Experience in Dyeing with Paranitrosodimethylaniline Hydrochloride

L. P. Kulev, G. M. Stepnova, and P. V. Kristalev

Para-nitrosodimethylaniline is one of the most important intermediates in the synthesis of quinoneimine dyes. Reacting in its quinoid form, para-nitrosodimethylaniline readily enters into condensation reactions with aromatic amines, diamines, phenols, aminophenols, and other compounds, as a result of which a variety of dyes are obtained, distinguished by deep shades and often considerable lightfastness. Condensation of hydrochloric p-nitrosodimethylaniline with gallic acid led to the synthesis of gallocyanine, which possesses mordant properties and forms fast lakes with metals. In view of the value of this dye, work began on developing methods for obtaining a considerable number of its homologues and derivatives, many representatives of which have found wide application in dyeing and calico-printing technology. Oxidation of the leuco compound of gallocyanine to the dye is carried out by the excess nitroso compound present in the reaction mixture. According to Academician A. E. Porai-Koshits [1], hydrochloric p-nitrosodimethylaniline is itself reduced in this process to p-dimethylaminophenylhydroxylamine, which, under the action of a dehydrating agent (methyl alcohol), first forms a dihydrodiphenazine derivative, and then, upon oxidation, an azine dye.

\[ \begin{aligned} &\text{hydrochloric }p\text{-nitrosodimethylaniline} +\text{ }p\text{-dimethylaminophenylhydroxylamine} \\ &\longrightarrow 2\,\mathrm{H_2O}+\mathrm{Cl} +\text{dihydrodiphenazine derivative} \ \xrightarrow{\text{oxidation}}\ \text{azine dye} \end{aligned} \]

This dye bears the name methylene gray or nigrizine and is formed in the preparation of all oxazine and azine dyes for which p-nitrosodimethylaniline serves as the starting material and oxidizing agent. I. M. Kogan [2] assigns a somewhat more complex structure to nigrizine, regarding it as a derivative of phenylphenazonium, formed as a result of condensation of hydrochloric p-nitrosodimethylaniline with p-aminodimethylaniline:

\[ \ce{ [p\text{-nitrosodimethylaniline hydrochloride}] + [p\text{-aminodimethyl-aniline}] ->[-H2O] [methylene\ gray\ intermediate] ->[\ +\ p\text{-aminodimethyl-aniline}\ ] [methylene\ gray] } \]

Both of these formulas are hypothetical, and the structure of methylene gray cannot be regarded as definitively established. In the production of gallol dyes and new blue P, nigrisine may be isolated as a by-product in a yield of about 20% of the weight of the dyes. Methylene gray, being a basic dye, dyes animal fibers and tanned cotton gray with a reddish cast, but the dyeings are not fast to light and are not of great interest for the textile industry. True, thanks to the work of our Soviet chemists, good results have been obtained in the use of nigrisine in calico printing; nevertheless, the field of application of this dye is very limited, and it still remains a little-valued by-product of the manufacture of oxazine and azine dyes.

While studying the conditions for the formation of methylene gray, we observed that hydrochloric p-nitrosodimethylaniline, even in the absence of alcohol—for example, simply on boiling in water—undergoes condensation with the formation of products colored dark brown or black. Attempts to isolate these products in dry form were unsuccessful, since on evaporation of the water, even under reduced pressure, resinous substances are obtained that do not pass into the solid state. The products of incomplete condensation of hydrochloric p-nitrosodimethylaniline are readily soluble in water. If a sample of wool or natural silk is placed in such a solution, then after boiling for a comparatively short period of time (20–30 minutes) the fibers are dyed from light brown to a deep black color. The dyeings are distinguished by exceptional fastness to washing, rubbing, and especially to light; they also proved sufficiently fast to the action of chlorine.

The very high light fastness of the dyes formed on the fiber and their color indicate that these dyes are not identical with methylene gray, but, in their ability to dye animal fibers and tanned cotton, like nigrisine, are also basic dyes. Brown dyeings are somewhat inferior to black ones in fastness to light; however, in all other respects—fastness to washing, ironing, alkalis and acids, fulling, etc.—they do not differ from the latter. In dyeing animal fibers with hydrochloric p-nitrosodimethylaniline, a gradual transition from a yellow-green color to black is observed, with various brown shades lying between these two extreme points. From this it may be concluded that p-nitrosodimethylaniline, condensing on the fiber, gives dyes of varying degrees of complexity, of which the most complex and at the same time the most fast is the black dye. Consequently, the process of formation of this final product of condensation of p-nitroso-

