Nitro dyes

Nitro dyes are special because the small dye molecules penetrate the hair and impart color throughout the hair or in an annular pattern [41], In this way, intense colors can be obtained even in hair with a large proportion of white. The dyes are 

The physical and chemical properties of dyes are very important for even coloring. First a hair dye should have comparable affinities for the roots, damaged areas, and tips. Second, the combined yellow-to-blue dyes used for shading must have similar properties so that color shifts will not take place when the hair is washed, for example. The importance of these problems is illustrated by the number of patent applications disclosing new, custom-tailored dyes [35] and suitable dye mixtures [42], The relatively good colorfastness and stability of some nitro dyes allow them to be used in oxidation hair dyes as well, especially for the shading of brilliant (mainly red) fashion colors. 

Preparation 392.—Naphthol Yellow S. (2 i-Dinitro-l-naphthd-1-sulphonic add) (K Salt). 

100 gms. of cone, sulphuric acid are warmed to 100° in a small flask, 50 gms. of powdered a-naphthol are added in one instalment. The mixture is raised to 120° by heating in an oil or sand bath and maintained at this temperature for 3—4 hours. The sulphonation mixture is then poured into 600 c.cs. of water, which are stirred mechanically. When the temperature of the mixture falls to 30° it is poured into a mixture of 23 gms. of cone, nitric acid and 8 c.cs. of water, which is well stirred mechanically the temperature is kept below 35° by cooling in water, if necessary. A further 21 gms. of cone, nitric acid are added at such a 

Orange-yellow powder dyes wool and silk from an acid bath. 299.) 

The manufacture of sulfur dyes involves sulfurisation processes, the chemistry of which remains rather mysterious and may arguably be considered still to be in the realms of alchemy The processes involve heating elemental sulfur or sodium polysulfide, or both, with aromatic amines, phenols or aminophenols. These reactions may be carried out either as a dry bake process at temperatures between 180 and 350 °C or in solvents such as water or aliphatic alcohols at reflux or at even higher temperatures under pressure. C. I. Sulphur Black 1, for example, is prepared by heating 2,4-dinitrophenol with sodium polysulfide. 

The nitro group is commonly encountered as a substituent in dyes and pigments of most chemical classes, but it acts as the essential chromo-phore in only a few dyes. Nitro dyes are a small group of dyes of some importance as disperse dyes for polyester and as semi-permanent hair dyes. Picric acid, 139, was historically the first nitro dye, although it was 

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Nitro-Disc Nitrodisc Nitro-Dur Nitro-dur Nitro-Dur Nitro dyes Nitroesters Nitroethane Nitroethane [79-24-3]... 

Also present but not essential in permanent hair colorants are nitro dyes which dye hair without oxidation. These dyes, nitro derivatives of aminophenols and benzenediamines, impart yellow, orange, or red tones. Although they have good tinctorial value, they are not as colorfast as the oxidative dyes. They also are used in semipermanent hair colorants. 

Nitro and Nitroso Dyes. These dyes are now of only minor commercial importance, but are of interest for their smaU molecular stmctures. The early nitro dyes were acid dyes used for dyeing the natural animal fibers such as wool and sUk. They were nitro derivatives of phenols, eg, picric acid [88-89-1] (73) (Cl 10305), or naphthols, eg. Cl Acid YeUow 1 [846-70-8] (74) (Cl 10316). 

The most important nitro dyes are the nitrodiphenyl amines of general stmcture (75). 

Chemical classification is based on chromogen. For example, nitro dyes have the chromophore -NOj. The Color Index (C.I.), published by the Society of Dyers... 

Nitro-bakterien, n.pl. nitrobacteria, -benzol, -benzin, n. nitrobenzene, -chinon, n. nitro-quinone. -cocussaure, /. nitrocoeeic acid, -derivat, n. nitro derivative, -farbstoff, m. nitro dye. -fettkorper, m. aliphatic nitro compound, -fettsaure, /. nitro fatty acid, -gruppe, /. nitro group, -halogenbenzole,... 

The valence-bond approach may be used to provide a qualitative account of the /lmax values, and hence the hues, of many dyes, particularly those of the donor acceptor chromogen type. The use of this approach to rationalise differences in colour is illustrated in this section with reference to a series of dyes which may be envisaged as being derived from azobenzene, although in principle the method may be used to account for the colours of a much wider range of chemical classes of dye, including anthraquinones (see Chapter 4), polymethines and nitro dyes. 

