Benzidine dyes

Direct dyes are defined as anionic dyes substantive to ceUulosic fibers (cotton, viscose, etc), when applied from an aqueous bath containing an electrolyte. Before the discovery of Congo Red in 1884, only mordanted cotton could be dyed. Congo Red [573-58-0] (62) (Cl Direct Red 28 Cl 22120) a primary symmetrical disazo dye, which is made readily from bisdiazotized benzidine and naphthionic acid [84-86-6] (4-arnino-l-naphthalenesulfonic acid), was the precursor of a most important line of dyes, including all shades, derived from benzidine and its homologues. Today, no benzidine dye is produced because benzidine is carcinogenic. 

In spite of the reactive dyes with their outstanding wet-fastness, the direct dyes continue to hold a large share of the market for inexpensive cellulose and paper dyes. Apart from the new developments to replace the old established benzidine dyes, con-... 

In contrast to some naturally occurring dyes like indigo or kermes, which must be vatted or mordanted to be applied in textile dyeing, direct or substantive dyes can be used on cellulosic fibers directly . Their use is widespread because of their easy handling. Today, direct dyes still account for ca. 10% of the world textile dye consumption. For use on paper, see Section 5.3. Recent research on direct dyes concentrated on the replacement of possibly carcinogenic benzidine dyes [44],... 

While the development of direct dyes was essentially completed in the 1930s, it was reactivated in the 1960s and 1970s, when it was found that benzidine, an important direct dye intermediate at the time, is carcinogenic. Benzidine dyes are used for some yellows and reds and mostly for black. In view of the ensuing concern, the leading dye producers withdrew benzidine-based dyes preemptively to protect their workers, dyers, and consumers. Studies to find alternative dyes were undertaken, and this work led eventually to many of today s most important black dyes for leather, e.g,C.I. DirectBlack 168, 30410 [85631-88-5] (16). 

At present nitration is one of the most widely applied direct substitution reactions. This is due to several factors. For example nitration usually proceeds easily, its products can readily be separated from the spent acid, said there is a wide range of possibilities in the practical use of nitro compounds, both as intermediates and end products. The presence of a nitro group in the starting product made it possible to obtain a number of basic organic intermediates such as aniline said benzidine. Dyes with more than one nitro group, such as picric acid were obtained. It has been found that higher nitrated nitro compounds and nitric acid esters have explosive properties and are of practical importance. Some nitro compounds are used in perfumes. Medicinal properties have lately been discovered in certain nitro compounds, eg. chloramphenicol. 

Benzidine Dyes. In the preparation of mixed disazo dyes from benzidine and similar bases, the first coupling reaction must be carried out under conditions which will prevent the formation of the symmetrical disazo dye. Hence, the first coupling reaction is done with the coupler which reacts less readily, and the more active coupler is used in the second coupling step. If there is not a large difference between the two couplers with respect to coupling reactivity, the first coupling reaction is carried out under the mildest conditions possible (low alkalinity or... 

Benzidine can be combined with all of the phenols and amines which are commonly used in preparing azo dyes. It happens that only one of the two diazo groups in tetrazobenzidine is highly reactive while the other is rather inactive. It is possible, therefore, to make not only the benzidine dyes employing two molecules of a single phenol or amine coupling component, but also, in many cases, the so-called mixed benzidine dyes. The mixed dyes are formed, however, only when the first coupling component is not too reactive so that the disazo dye is not formed immediately from the intermediate compound ... 

Technical Observations. Congo red, the first of the benzidine dyes, is widely ascd despite its sensitivity to acids. It is not surpassed in beauty by any other direct dye. Congo red is manufactured by only a few firms because its low price does not allow for much margin of profit. 

Benzidine Dyes.— The importance of hydrazo compounds in connection with dyes is not on their own account for, as has been stated, they are colorless compounds but because they are easily oxidized to azo compounds which are dye compounds and because of the above rearrangement into compounds like benzidine which yield dyes known as benzidine dyes (p. 787). 

The diazotization of benzidine yielding diazo and tetrazo compounds, the coupling of these with phenols and amines yielding azo compounds, which are the benzidine dyes, should be recalled here (p. 569). 

Synthetic Dyes.—Ip common usage the term aniline dye is applied to any dyestuff prepared from organic chemical substances. As the first dye made and many of those made at present are derived from aniline the above name is significant. It is not a true name, however, in many cases for though some of the azo and benzidine dyes (p. 573) may be considered as aniline derivatives those derived from naphthalene can not be so considered. Other dyes which we shall study later. 

Use Organic synthesis, heat-transfer agent, fungistat in packaging of citrus fruit, plant disease control, manufacture of benzidine, dyeing assistant for polyesters. 

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