dimethylaniline may be compared with the multistage process of formation of aniline black, where there is likewise a gradual complication of the molecule through a long series of intermediate compounds (phenylquinonediimine, imine-red emeraldine, nigraniline, pernigraniline, etc.). It is known that elucidating the structure of aniline black is one of the most difficult problems in the chemistry of dyes, owing to the high molecular weight, insolubility, and amorphous state of this dye. This task is further complicated by the circumstance that aniline black is apparently not a homogeneous product, but a mixture of various substances, the separation and study of which involve great difficulties. Therefore all the formulas proposed for this dye, including Green’s formula, may be regarded only as the expression of one of the condensation products, but not of aniline black itself. The elucidation of the structure of the black dye formed during the condensation of hydrochloric p-nitrosodimethylaniline appears to us to be an equally complex problem. Leaving aside for the time being the question of isolating and analyzing the dyes, we considered it necessary to develop a dyeing procedure with hydrochloric p-nitrosodimethylaniline and to determine the coloristic properties of the dyes formed.

Experimental Part

Hydrochloric p-nitrosodimethylaniline was obtained by us in the usual manner, i.e., by nitrosation of dimethylaniline in hydrochloric acid solution with sodium nitrite [3]. Following the indication of N. N. Vorozhtsov that, in preparing nitroso compounds by the method of liberating nitrous acid from a nitrite solution, the oxidizing action—which probably depends on the admixture of a nitric-acid salt in NaNO₂—is harmful, we carried out the interaction of the solutions under a layer of liquid. For this purpose the sodium nitrite solution was slowly added through a tube immersed to the bottom into a vessel containing hydrochloric acid and dimethylaniline. The hydrochloride of p-nitrosodimethylaniline that precipitated as a yellow sediment was filtered off and washed several times with dilute hydrochloric acid.

The moist salt darkens rather quickly on storage and subsequently turns into a resin, which no longer possesses dyeing properties. Drying the nitroso product is therefore essential if it is intended to be used for dyeing not immediately after preparation.

Experiments on dehydrating hydrochloric p-nitrosodimethylaniline showed that heating to 100° does not affect the quality of the product, whereas at higher temperatures a more or less considerable resinification is observed.

Table 1

It proved still more rational to dry the salt under reduced pressure, for example, in a vacuum drying cabinet. Under these conditions, moisture is removed more completely, and the preparation produced in this way can be stored in hermetic packaging for an indefinitely long time without any noticeable change.

Dyeing of Wool and Silk

Experimentally we established that the minimum amount of hydrochloride of p-nitrosodimethylaniline required to obtain a deep black color without grayish-green shades corresponds to 5%. The salt is dissolved in a small amount of water, and the solution is heated until its yellow color changes to dark brown (15–20 minutes). The solution is diluted with hot water in such a way that the liquor ratio, i.e., the ratio of the weight of the entire dye solution to the weight of the fibrous material being dyed, reaches 30:1. A sample of wool (silk) well wetted with water, or preferably treated by brief (2–3 minute) boiling with a 5-percent solution of \(NH_4OH\), is immersed in the dye bath, the temperature of which is brought to 80–90°. Dyeing is carried out at this temperature for 1–1.5 hours; then heating is stopped, the solution is allowed to cool somewhat, the dyed sample is removed, and it is thoroughly washed with warm water until the yellow coloration disappears from the wash waters. It should be borne in mind that, in order to obtain deep-black and fast dyeings, it is necessary to remove thoroughly from the dyed material the hydrochloride of p-nitrosodimethylaniline that has not entered into reaction; for this purpose it is sometimes useful to apply an additional washing of the material with hot water, especially in those cases where thick woolen fabrics, felt, etc., are being dyed.

It has been observed that the dyeing time with hydrochloride of p-nitrosodimethylaniline is somewhat shortened if 3–5% of \(CuSO_4\), \(ZnSO_4\), or \(ZnCl_2\) is added to the dye bath.
Of these salts, copper sulfate gives the best results.

Table 2

Dyeing of wool

Zinc salts act noticeably more weakly, although zinc chloride is little inferior in this respect to copper sulfate.

The salts indicated apparently play the role of condensing agents, accelerating the formation of the dye. Some of the salts tested by us, for example \(FeSO_4\), had a negative effect, causing strong tar formation in the product.

In dyeing silk, the same conditions are observed as in dyeing wool. Possessing a considerably more acidic character than wool, natural silk has a greater “affinity” for basic dyes and is dyed more rapidly with hydrochloride of p-nitrosodimethylaniline. Good black dyeings are obtained in this case even at lower dye concentrations, beginning with 3-percent solutions (liquor ratio 30:1).

