Component Coupling

The most important coupling components can be divided into the following groups. Where there are several possibilities for coupling, the preferred coupling positions are marked by bold arrows and the other possible coupling positions by ordinary arrows. 

When treated with aniline, diazotized aniline (benzenediazonium chloride) yields, apart from a small quantity of 4-aminoazobenzene (33), diazoaminoben-zene (34) as the main product  

4-Aminoazobenzene (33) is obtained by coupling in a more strongly acid medium, but better still by heating 34 in aniline with addition of aniline hydrochloride. 

Electron-donating substituents in the aniline, such as methyl or methoxy groups, especially at the meta position, promote coupling the tendency to couple thus increases in the order aniline o -to 1 uidine w -toluidinc m -anisidi ne cresi-dine l-amino-2,5-dimethoxybenzene (aminohydroquinone dimethyl ether) to the extent that without formation of the diazoamino compound, the last three bases are attached almost quantitatively in the desired position, i.e., at the 4-position relative to the amino group. 

Phenols, Naphthols (e.g., 39-47). Phenols mainly couple at the 4-position, or at the 2-position if the 4-position is occupied, p-Hydroxybenzoic acid couples with elimination of C02 resorcinol couples twice initially at the 4-position, and with a second equivalent of diazonium compound at the 2-position under acid conditions or at the 6-position under alkaline conditions. a-Naphthols mainly couple at the 4-position, in addition to which varying quantities of 2- and 2,4-coupling products are obtained, depending on the diazo component. p-Naphthol couples at the 1-position. Substituents in the 1-position, such as S03H, COOH, Cl, CH2OH, or 

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An example of an a2o dyestuff in which y-acid is the coupling component is Acid Red 337 (19), which is obtained through the reaction of dia2oti2ed -trifluoromethylaniline and y-acid (52). 

The principal uses for 2-naphthalenol are in the dyes and pigments industries, eg, as a coupling component for azo dyes, and to make important intermediates, such as 3-hydroxy-2-naphthalenecarboxyhc acid (BON) (28) and its anilide (naphthol AS), 2-naphtholsulfonic acids, aminonaphtholsulfonic acids, and l-nitroso-2-naphthol/77/-5 /-5(/ (29). 

Naphthalenediol. 1,5-Dihydroxynaphthalene or Asurol is a colorless material which darkens on exposure to air. It is manufactured by the fusion of disodium 1,5-naphthalenedisulfonate with sodium hydroxide at ca 320°C in high yield. 1,5-Naphthalenediol is an important coupling component, giving ortho-a2o dyes which form complexes with chromium. The metallised dyes produce fast black shades on wool. 1,5-Naphthalenediol can be aminated with ammonia under pressure to 1,5-naphthalenediamine. 

Naphthalenediol. This diol is prepared by the alkah fusion of 2-hydroxynaphthalene-6-sulfonic acid (Schaffer acid) at 290—295°C. Schaffer acid is usually produced by sulfonation of 2-naphthol with the addition of sodium sulfate at 85—105°C. This acid is also used as a coupling component in the production of a2o dyes such as Acid Black 26. 2,6-Naphthalenediol is used as a component in the manufacture of aromatic polyesters which, as is also tme of the corresponding amides, display Hquid crystal characteristics (52). 

Hydroxynaphthalenesulfonic acids are important as intermediates either for coupling components for a2o dyes or a2o components, as well as for synthetic tanning agents. Hydroxynaphthalenesulfonic acids can be manufactured either by sulfonation of naphthols or hydroxynaphthalenesulfonic acids, by acid hydrolysis of arninonaphthalenesulfonic acids, by fusion of sodium naphthalenepolysulfonates with sodium hydroxide, or by desulfonation or rearrangement of hydroxynaphthalenesulfonic acids (Table 6). 

These amides (45), which are of the Naphthol AS type, are important coupling components that are appHed to fiber, eg, cotton (qv). They then react with a dia2o component on the fiber to produce insoluble a2o dyes of high washfastness and lightfastness. A wide range of arylamides of... 

Pigment Yellow CAS Registry Number Substituents on diazonium component Substituents on coupling component... 

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) ... 

Before coupling, excess nitrous acid must be destroyed. Nitrite can react with coupling components to form nitroso compounds causiag deHterious effects on the final dyestuff. The presence of nitrite can be detected by 4,4 -diamiQO-diphenyHnethane-2,2 -sulfone [10215-25-5] (Green reagent) or starch—iodide. Removal of nitrite is achieved by addition of sulfamic acid or urea [57-13-6], however, sulfamic acid [5329-14-6] has been more effective ia kinetic studies of nine nitrous acid scavangers (18). 

Azo Coupling. The coupling reaction between an aromatic diazo compound and a coupling component is the single most important synthetic route to azo dyes. Of the total dyes manufactured, about 60% are produced by this reaction. Other methods iaclude oxidative coupling, reaction of aryUiydraziae with quiaones, and oxidation of aromatic amines. These methods, however, have limited iadustrial appHcations. 

The acid—base equiUbtia are fundamental to the kinetics of azo coupling and of practical significance for azo technology. Thus it is important that coupling reactions be carried out in a medium such that the acid—base equiUbtia of the diazo and coupling components favor as much as possible the diazonium ions and the phenolate ions or the free amine, respectively. 

Broad principles governing the activity of coupling components may be summarized as follows ... 

Electron-attracting substituents in the coupling components such as halogen, nitro, sulfo, carboxyl, and carbonyl, are deactivating and tend to retard coupling. 

It is possible for diazo compounds to attack both the ortho and para position of hydroxyl and amino coupling components when these positions are not already occupied. 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Resorcine Brown R [5850-16-8] (27) (Cl Acid Brown 14, Cl 20195) is a symmetrical primary disazo dye with bihmctional coupling component (resorcinol). 

Calcocid Blue Black Ex [1064-48-8] (28) (Cl Acid Black 1 Cl 20470) is an unsymmetrical primary disazo dye with bihmctional coupling component (H-acid). 

Secondary Disazo Dyes. There are about 250 dyes of known constitutions in this group. They are made by diazotizing an aminoazo compound, the amino group of which derives from the original coupling component and coupling it to a suitable intermediate. Intrasil Orange YBLH... 

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