteria for classification of dangerous substances, is rather low. Direct toxic levels of azo dyes will never be reached by consuming azo dye coloured food. The majority of azo dyes (food and textile) have LD50 values between 250-2,000 mg/kg body weight, indicating that for a lethal dose many grams of azo dyes have to be consumed in a singe dose. As azo dyes are highly water soluble, they do not accumulate in the body, but are metabolised in the liver and excreted in the urine. As azo dyes are very strong colour, foods normally are coloured with dyes in levels of mg dye/kg food. To reach a lethal dose an average adult person thus need to consume over 100 kg of azo coloured food in a single day.
The azo linkage is the most labile portion of an azo dye molecule and may easily undergo enzymatic breakdown in mammals, including man. The azo linkage may be reduced and cleaved, resulting in the splitting of the molecule in two parts. This reaction is carried out by an enzyme named azo-reductase. It is a non-specific enzyme, found in various micro-organisms (such as in intestinal bacteria) and in all tested mammals.
In mammals azo-reductases are, with different activities, present in various organs like liver, kidney, lung, heart, brain, spleen and muscle tissues. The azo-reductase of the liver, followed by the azo-reductase of the kidneys possesses the greatest enzymatic activity.
After cleavage of the azo-linkage, the component aromatic amines are absorbed in the intestine and excreted in the urine. However, the polarity of azo dyes influences the metabolism and consequently the excretion. Sulphonation of azo dyes appears to decrease toxicity by enhancing urinary excretion of the dye and its metabolites. Sulphonated dyes, mainly mono-, di- and trisulphonated compounds are world-wide permitted for use in foods, cosmetics and as drugs for oral application.
It is claimed that some food colours increase, or even cause, hyperactivity in children. This is especially claimed for azo dyes. Since the late 1970s the effects of azo dyes on hyperactivity have been studied. Most studies were non conclusive and several studies are contradictory. A main drawback for these studies is that there are no clear indicators for hyperactivity and that in many cases parental reports were used. Often parental reports are biased, which makes interpretation difficult. Also, several studies that have shown an effect on hyperactivity based on parental reports, fail to show an effect based on physiological parameters. A typical example thereof is the recent article of Bateman et al. (see references below) from 2004 and the reactions on this article published in the Lancet afterwards (references see below). In nearly all studies the azo dyes itself had no effect, but the strongest effects were observed in children receiving azo dyes and benzoic acid combinations. 041b061a72