Diazo-components

Diazo components for the production of azo dyes can be divided into the following groups of aromatic amines  

Aniline and Substituted Anilines. Methyl, chloro, nitro, methoxyl, ethoxyl, phe-noxyl, hydroxyl, carboxyl, carbalkoxyl, carbonamide, and sulfonic acid groups are primarily used as substituents. 

Examples Toluidines, nitroanilines, amino benzoyl amides, and the sulfonic acids thereof. 

Many of these compounds can also be used as coupling components (see Section 2.2.2), as shown by the arrow indicating the coupling position. 

Diamines H2N-A -NH2. The diamines can be divided into different groups  

The diazotization reaction typically involves an aromatic amine, such as a mono-, di-, or trisubstituted aniline, as a diazo component. Coupling components are also frequently based on aniline or its derivatives. The following examples are thus important diazo components  

Another family of technically important diazo components for pigment formation includes a series of aromatic diamino compounds, primarily 3,3 -dichlorobenzidine, and to a lesser extent 3,3 -dimethoxybenzidine (o-dianisidine), 3,3 -dimethylbenzidine (tolidine) and 2,2, 5,5 -tetrachlorobenzidine  

Polycyclic amines, such as a-aminoanthraquinone, and heterocyclic amines, such as aminophthalimide, aminobenzazoles, or aminoquinazolines are less interesting coupling components for azo pigments. 

Aromatic aminosulfonic acids, which play a major role in connection with pigment lakes, are produced by sulfonating the corresponding nitro compound and then reducing it to an aminosulfonic acid. An alternative technique, known as baking process, involves exposing an amine/dihydrosulfate to a temperature of 200 to 300°C in order to effect rearrangement to p-aminosulfonic acid. Ortho-sulfonation prevails if the para position is occupied. In contrast to sulfonation techniques with sulfuric acid, this method avoids wastewater contamination with sulfuric acid. 

The synthesis of an aromatic amine generally starts from the corresponding nitro compound. Nitration and subsequent reduction are key reactions in the synthesis of intermediates for azo pigments. 

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The ortho para isomer ratio depends on several factors, including the nature of the diazo component, the nature of the solvent, the pH of the medium, the temperature of coupling, the presence of catalysts, and the position of substituents. Simple benzenediazonium compounds couple to... 

The ratio between the isomers obtained in coupling with 1,3- and 1,5-naphtholsulfonic acids depends on the reactivity of the diazo component. Energetic ones, such as the 2,4-dinitrobenzenediazonium compound, essentially couple only with l-naphthol-3-sulfonic acid [3771-14-0] in the para position, but 4-chloro-benzenediazonium salt (a weaker diazo) attacks the ortho position. Both isomers result when mononitrobenzenediazonium compounds are used. The tendency to couple para is greater in l-naphthol-5-sulfonic acid [117-59-9] C QHgO S (21). For the combination of... 

The corresponding diazo component with a 6-nitro substituent gives Eriochrome Black T /7787-61-7] (18b, R = NO2) (Cl Mordant Black 11 Cl 14645) which is supposed to consist almost exclusively of the ortho compound. 

Naphthol AS Coupling Components. Naphthol AS components are the aryhdes of either o-hydroxyarylcarboxycHc acids or acylacetic acids. They are free of sulfo and carboxyl groups, but form salts with bases these salts dissolve in water to give colloidal solutions, which couple with diazo components to form colored pigments. The whole class derives from the anilide of 3-hydroxy-2-naphthoic acid [92-70-6] Naphthol AS (85) (Cl Azoic Coupling Component 2). 

Commercially, the most important fast color bases (Table 10) and salts include Azoic Diazo Component 3, produced by the reduction of... 

Cl azoic diazo component Amine stmctuie CAS Registry Number Cl number Fast base or salt... 

Commercial Disperse Azo Dyes. The first proposal to use insoluble dyes in suspension in an aqueous foam bath, ie, disperse dyes, to dye cellulose acetate was in 1921 (60). Commercialization of disperse dyes began in 1924 with the introduction of the Duranol dyes by British Dyestuffs Corporation (61) and the SRA dyes by British Celanese Company (62). In contrast to the acid monoazo dyes, derivatives of benzene rather than of naphthalene are of the greatest importance as coupling components. Among these components mono- and dialkylariifines (especially A/-P-hydroxyethyl-and A/-(3-acetoxyethylanifine derivatives) are widely used couplers. Nitrodiazobenzenes are widely used as diazo components. A typical example is CeUiton Scarlet B [2872-52-8] (91) (Cl Disperse Red 1 Cl 11110). 