The chemistry of the three most important chemical classes of organic colorants, the azo, carbonyl and phthalocyanine classes, has been dealt with individually in Chapters 3-5 respectively. In this chapter, the chemistry of a further five chemical classes which are of some importance for specific applications is discussed. These classes are the polymethines, arylcarbonium ion colorants, dioxazines, sulfur dyes and nitro dyes. A section of this chapter is devoted to each of these, each individual section contains a description of the principal structural features which characterise the particular colorant type, together with an outline of the chemistry of the main synthetic routes. There are many other chemical types of dyes and pigments that do not fall into the categories previously mentioned, but which are neglected in this text either because they are commercially of little importance or because they have been less extensively investigated. 

The synthesis of nitro dyes is relatively simple, a feature which accounts to a certain extent for their low cost. The synthesis, illustrated in Scheme 6.5 for compounds 140 and 141, generally involves a nucleophilic substitution reaction between an aromatic amine and a chloronitroaromatic compound. The synthesis of C. I. Disperse Yellow 14 (140) involves the reaction of aniline with l-chloro-2,4-dinitroaniline while compound 141 is prepared by reacting aniline (2 mol) with compound 144 (1 mol). 

Nitro dyes exhibit benzenoid-quinonoid tautomerism (1.25) and their colour is attributed mainly to the o-quinonoid form, since this can be stabilised by hydrogen bonding. The tautomeric o-nitrosonaphthols (1.26) readily form chelate complexes with metals. A few yellow nitro disperse dyes, including Cl Disperse Yellow 1 (1.25), and brown acid dyes remain of significance. The remaining nitro and nitroso colorants, such as (1.26) and its 1 3 iron (II) complex (1.27), are no longer of commercial interest. 

The hydroxy-nitro dye Naphthol Yellow S (6.233 Cl Acid Yellow 1) was discovered in 1879 by Caro and is still manufactured. It is produced by sulphonation of 1-naphthol to give l-naphthol-2,4,7-trisulphonic acid, followed by replacement of the 2- and 4-sulpho groups in nitric acid medium. Nucleophilic substitution of l-chloro-2,4-dinitrobenzene with 4-aminodiphenylamine-2-sulphonic acid gives Cl Acid Orange 3 (6.234). 

Xitro and Nitraso Dyes. These dyes are now of only minor commercial importance, hut are of interest for their small molecular structures. The most important nitro dyes are the nitrodiphcnylamines. Their small molecules arc ideal for penetrating dense libers such as polyester, and are therefore used as disperse dyes for polyester. All the important dyes are yellow. Although the dyes are not terribly strong (fW- 20- 000). they are cosi-effective hecatise of their easy synthesis from inexpensive intermediates... 

Industrially applied disperse dyes are based on numerous chromophore systems. Approximately 60 % of all products are azo dyes, and ca. 25 % are anthraqui-none dyes, with the remainder distributed among quinophthalone, methine, naphthalimide, naphthoquinone, and nitro dyes [9],... 

Nitro Dyes. 2-Nitrodiphenylamines are readily obtained by condensation of derivatives of 2-nitrochlorobenzene 88-73-3] with suitable aromatic amines. Because of their accessibility and good lightfastness, these dyes became very important for dyeing cellulose acetate and, more recently, have gained a solid position as disperse dyes for polyester fibers. This is especially true for the reaction product of 1 mol of 3-nitro-4-chlorobenzenesulfonyl chloride [97-08-5] and 2 mol of aniline. An exhaustive review of the constitution and color of nitro dyes is given by Merian [40], The yellow nitroacridones may also be classified in this group. 

Anionic dyes include many compounds from the most varied classes of dyes, which exhibit characteristic differences in structure (e.g., azoic, anthraquinone, triphenylmethane, and nitro dyes) but possess as a common feature water-solubilizing, ionic substituents. The anionic azo dyes which are discussed here constitute the most widely used group of this class of dyes. 

Nitro dyes are the most important class of direct hair dyes they are substituted derivatives of nitrobenzene or nitrodiphenylamine [3, Nitro and Nitroso Dyes]. By proper selection of donor groups and substitution site on the benzene ring, a spectrum of dyes from yellow to blue violet can be prepared [9, pp. 247-250], [40] (Scheme 3). 

Scheme 5.3 Influence of substituents on the color of nitro dyes...

As the name suggests, this very small class of organic dyes has at least one nitro group as the chromophore. Nitro dyes invariably are yellow or orange and are important for their economical cost and good lightfastness. Examples include the dyes shown in Fig. 13.78-C.I. Acid Orange 3 (A), C.I. Disperse Yellow 42 (B), C.I. Acid Yellow 1 (C), and... 

C.I. Disperse Yellow 70 (D). A key disadvantage of nitro dyes is their low color strength (emax = 5000-7000). Improvements in color strength have been achieved by incorporating an azo group, as illustrated in dye D. 

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