Leuco form

Both the m- and -phenylenediamines are used to manufacture sulfur dyes, either by refluxing in aqueous sodium polysulfide, or heating with elementary sulfur at 330°C to give the leuco form of the dye. These dyes are polymeric, high molecular weight compounds, and soluble in base. The color is developed by oxidation on the fabric. 2,4-Toluenediamine and sulfur give Sulfur Orange 1 (14). 

Chemical reactions for the negative systems in CopiArt can be generalized by the reaction of the leuco form (9) of crystal violet [548-62-9] to produce the colored form (10). 

Addition of sodium dithionite to formaldehyde yields the sodium salt of hydroxymethanesulfinic acid [79-25-4] H0CH2S02Na, which retains the useful reducing character of the sodium dithionite although somewhat attenuated in reactivity. The most important organic chemistry of sodium dithionite involves its use in reducing dyes, eg, anthraquinone vat dyes, sulfur dyes, and indigo, to their soluble leuco forms (see Dyes, anthraquinone). Dithionite can reduce various chromophores that are not reduced by sulfite. Dithionite can be used for the reduction of aldehydes and ketones to alcohols (348). Quantitative studies have been made of the reduction potential of dithionite as a function of pH and the concentration of other salts (349,350). 

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 ... 

Thus, the presence of a thiamine ring in Cl Sulfur Blue 9 was conclusively proved. The thiamine ring is the fundamental chromophore that accounts for the high color value of both the sulfur dye and Methylene Blue [61-73-4] including their abiUty to form pale yeUow leuco forms on reduction. Methylene Violet (15) is obtained from Methylene Blue (16) by hydrolysis in boiling alkah. 

Sulfur dyes are appHed to leuco form. In this form, the dye has affinity for the fiber. After the dye is completely absorbed by the fiber, it is reoxidized in situ. In dyes, such as the bright blues which contain quinonimine groups, further reduction takes place in a manner similar to the reduction of the keto group in vat dyes. 

Logwood extract—A reddish brown-to-black soHd material extracted from the heartwood of the leguminous tree Haemato>y/on campechianum. The active colorant substance is principally hematein. The latent coloring material is the unoxidized or leuco form of hematein called hematoxylin. The leuco form is oxidized by air. 

Fig. 6. Color-shifting dye developers (a) yeUow dye developer [16044-30-7] that becomes magenta upon hydrolysis (b) cyan dye [50695-79-9] that becomes colorless upon protonation and (c) leuco form of a dye developer [50481-86-2] that becomes yellow upon hydrolysis and oxidation.

Vat Dyes. These water-iasoluble dyes ate appHed mainly to ceUulosic fibers as soluble leuco-salts after teductioa ia an alkaline bath, usuaUy with sodium hydrosulfite. FoUowiag exhaustion onto the fiber, the leuco forms ate reoxidized to the iasoluble keto forms and aftertreated, usuaUy by soapiag, to redevelop the crystal stmcture. The principal chemical classes of vat dyes ate anthraquiaone and iadigoid. 

Sulfur dyes are used for dyeing ceUulosic fibers. They are insoluble in water and are reduced to the water-soluble leuco form for appHcation to the substrate by using sodium sulfide solution. The sulfur dye proper is then formed within the fiber pores by atmospheric oxidation (5). Sulfur dyes constitute an important class of dye for producing cost-effective tertiary shades, especially black, on ceUulosic fibers. One of the most important dyes is Cl Sulfur Black 1 [1326-82-5] (Cl 53185), prepared by heating 2,4-dinitrophenol with sodium polysulfide. 

Flavanthrone. Flavanthrone [475-71-8] (177) (Cl 70600) has exceUent dyeiag properties, which ate due to the stabUity of the leuco form, but its fastness is not satisfactory. Only the unsubstituted flavanthrone is used as a vat dye, ie. Cl Vat YeUow 1 (177). It is mainly used as a pigment, ie. Cl Pigment YeUow 24. Synthetic routes ate iUustrated ia Figure 10. 

The almost colourless leuco form of the base passes upon oxidation into the strongly coloured indamine. When titrating iron(III) at a pH of about 3 and the colourless hydrochloride of the leuco base is added, oxidation to the violet-blue indamine occurs with the formation of an equivalent amount of iron(II). At the end point of the EDTA titration, the small amount of iron(II) formed when the indicator was introduced is also transformed into the Fe(III)-EDTA complex FeY-, whereupon the blue indamine is reduced back to the leuco base. 

At concentrations above 10 M a complex series of reactions, ending with the formation of 4,4-methylenebis (N,N-dimethylaniIine), occur. The extra methylene group comes from a further molecule of starting material. Finally, at even higher concentrations, the product becomes the dye, crystal violet or its leuco-form. 

The chemistry of quinone dyes has been discussed in a series of books entitled The Chemistry of Synthetic Dyes, edited by Venkataraman.1 The general chemistry of quinoid compounds has been discussed by Patai.2 There have been many books that cover quinoid compounds as dyes and pigments but very few discuss the chemistry of the corresponding leuco dyes. Traditional vat dyes are applied to cellulosic fiber in the leuco form. The chemistry of the leuco form of vat dyes is rather simple. Some leuco quinones are quite stable in the solid state and can be stored for a year. Other leuco dyes are unstable in solution and gradually undergo aerial oxidation. 

Anthraquinone leuco dyes are widely known as vat dyes.10 Vat dyes possess extensively conjugated aromatic systems containing two or more carbonyl groups, e.g., anthraquinone, indigoid chromophores. The colored form of vat dyes are insoluble in water. The dyes are applied by a process whereby the dye is converted to the reduced form (leuco dye) which is soluble in water and can penetrate into a cellulosic fiber. On exposure to the atmosphere the leuco form is oxidized to the original quinoid form which then precipitates as an aggregate. Vat dyes generally have excellent chemical and photochemical stability. 

When Methylene Blue is reduced, the yellowish leuco cannot be isolated due to instant air oxidation. Benzoylation of the leuco form provides stabilization. There are also leuco thiazine dyes stable enough to be isolated without the need for aroylation. 

Benzo[l,2-fl]-8-methyl-9-azaphenothiazinone (14) was reduced to a leuco form 15 which was too unstable to be isolable.10 The leucos 16 and 18 obtained from thionation of iV,iV-diphenyl-p-phenylenediamine and j9,//-dianilinodiphenylamine, respectively, are also air sensitive. 11 They are oxidized to thiazine dyes 17 and 19 which are reported to absorb in the near infrared. 

Phenazine leucos are generally more reactive and more susceptible to air oxidation than the thiazines and oxazines. Incorporation of electron-withdrawing groups on the acyl substituent at the 10-position of the leuco dye can provide a substantial improvement in the thermal and light stability of the leuco form and it is found that in general the stronger the electron-withdrawing character of the acyl substituents the more stable the leuco is.18... 

Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic fibres they may also be similarly used with vat or sulphur dyes in their leuco forms. In the case of anionic dyes on wool or nylon, however, their role is different as they are used to facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this does not create a problem and so most adsorption isotherms of water-soluble dyes are interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however, some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes that cannot be explained by ionic interactions alone. 

Figure 12.24 Schematic diagram of the cathodic reduction of the mediator that converts the insoluble vat dye into its soluble leuco form [239]...

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