Hydroxyazo compounds

Oxy-aldehyd, n, hydroxy aldehyde, -ammo-niak, n, oxyammonia (hydroxylamine), -azoverbindung, /. hydroxyazo compound, -benzol, n, hydroxybenzene (phenol), -bem-steinsaure. /, hydroxysuccinic acid (malic acid). -biazol, n. oxadiazole, oxdiazole. -bitumen, n, oxidized bitumen, -carbon-s ure, /, hydroxycarboxylic acid, -chlnoltn, n. hydroxyquinoline, -clunon, n. hydroxy-quinone. -chlorid, n. oxychloride, -chlor-kupfer, n. copper oxychloride, -cyan, n. oxycyanogen. 

Some examples of metal ion indicators. Numerous compounds have been proposed for use as pM indicators a selected few of these will be described. Where applicable, Colour Index (C.I.) references are given.12 It has been pointed out by West,11 that apart from a few miscellaneous compounds, the important visual metallochromic indicators fall into three main groups (a) hydroxyazo compounds (b) phenolic compounds and hydroxy-substituted triphenylmethane compounds (c) compounds containing an aminomethyldicarboxymethyl group many of these are also triphenylmethane compounds. 

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. 

Whilst azo compounds prepared from diazonium salts and phenolic or keto-enol coupling components are often depicted in the hydroxyazo form (4.11), an alternative tautomeric structure can be drawn for such compounds (Scheme 4.19). This ketohydrazone tautomer (4.21) can, in cases where the azo and hydroxy groups are located on adjacent carbon atoms, exhibit hydrogen bonding between the two groups as shown. Similar pairs of structures, but without hydrogen bonding, can be drawn for p-hydroxyazo compounds. 

The converse of this situation is presented by diazotised arylamines coupled to phenols, where assumption of a ketohydrazone structure (4.21) would necessitate loss of the aromatic character of the phenolic ring. Consequently, these compounds exist solely in the hydroxyazo form. 

A totally different picture is presented by 3-phenylazo-2-naphthol. This unusual isomer cannot be prepared by a normal coupling procedure but has been obtained by reaction of 3-amino-2-naphthol with thionyl chloride to give the N-sulphinylamine (4-24), condensation of which with N-phenylhydroxylamine yields the desired product [59]. Here, assumption of a ketohydrazone form would entail loss of aromatic character in both rings of the naphthalene nucleus and the energetic unfavourability of this situation ensures that the compound exists solely in the hydroxyazo form. 

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]. 

The halogen atom in an o-halogeno-o -hydroxyazo compound may be replaced by a hydroxy group under mildly alkaline conditions, provided that the halogeno substituent is activated by the presence of electron-withdrawing groups (acetyl, cyano, nitro) in the o- and/or p-positions [26]. The mechanism is believed to involve formation of an intermediate complex (5.52 R = electron-withdrawing substituent) of low stability in which chlorine is coordinated with the copper atom [27]. This facilitates attack by hydroxide ion at the... 

During the oxidative copperizing process, an o-hydroxyazo compound reacts with a copper(II) salt in the presence of hydrogen peroxide. Both methods broaden the scope of metal complexation reactions by extending the selection of suitable diazo components [8,9]. 

Reaction LXXIV. (b) Transformation of Azoxy Compounds to Azophenols. Azoxy compounds (see Reaction CLXX.) are converted by cone, sulphuric acid into p-hydroxyazo compounds (B., 13, 525). 

Most azoxy compounds under the influence of ultra-violet light are converted into o-hydroxyazo compounds. (J. C. S., 127, 2374.)... 

The unsym. azoxy compound has been isolated by the action of light on ococ -azoxynaphthalene (J. C. S., 123, 2466). When the latter is heated with benzaldehyde the unsym. azoxy compound is also formed, whilst if a base such as quinoline is present, the further transformation to the o-hydroxyazo compound takes place. 

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 ... 

It is generally accepted that the ICC of nickel, copper, and ruthenium, on the basis of ligands of type 426, have trans-planar configuration 427, while a tetrahedral polyhedron is characteristic for cobalt and zinc chelates [270,401,751-754], It was also accepted that a cis-planar configuration for ICC of o-hydroxyazo compounds and chelates of o-hydroxyazomethines in 422 (R = Aik, Ar, Het) is not likely or, at best, only scarcely possible. However, a cis-planar complex 428 has recently been prepared and characterized [755] ... 

In a marked difference with respect to complexes of o-hydroxyazo compounds containing a six-member cycle of the type shown by 427, the existence of five- [751,857-862] or five- and six-member [751,863] cycles, shown by structures 482 and 483, respectively, is characteristic for coordination compounds obtained from analogous 2-mercapto derivatives ... 

Nickel and copper were used as a source of complex-former in the syntheses above. Additionally to o-hydroxyazo compounds, other N,0-donors were applied (Sec. 2.2.5.3), in particular o-hydroxyazomethines, 8-hydroxyquinoline, and 2-hydroxy-phenylbenzoxazol, which form ICC 422, 445, and 446, respectively. Yields of the products are from 10 to 60% [541]. 

As seen from (58), the primary product of arylazo substitution is the anion Ar—Nj—R . It is protonated in a subsequent step either at a basic center of the residue R (preferentially at an oxygen atom, for example, in coupling with phenol), or at the azo nitrogen next to the group Ar forming a hydrazone. The problem as to whether the final product k a hydroxyazo- or a quinonehydrazone-type compound is therefore not a question of the substitution proper but of the consecutive tautomeric equilibrium. 

The absorption spectrum of l-phenylazo-2-naphthol in various solvents and the spectra of the photoinduced colored species at low temperature generated by the exposure of this compound to light at various wavelengths was studied in detail, For any o-hydroxyazo compound, e.g., l-phenylazo-2-naphthol, the combined results of tautomerization and rotation around single and double bonds lead to the eight configurations ATI, AT2, ACl, AC2, HTl, HT2, HCl, and HC2, where H and A represent the hydrazone and azo forms, respectively, and T and C denote the trans and cis configurations, respectively. 

Tautomerism involving phenols is most often seen for Schiff bases, Mannich bases, or o-hydroxyazo aromatics, but is also discussed for o-hydroxynitroso compounds. This is relatively seldom for phenols not having a nitrogen-containing substituent. An exception is 30 and other cases mentioned later. 

METAL COMPLEXES OF AZO DYES Metal complexes of certain 0,0 -dihydroxyazo, 0-carboxy-0 -hydroxyazo, o -amino-o hydroxyazo, arylazosalicyclic acid, and formazan compounds are used as dyes for wool, nylon, and cotton with generally much improved washfastness and lightfastness properties when compared to their respective unmetallized precursors. Dyes that are chelated with the metal on the substrate during the dyeing process are termed metallizable or mordant dyes. Conversely, those dyes that have been metallized by the dye manufacturer prior to use by the dyer, are classified as premetallized dyes. The two important types of premetallized dyes are the 1 1 and 2 1 complexes, eg, complexes with 1 1 and 2 1 ligand-to-metal ratios, respectively. 

---