Azo-dyes, production

Many aminonaphthalenesulfonic acids are important in the manufacture of azo dyes (qv) or are used to make intermediates for azo acid dyes, direct, and fiber-reactive dyes (see Dyes, reactive). Usually, the aminonaphthalenesulfonic acids are made by either the sulfonation of naphthalenamines, the nitration—reduction of naphthalenesulfonic acids, the Bucherer-type amination of naphtholsulfonic acids, or the desulfonation of an aminonaphthalenedi-or ttisulfonic acid. Most of these processes produce by-products or mixtures which often are separated in subsequent purification steps. A summary of commercially important aminonaphthalenesulfonic acids is given in Table 4. 

H-acid, l-hydroxy-3,6,8-ttisulfonic acid, which is one of the most important letter acids, is prepared as naphthalene is sulfonated with sulfuric acid to ttisulfonic acid. The product is then nitrated and neutralized with lime to produce the calcium salt of l-nitronaphthalene-3,6,8-ttisulfonic acid, which is then reduced to T-acid (Koch acid) with Fe and HCl modem processes use continuous catalytical hydrogenation with Ni catalyst. Hydrogenation has been performed in aqueous medium in the presence of Raney nickel or Raney Ni—Fe catalyst with a low catalyst consumption and better yield (51). Fusion of the T-acid with sodium hydroxide and neutralization with sulfuric acid yields H-acid. Azo dyes such as Direct Blue 15 [2429-74-5] (17) and Acid... 

Uses. (9-Nitrochlorobenzene is used in the synthesis of azo dye intermediates such as o-chloroaniline (Fast YeUow G Base), i9-nitroani1ine (Fast Orange GR Base), o-anisidine (Fast Red BB Base), o-phenetidine, and (9-aminophenol (see Azo dyes). It also is used in corrosion inhibitors, pigments, and agriculture chemicals. -Nitrochlorobenzene is used principally in the production of intermediates for azo and sulfur dyes. Other uses include pharmaceuticals (qv), photochemicals, mbber chemicals (qv), and insecticides (see Insectcontroltechnology). Typical intermediates manufactured from the para isomer are -lutioaruline (Fast Red GC Base), anisidine, -aminophenol, -nitrophenol, -phenylenediamine, 2-chloro-/)-anisidine (Fast Red R Base), 2,4-dinitrochlorobenzene, and l,2-dichloro-4-nitrobenzene. 

Uses. (9-Nitrotoluene is used in the synthesis of intermediates for azo dyes, sulfur dyes, mbber chemicals, and agriculture chemicals. Typical intermediates are o-toluidine, o-nitrobenzaldehyde, 2-nitro-4-chlorotoluene, 2-nitro-6-chlorotoluene, 2-amino-4-chlorotoluene (Fast Scarlet TR Base), and 2-amino-6-chlorotoluene (Fast Red KB Base). -Nitrotoluene is used principally in the production of intermediates for azo and sulfur dyes. Typical intermediates are -toluidine, -nitrobenzaldehyde, and 4-nitro-2-chlorotoluene. 

Industrial production is by reduction of the corresponding nitrophenol with iron or hydrazine (167,168). 2-Amino-4,6-dichlorophenol finds important use as an azo-dye intermediate (see Azo dyes). 

Azo-stilbene dyes formed by diazotization of a condensation product containing primary amino groups and coupling with azo dye coupling components, eg. Direct Brown 29 (Cl 40505) (6) ... 

Stilbene-azo dyes of more precise constitution prepared in the usual way by tetrazotization and coupling of 4,4 -diamino-2,2 -stilbenedisulfonic acid [81-11-8] (7). The product in this example is ZDkect Yellow 4 (Cl 24890) [3051 -11 -4] (8) ... 

Alkaline Coupling Process. Orange II [633-96-5] (21) (Cl Acid Orange 7 Cl 15510) a monoazo dye discovered ia 1876, serves as an example of the production of an azo dye by alkaline coupling. A suspension of diazotized sulfanilic acid (0.1 mol) is added to a solution (cooled to about 3°C) of 14.4 g 2-naphthol dissolved ia 15 g 30% sodium hydroxide, 25 g sodium carbonate, and 200 mL of water. The temperature should not be allowed to rise above 5°C. The reaction is heated until solution occurs and the dye is precipitated with 100 g sodium chloride. The mixture is cooled and filtered, and the product is dried. 