Table 3

Dyeing of Cotton

Hydrochloric acid p-nitrosodimethylaniline dyes cotton without mordant only gray, a color not fast to washing or light. On the other hand, on tannin-mordanted cotton one obtains black dyeings which, in quality, are not inferior to dyeings on wool and silk. For mordanting cotton the usual method is used, i.e., the specimen of fabric or yarn is treated first with a solution of tannin and then with tartar emetic, taken in half the amount by weight of the mordant. Good results were obtained by us when using 8% tannin and 4% tartar emetic, relative to the weight of the fibrous material, while observing all the other conditions recommended for laboratory tannin mordanting of cotton [4].

The mordanted fabric is immersed in a solution of hydrochloric acid p-nitrosodimethylaniline (3% relative to the weight of the material being dyed), preliminarily heated until a black coloration appears.

The bath ratio, as in the preceding cases, is 30 : 1. Dyeing is carried out at the boil for 1 hour; the dyed material is removed, washed well with cold water, and dried. Dyeings on cotton are distinguished by high fastness to washing, rubbing, ironing, and especially to light, in this respect not yielding to aniline black.

Dyeing of Leather

The black dye from p-nitrosodimethylaniline proved quite suitable also for dyeing leather. For this purpose one may use a 2–3-percent aqueous solution of hydrochloric acid p-nitrosodimethylaniline, which should be boiled until it acquires a deep black coloration. It is better to dye the leather “by spreading,” i.e., to cover its surface with this solution using a brush, paintbrush, or sponge. Like other basic dyes, our dye dyes leather treated with tanning acids better; moreover, when 2-percent solutions of hydrochloric acid p-nitrosodimethylaniline are used, no “bronzing” shade is observed.

Testing Dyeings with Hydrochloric Acid p-Nitrosodimethylaniline for Fastness

It is known that, in evaluating the quality of a dye, tests for fastness to light, washing, fulling, perspiration, acids, alkalis, chlorine, and other kinds of action to which the dyed material may be subjected are important. The dyeings prepared by us were tested first of all for ...

fastness to light. For this purpose, samples of fabrics dyed with \(p\)-nitrosodimethylaniline hydrochloride were fastened to a board and exposed under identical conditions alongside samples dyed with aniline black and cryogen black. Some of the samples were exposed to outdoor light, with the calculation that for a prolonged period of time (6 weeks) they would remain in the air; another portion was irradiated with artificial light by means of a lamp giving rays with \(\lambda = 3650\ \text{Å}\). In this case the duration of illumination corresponded to 55 days of continuous action of sunlight. In both cases no difference was observed between the tested dyeings and the standard samples taken for comparison. Thus, the lightfastness of the black dye on wool, silk, and tanned cotton may be rated at the highest grade (7–8 on the eight-grade scale).

It should be noted that prolonged wear of outer garments made of woolen fabrics dyed with \(p\)-nitrosodimethylaniline hydrochloride confirmed the high lightfastness of the dye and its resistance to atmospheric influences. Fastness to washing was tested by us under ordinary conditions, i.e., by vigorous treatment of the dyed fabric for one hour in a solution of 5 g of liquid soap and 3 g of soda in 1 l of water at \(T^\circ 40–50^\circ\). The dyeing did not change in shade, and the soap-soda bath remained colorless.

Positive results were also obtained in testing the dyeings for perspiration (heating in dilute acetic acid or in a solution of ammonium carbonate), rubbing (with white cotton fabric), and chlorine (the action of gaseous chlorine on dry and wet dyed fabrics). The dyeings on tanned cotton are inferior in resistance to acids and chlorine to dyeings on wool and silk, since here destruction occurs of the tannin lake formed by antimony and the tanning substance. In all other respects no noticeable difference is observed between the various samples.

Conclusions

1. It has been established that \(p\)-nitrosodimethylaniline hydrochloride has the ability to condense on the fiber with the formation of brown and black dyes.

2. The possibility of using \(p\)-nitrosodimethylaniline hydrochloride for dyeing wool, silk, tanned cotton, and leather has been demonstrated. A recipe for dyeing these materials has been developed.

3. Experiments have been carried out to test the fastness of the dyeings to light, washing, perspiration, rubbing, and chlorine. The black dye is distinguished by high lightfastness and is fast in all other respects.

LITERATURE

1. Porai-Koshits A. E. Intermediates and Organic Dyes. ONTI, 750, 1936.
2. Kogan I. M. Chemistry of Dyes (Synthetic). ONTI, 381, 1938.
3. Vorozhtsov N. N., Kasatkin N. M. Izv. IVPI, XI, 79, 1928.
4. Ruggli P. Practicum on Dyeing and Analysis of Dyes. NKHTI, 45, 1929.