Azo Coupling Components

There are apparent exceptions to the rule that aromatic compounds are azo coupling components only if they contain a hydroxy or an amino group. A long time ago Meyer and Tochtermann (1921) demonstrated that the 2,4,6-trinitrobenzenedi-azonium salt couples with mesitylene, isodurene (1,2,3,5-tetramethylbenzene), and pentamethylbenzene (see also Smith and Paden, 1934). That result was surprising at the time, but today it is, of course, understandable the diazonium salt used is pro-... 

The fact that adenosine and its derivatives are azo coupling components is used for immobilizing nicotinamide-adenine nucleotide (NAD+) for affinity chromatography purposes. In 12.58 NAD+ is bonded to a matrix through an azo bond. Compound 12.58 is used for the purification of dehydrogenase enzymes (Hocking and Harris, 1973). 

Substituted 2-naphthols, as azo coupling components 356ff., 362 Sulfanilic acid 14, 71 Sulfidomolybdenum dimer anion, complex with diazonium ions 117 Surface-enhanced Raman spectroscopy 280... 

The direct dyes that are most suitable for leather have the following features predominantly long-chain dye molecules, sulfonate groups at the ends of the molecule, additional non-dooble-bonded groups, a balanced ratio of double bonds to solubilizing groups, and predominantly meta -substituted azo coupling components. These empirical observations have been used to enhance the technical properties of anionic polyazo leather dyes. 

Diazo-l,3-dicarbonyl compounds are electrophilic enough to give azo coupling products with reactive aromatic azo coupling components as well as with CH acidic compounds such as P-diketones and p-keto-esters. 

Stfirba et al. were successful, however, in solving that problem for another azo coupling component, namely l-phenyl-3-methyl-5-pyrazolone 119), which in solution exists as a mixture of tautomeric forms A-C. The conjugate base 120 reacts 10 times faster than the neutral compound. 

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). 

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) ... 

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. 

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). 

With azo dyes derived from 4,5-dihydroxy-2,7-naphthalenedisulfonic acid [148-25-4] (chromotropic acid) as the coupling component, metal complex formation occurs with the perihydroxy groups without oxidation. 

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). 

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. 

Ah. the heterocychc coupling components that provide commercially important azo dyes contain only nitrogen as the hetero atom. They are indoles (31), pyrazolones (32), and especially pyridones (33) they provide yeUow to orange dyes for various substrates. 

Pyrazolones. Pyrazolones are used as coupling components since they couple readily at the 4-position under alkaline conditions to give important azo dyes in the yeUow-orange shade area. 

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 rate constants in Figure 5-3 were measured by injecting the solution of the (E)-diazoate into a buffer solution that also contained a highly reactive coupling component (2-naphthol-3,6-disulfonic acid, except at pH values below 2.5, where l,8-dihydroxynaphthalene-3,6-disulfonic acid was used instead). The diazonium ion formed reacts rapidly with these naphthols, and the concentration of the corresponding azo compounds was determined spectrophotometrically. 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

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