Basic Orange 1 (130) (aniline coupled to 2,4-diamiaotoluene) and Basic Orange 2 (22) (aniline coupled to y -phenylenediamiae) are examples of amine salt type cationic azo dyes. The cation is formed by protonation under acidic conditions. Under neutral or alkaline conditions, these dyes behave more like disperse dyes. In 1988 the U.S. production of Cl Basic Orange 2 amounted to 132 tons. 

Consumption. Anthraquinone dyes are the most important dye class after azo dyes. Wodd textile production is estimated in Table 14. Estimates of the consumption of dyes for textiles ate given in Figure 14, together with the figures for fiber consumption. This shows that the consumption of each dye class or classes is approximately parallel to the consumption of fibers to which they ate apphed. 

The only practical method of preparing 1,4-aminonaphthol is from a-naphthol through an azo dye, the nitroso compound not being readily available. The majority of investigators have reduced technical Orange I with stannous chloride Mi.is.is.ir.is by the procedures discussed above, and benzeneazo-a-naphthol has been reduced by the same reagent. In order to make possible the use of crude, technical a-naphthol a method has been developed for the preparation of the benzeneazo compound, its separation from the isomeric dye coming from the d-naphthol present as well as from any disazo compound by extraction with alkali, and the reduction of the azo compound in alkaline solution with sodium hydrosulfite. The process, however, is tedious and yields an impure product. 

NaN02, in addition to its use with nitrates in heat-transfer molten-salt baths, is much used in the production of azo dyes and other organo-nitrogen compounds, as a corrosion inhibitor and in curing meats. 

Azo-coupled products are widely used as dyes for textiles because their extended conjugated tt electron system causes them to absorb in the visible region of the electromagnetic spectrum (Section 14.9). / -(Dimethylamino)azobenzene, for instance, is a bright yellow compound that was at one time used as a coloring agent in margarine. 

Metal exposure is a critical factor in color retention of most azo dye formulations. However, improved coating techniques have made it possible to attain the desired shelf life of products colored with azo dyes. 

The dyes on the group above are known as azo dyes, because they have the two nitrogen atoms together in the center (called an azo group). These dyes were originally derived from coal tar, but are now mostly made from petroleum. Azo dyes come in many colors besides those allowed in food (listed above). The other colors find use in fabrics, paper products, and plastics. 

By the use of microstructured mixers, pigment and other particulate syntheses can be improved. In this way, finer particles with more uniform size distribution were yielded for the commercial azo pigment Yellow 12 (see Fig. 2) [11]. The particles formed in the microstructured mixer have better optical properties such as the glossiness or transparency at similar tinctorial power. Since the micro-mixer made pigments have more intense colour, lower contents of the costly raw material in the commercial dye products can now be employed which increases the profitability of the pigment manufacture. 

HPLC is often reported to be the technique of best choice for the quantification of food colorants. According to European Directive 94/36/EC, the quantities of synthetic colorants to be added to foods are restricted and thus reliable methods for their quantification must be established. Approved colorants, defined by E-coded numbers (Table 6.6.2), are permitted for non-alcoholic beverages, confectionery products, and even for caviar (dying fish roe). For example, a specific HPLC chromatographic method for the quantization of 14 synthetic food colorants belonging to azo dye, triphenyhnethane, or quinophthalone classes (E 102,104, 110, 122,123, 124, 127, 128, 129, 131, 132, 133, 142, 151) was reported to check their contents in caviar. ... 

Possibly the most significant discovery in the metabolism of aromatic azo compounds had implications that heralded the age of modem chemotherapy. It was shown that the bactericidal effect of the azo dye Prontosil in vivo was in fact due to the action of its transformation product, sulfanilamide, which is an antagonist of 4-aminobenzoate that is required for the synthesis of the vitamin folic acid. Indeed, this reduction is the typical reaction involved in the first stage of the biodegradation of aromatic azo compounds. 

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