A number of other heterocycHc diazo components such as thiazole, iadazole, thiophenes, and thiadiazole types (see Fig. 1), as well as heterocycHc couplers, ie, 6-hydroxy-2-pyridinone [626-06-2] barbituric acid [67-52-7] and tetrahydroquiaoline [25448-04-8] h.2L e been cited ia the Hterature (90,91). Reviews on disperse dyes have been pubUshed (92,93). 

Basic Red 18 (131), Basacryl Red X-NL [14097-03-17 is an example of a pendant cationic azo dye, ia which a localized positive charge is not conjugated with the chromophoric system. /V-Kthy1-/V-(2-ch1oroethy1)ani1ine [92-49-9] reacts with trimethyl amine to form the ammonium salt coupler. The diazo component ia Basic Red 18 is 2-ch1oro-4-nitroani1ine [121 -87-9]. 

Heterocyclic Azo Dyes. One long-term aim of dyestuffs research has been to combine the brightness and high fastness properties of anthraquinone dyes with the strength and economy of azo dyes. This aim is now being realized with heterocychc azo dyes, which fall into two main groups those derived from heterocychc coupling components, and those derived from heterocychc diazo components. 

AminothiaZoles. In contrast to the pyrazolones, pyridones, and indoles just described, aminotliiazoles are used as diazo components. As such they provide dyes that ate more bathochromic than their benzene analogues. Thus aminothiazoles are used chiefly to provide dyes in the red-blue shade areas. The most convenient synthesis of 2-aminothiazoles is by the condensation of thiourea with an a-chlorocarbonyl compound for example, 2-aminothiazole [96-50A-] (94) is prepared by condensing thiourea [62-56-6J with a-chloroacetaldehyde [107-20-0J both readily available intermediates. 

Chemists working with azo dyes use a shorthand designation based on the synthesis by azo coupling. It is characterized by an arrow from the amine used for diazotization (the so-called diazo component) to the coupling component, e. g., for 1.8 sulfanilic acid - 2-naphthol. This designation is especially useful for polyazo dyes (see Zollinger, 1991, p. 109). 

The diazotization of heteroaromatic amines is a ticklish procedure. In spite of the great increase in interest for disperse dyes based on heterocyclic diazo components,... 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3, so-called neutral diazotization) or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. Kozlov and Volodarskii (1969) measured the rates of diazotization of l-amino-2-naphthol-4-sulfonic acid in the presence of one equivalent of 13 different sulfates, chlorides, and nitrates of di- and trivalent metal ions (Cu2+, Sn2+, Zn2+, Mg2+, Fe2 +, Fe3+, Al3+, etc.). The rates are first-order with respect to the added salts. The highest rate is that in the presence of Cu2+. The anions also have a catalytic effect (CuCl2 > Cu(N03)2 > CuS04). The mechanistic basis of this metal ion catalysis is not yet clear. 

Originally, 2-diazophenols were used as diazo components. Nowadays the most important compound is 2,5-diethoxy-4-morpholinobenzenediazonium tetrafluoro-borate. Typical coupling components are acetoacetic anilide, l-phenyl-3-carbamido-... 

About 50% of all industrial dyes manufactured during the last 120 years were produced using this reaction. The terms diazo component and coupling component originated in azo dye technology. 

In spite of the large amount of scientific and industrial activity on diazo compounds and azo coupling reactions since the 19th century, heteroaromatic diazo components have been studied intensively only since the 1950s. 

In the industrial sector, dyes with heterocyclic diazo components are prepared on a large scale and are very important, particularly for dyeing man-made fibers, because of their excellent brightness and high tinctorial power. The volume of patent literature on that subject bears witness to its importance (Weaver and Shuttleworth, 1982 see also Butler, 1975, and Zollinger, 1991, p. 140). 

In the scientific sector, the understanding of the generally higher reactivity of heteroaromatic diazo components relative to that of aromatic diazonium salts has increased. The number of heterocyclic nitrogen atoms in azolediazonium ions has a marked influence on the N-H acidity of these ions. The pvalues of a series of such ions in aqueous solution at 0 °C (Scheme 12-4) indicate that the electrophilicity of the diazonio group in these compounds increases with the number of nitrogen atoms in the ring. ... 

The pK values for azolediazonium ions (Scheme 12-4) refer to the heterolysis of the NH bond, not to the addition of a hydroxy group. Therefore, these heteroaromatic diazo components may react either as a cation (as shown in Scheme 12-4) or as the zwitterion (after loss of the NH proton). Diener and Zollinger (1986) investigated the relative reactivities of these two equilibrium forms (Scheme 12-5) in the azo coupling reaction of l,3,4-triazole-2-diazonium ion with the tri-basic anion of 2-naphthol-3,6-disulfonic acid. 

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