Leuco indophenol

This reaction is also applicable to 1,8-diamino-4-naphthalenesulfonic acid, t,8-diamino-3,6-naphthalenedisulfonic acid (24) and to the indo-phenol dyes of 1,8-naphthalenediamine with p-aminophenol and 4-amino-2,6-dichlorophenol. Owing to the liberation of hydrogen sulfide during the cyclization, the leuco-indophenol dyes of 2,3-dihydro-fH-periniidine-2-thione are obtained (25). 

Polysulfide Melt. Cl Sulfur Black 1 [1326-82-5] (Cl 53185), derived from 2,4-dinitrophenol, is the most important dye in this group which also includes the indophenol-type intermediates. The latter are appHed in the stable leuco form. The derived dyes are usually confined to violet, blue, and green shades. Other members of this group are intermediates capable of forming quinoneimine (10) or phenazone stmctures (11) that produce red-brown or Bordeaux shades ... 

Sulphur dyes in the insoluble disulphide form and the Cl Solubilised Sulphur brands are reduced by the dyer as part of the application procedure. In the case of the Cl Leuco Sulphur brands reduction has already been carried out by the manufacturer, so that they are substantially in a form suitable for immediate application (section 1.6.2) The chemistry of the reduction of sulphur dyes is complex, as is the chemistry of the dyes themselves it has been well described elsewhere [204]. It is possible to describe the state of a reduced sulphur dye in alkaline sulphide or polysulphide solution by the general formula 12.42, but there are certain complications. In many cases the chromogen is not itself reduced, but in others, notably reddish browns, blues and navy blues based on indophenols, the chromogenic... 

The starting products for reflux thionation in aqueous or solvent media are chiefly derivatives of indophenol (10, Y = OH) and indoaniline (10, Y = NH2). The starting compounds are often in the leuco form 11. 

In this process, the reaction of the intermediates-essentially indophenols, chiefly in leuco form, or indophenol-like substances-with sulfur is effected by heating with alkali metal polysulfide in an aqueous or alcoholic medium under reflux. When monoethers of ethylene glycol or diethylene glycol (such as Carbitol) are used, the dye can be obtained in solution form after melting, without distillation and elimination of the solvent. Accordingly, these solvents are used chiefly in the preparation of ready-to-dye sulfur dye solutions. The addition of hydrotropic substances such as sodium xylenesulfonate improves the homogeneity of the melt and hinders sedimentation of the dye on storage. 

Because indophenols are unstable and reactive (they are hydrolyzed by dilute acids), iron, sodium sulfide or other reducing agents are used to reduce them to the more stable leuco form, either in situ or before application. 

Oxidative coupling reactions of p-phenylenedia-mines with amines and phenols are widely used. As can be seen in Scheme 1 p-phenylene diamine is oxidized to its quinone diimine (QDI) by potassium hexacyanoferrate(III) or a similar oxidant in a weakly basic medium. In the rate-limiting step, QDI reacts with the amine or phenol to give leuco-indamines (indophenols), which are rapidly oxidized to colored indamines (indophenols) with the aid of a QDI molecule. This reaction is chiefly used for resolving a large variety of mixtures of aromatic amines, phenols, and chlorophenols. 

Of the various oxidation-reduction methods those based upon the reduction of indophenol have been found to be the most satisfactory. 2,6-Dichlorophenolindophenol is a dye that is blue in alkali and pink in acid. It is reduced by ascorbic acid to the colorless leuco form, as shown by following reaction (1). 

Among the donors known to be oxidized by the peroxidase-hydrogen peroxide system are mono- and polyphenols, aminophenols, diamines, leuco bases of several dyes, indophenols, amino acids with reactive groups, ascorbic acid, etc. Extensive lists of donors which give colored oxidation products can be found in the papers of Kastle (202) and Josl (198). The oxidation of amino acids and proteins is discussed by Sizer (321) and reports on other donor substances are found scattered over numerous publications. 

Jacoby (190), and later Glavind etal. (163) used leuco-2,6-dichlorophenol-indophenol for histological localization of peroxidase. Ritter and Oleson (290) employ o-naphthol, and De Robertis and Grasso (114) recommend a variety of donor substances including guaiacol and pyrogallol. 

Quantitative determination of ascorbic acid in biological fluids and processed foods is hindered by interference by ferrous, cuprous and stannous ions, phenols, sulphydryl compounds, reductone (LXXIX), reductic acid (LXXVIII) and tartronaldehyde (LXXX). The most commonly used method of estimation is based on the rapid reduction of 2,6-dichlorophenol-indo-phenol ( indophenol ) to its leuco form by ascorbic acid. This method, which Wcisfirst introduced by Tillmanns, Hirsch and Hirsch , involves the titration of an acid extract of the tissue or vitamin with a dilute solution of the dye. 

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