Nitro and Nitroso Dyes

These dyes are now of only minor commercial importance, but are of interest for their small molecular structures. The early nitro dyes were acid dyes used for dyeing natural animal fibers such as wool and silk. They are nitro derivatives of phenols, e.g., picric acid (4) or naphthols, e.g., C.I. Acid Yellow 1, 10316 [846-70-8] (5). 

The most important nitro dyes are the nitrodiphenylamines of general structure (6). 

These small molecules are ideal for penetrating dense fibers such as polyester and are therefore used as disperse dyes for polyester. All the important dyes are yellow (6) (X = H) is C.I. DisperseYellow 14, 10340 [961-68-2] and (6) (X = OH) is C.I. DisperseYellow 9, 10375 [6373-73-5]. Although the dyes are not terribly strong ( max ca. 20 000), they are cost-effective because of their easy synthesis from inexpensive intermediates. C.I. DisperseYellow 42 and C.I. DisperseYellow 86 are important lightfast dyes for automotive-grade polyester. 

Nitroso dyes are metal-complex derivatives of o-nitrosophenols or -naphthols. Tautomerism is possible in the metal-free precursor between the nitrosohydroxy tautomer (7) and the quinoneoxime tautomer (8). 

The only nitroso dyes important commercially are the iron complexes of sulfo-nated l-nitroso-2-naphthol, e.g., C.I. Acid Green 1, 10020 [19381-50-1] (9) these inexpensive colorants are used mainly for coloring paper. 

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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). 

R. Raue and J. F. Corbett, Nitro and nitroso dyes, in Ullmann s Encyclopedia of industrial Chemistry, Wiley-VCH Verlag GmbH, Weinheim, 2002. 

Nitro and nitroso dyes. These are dyes that contain NO2 functionality. 

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. 

NITRO- AND NITROSO-COMPOUNDS. Nitro-compounds contain the nitro group (-NOil attached directly to a carbon atom nitroso-compounds contain the nitroso-group (—NO) similarly attached. A very important member of this group is nitrobenzene, which upon reduction yields a variety of products, important in the synthesis of drugs and dyes. See Table I. 

The nitro and nitroso derivatives of a few aromatic hydroxy compounds are used as yellow-green dyes. The nitro compounds are prepared by direct action of nitric acid in presence of sulfuric acid, or by first sulfonating and then nitrating the sulfonic acid. In the preparation of naphthol yellow, a-naphthol is first sulfo-nated to the 2,4,7-trisulfonic acid, which on nitration undergoes replacement of the sulfonic acid groups at positions 2 and 4 by nitro groups. 

NDMA is not an industrially or commercially important chemical nevertheless, it can be released into the environment from a wide variety of manmade sources. This is due to the inadvertent formation of NDMA in industrial situations when alkylamines, mainly dimethylamine and trimethylamine, come in contact and react with nitrogen oxides, nitrous acid, or nitrite salts, or when trans-nitrosation via nitro or nitroso compounds occurs. Thus, potential exists for release into the environment from industries such as tanneries, pesticide manufacturing plants, rubber and tire manufacturers, alkylamine manufacture/use sites, fish processing industries, foundries and dye manufacturers. At this time, NDMA has been found in at least 1 out of 1177 hazardous waste sites on the National Priorities List (NPL) in the United States (VIEW Database 1989). 

Naphthol, alpha and beta Nitro dyes Nitroaniline Nitrobenzene Nitrophenol Nitroso dyes Oils light, medium, and heavy made in chemical plants... 

Nitro dyes (e.g., Martins yellow, naphthol yellow) are excreted in bile and urine whereas the nitroso dye, Alsatian green, is found in the urine only. Monoazo dyes are rapidly removed from the circulation but are eliminated in the urine rather than in bile (A4). Azorubine S and Ponceau 3R, however, are excreted mainly in the bile. Diazo dyes tend to accumulate in tissues, but there is a slow excretion of these dyes in the bile (Tl) and in the urine (W20). However, Palatine black 4B and benzo-... 

