Hydroxyazo dyes

The vast majority of azo dyes are azo compounds containing hydroxy or amino groups in the 2- or 4-position with respect to the azo group (e.g., 1.8). They are in equilibrium with their tautomers, the quinone hydrazones (quinone monoimine hydrazones). In spite of the fact that in most hydroxyazo dyes the equilibrium is shifted in favor of the quinone hydrazone, they are still called azo compounds. 

Scheme 3.1 Reaction scheme which provides evidence for tautomerism in some hydroxyazo dyes...

Hydroxyazo dyes fall into four main categories depending upon the nature of the coupling component used to make them, namely acyclics, phenols, heterocyclics, and naphthols. Those derived from naphthols comprise the largest and most important group commercially. 

The pXa of the hydroxy group determines the ease of formation of the dye anion. For many hydroxyazo dyes, the pKa is typically 8 10.7 21 Therefore, pH values of 8-10, which are not uncommon in many applications, will form the dye anion. In contrast, provided it is stable under the usage conditions and does not undergo de-metallization, a metal complex azo dye cannot form a... 

This is an alternative method of introducing copper into an o-hydroxyazo dye structure. The azo compound is treated with a copper(II) salt and an oxidant in an aqueous medium at 40-70 °C and pH 4.5-7.0. Sodium peroxide, sodium perborate, hydrogen peroxide or other salts of peroxy acids may be used as oxidants, the function of which is to introduce a second hydroxy group in the o -position [25]. This process is reminiscent of earlier work on Cl Acid Red 14 (5.51 X = H), an o-hydroxyazo dye that will not react with a chromium (III) salt to form a 1 1 complex but will do so by oxidation with an acidified dichromate solution. This oxidation product was later found to be identical with that obtained by conventional reaction of Cl Mordant Black 3 (5.51 X = OH) with a chromium(III) salt [7]. 

In 1941 two different isomers of the 1 2 chromium complex of an o-carboxy-o -hydroxyazo dye were isolated. It was concluded that the three donor ligand atoms of each... 

Tautomerism plays an important role in hydroxyazo dyes, having a marked influence on their tinctorial strengths and other properties cmcial in a commercial dye, such as fastness to light. The hydroxy azo dyes can be grouped according to whether... 

In theory, azo dyes can undergo tautomerism azo/hydrazone for hydroxyazo dyes azo/imino for aminoazo dyes, and azonium/ammonium for protonated azo dyes. A more detailed account of azo dye tautomerism can he found elsewhere. 

The reaction of diazo cation with phenolate yielding the azo dye may proceed through the formation of the diazo ether. Kekule came to this conclusion in 1870. Zollinger (1958), considering this conclusion, proposed and explained the mechanism by which the diazo ethers convert into the C-diazo compounds, that is, into the hydroxyazo dyes. The diazo ether preliminarily dissociates into the phenolate ion and the diazonium ion i.e., a two-stage intermolecular reaction takes place. The CIDNP effect suggests that the diazo ether may reversibly convert into the radical pair ... 

Hydroxyazo dyes vary in the proportion of tautomers present, from pure azo tautomer to mixtures of azo and hydrazone tautomers, to pure hydrazone tautomer. Almost all azo phenol dyes (64) exist totally in the azo form, except for a few special cases [4],... 

Deuteration115 of the acidic proton in or/ho-hydroxyazo dyes leads to a small shift in the azo-hydrazone equilibrium. 

Some researcher reported that photofading behaviour of a series of disperse azo dyes in polyester and nylon films may involve the initial oxidation of dye to the azoxy form, followed by a Wallach photorearrangement (Leaver I.H., 1980) to give an o-hydroxyazo dye (see scheme II) ... 

The reduction potential of Al(III) is similar to those of the alkali and alkali earth metals making its determination difficult. However, A1 may be determined indirectly by measuring an anodically shifted reduction current of an adsorbed di-o-hydroxyazo dye complexed with aluminum. The complexed dye would have a different reduction potential compared to the uncomplexed dye. 

For compounds or papers dealing with tautomeric equilibria not mentioned above, see the following o-hydroxyazo dyes [74], 3-acyltetronic acids [75], a-heterocyclic ketones [76], P-diketones [77], Schiff bases [78-80], P-ketothioamides [81, 82], perylene quinones [83], meldrum acid derivatives [84], nitromalonamide [85], and piroxicam [36], For other, possibly older, data, see also reviews [2, 4-13]. 

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