Dye Classification Based on chemical structure or chromophore, 20-30 different groups of dyes can be discerned. Azo (monoazo, disazo, triazo, polyazo), anthraquinone, phtha-locyanine and triarylmethane dyes are quantitatively the most important groups. Other groups are diarylmethane, indigoid, azine, oxazine, thiazine, xanthene, nitro, nitroso, methine, thiazole, indamine, indophenol, lactone, aminoketone and hydroxyketone dyes and dyes of undetermined structure (stilbene and sulphur dyes). The vast array of commercial colourants is classified in terms of colour, structure and application method in the Colour Index (C.I.), which has been edited since 1924 (and revised every three months) by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. Each dye is given a C.I. generic name determined by... 

Acid Dyes. These water-soluble anionic dyes ate appHed to nylon, wool, sUk, and modified acryHcs. They ate also used to some extent for paper, leather, food, and cosmetics. The original members of this class aU had one or mote sulfonic or catboxyHc acid groups in thein molecules. This characteristic probably gave the class its name. Chemically, the acid dyes consist of azo (including preformed metal complexes), anthraquiaone, and ttiaryHnethane compounds with a few azHie, xanthene, ketone imine, nitro, nitroso, and quHiophthalone compounds. 

This energetic reducing agent can be maintained at constant strength in aqueous hydrochloric acid solution for a reasonable period. It is advisable, however, to re-standardise it after 24 hours standing. It serves for the reduction of aromatic nitro compounds, some nitroso bodies, many azo dyes, and of nearly all the dyes which yield leuco-compounds. It is easily standardised against a ferric salt—say ferric alum—using potassium thiocyanate as indicator. From the equations —... 

In principle, the nitroso, hydroxylamine, and/or amine products of nitro reduction might undergo coupling to form azoxy, azo, and/or hydrazo dimers, but no evidence for these products has been found under the conditions that have been studied to date. One reason that these dimers do not accumulate may be that they are rapidly reduced by Fe°. In fact, Fe° reduces azo groups to amines [Eq. (12)] very rapidly [111-113], and this reaction may prove to be useful in the remediation of wastewaters contaminated with azo dyes. 

Nitroso Benzene.—The nitroso or nitrous acid derivatives are exactly analogous to the nitro or nitric acid derivatives. As the nitro radical is (AO2), so the nitroso radical is NO) and whenever this radical is present, as we found in the nitroso-amines (p. 61), and as we shall find in some more complex compounds of the dye class, it means nitroso derivative. The simplest representative, viz., nitroso benzene, CeHs—NO, differs from nitro benzene in that it is not formed by the direct action of the acid on the hydrocarbon nor, as shown above, is it able to be isolated as a reduction product of nitro benzene. It is prepared, however, by the oxidation of phenyl hydroxyl amine, either by means of ferric chloride, FeCU, or of chromic acid, CrOa. 

Anionic dyes are selected from the group consisting of nitroso compounds, nitro compounds, azo compounds, stilbenes, triaryl-methane compounds, xanthene compounds, quinoline compoimds, thiazole compounds, azine compounds, oxazine compoimds, thi-azine compounds, aminoketone compounds, anthraquinone compounds, indigoid compounds and phthalocyanine compoimds. 

Tertiary aliphatic amines and tertiary aromatic amines with substituted para position do not react with nitrous acid under these conditions. On alkalization the unchanged base precipitates, if insoluble in water. Tertiary amines with a free para position give nitroso derivatives which sometimes precipitate as insoluble hydrochlorides in contrast to nitro-soamines, these hydrochlorides are insoluble in ether. Upon alkalization of the solution the green nitroso derivative is set free, which can be extracted with ether (a blue extract). m-Phenylenediamine reacts with HNO2 under the formation of a brown dye (vesuvin), mercaptans with HNO2 give strongly colored thionitrous acids (esters are usually red). 

The relatively low quantity of nitroso and nitro compounds is noteworthy. The charged nitrene, generated from azido dye, appears to be less reactive towards oxygen, as compared to a neutral analogue, since comparative photolysis of neutral 4-(4-azidostyryl)quinoline under the same conditions resulted in the final reaction mixture contained much more nitroso (60%) and nitro (40%) compounds. 

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