Gallamine Blue from Gallamide
Nitrosodialkylanilines react with gallic acid or gallamide under the influence of heat to form well defined compounds designated as oxa- zines. The gallic acid is obtained exclusively from natural tannins.
p-Nitrosodi- Gallamide Gallamine blue methylaniline
(One molecule of nitrosodimethylaniline serves as an oxidizing agent.)
(a) Nitrosodimethylaniline
A mixture of 100 grams of dimethylaniline and 200 grams of 30 per cent hydrochloric acid is cooled, and 300 grams of ice is added. The
* At this point, the solution should not change color from blue to green; if it does, too much acid has been added. Excess acid prevents complete precipitation of the dye and should be neutralized by the addition of sodium acetate.
OXAZINE AND THIAZINE DYES 309 beaker containing the mixture is placed in an ice bath, and a solution of 60 grams of 100 per cent sodium nitrite in the least possible amount of water is added dropwise over a period of 5 hours. The presence of free nitrous acid in the reaction mixture cannot be tested for by starch- iodide paper, since nitrosodimethylaniline itself gives a positive test.
Excess nitrous acid, therefore, must be detected solely by smell. The mixture should have an acid reaction to Congo red, of course. After 6 hours, the mixture is filtered, and the precipitate, rinsed into the funnel by some of the mother liquor, is sucked as dry as possible, pressed out in a screw press, and then pulverized. The resulting nitrosodimethylaniline hydrochloride should not be dried, but used while still moist. In large scale operations, the product is dried sufficiently merely by centrifuging.
p-Nitrosodiethylaniline is prepared in a similar way, except that no water is added to the nitrosation mixture because of the very high solubilty of the hydro- chloride. Concentrated hydrochloric acid and saturated sodium nitrite solution are used. Industrial preparations are carried out in enameled equipment such as is used in tropaeoline coupling.
(b) Gallamine Blue
In a glass vessel equipped with reflux condenser and stirrer (Fig. 6), a solution of 20 grams of gallamide (page 170) of about 92 per cent purity (the purity is determined by distilling off the ammonia from a sodium hydroxide solution and titrating) in 500 cc. 90 per cent alcohol is heated to boiling, and the nitrosodimethylaniline hydrochloride, pre- pared from 75 grams of dimethylaniline, is added in three portions, al- lowing a 15-minute interval between additions. The mixture is then refluxed for 4 hours and allowed to stand for 12 hours more. The galla- mine blue comes out as a lustrous, bronzy precipitate which is filtered off and washed with water. The alcohol is recovered and rectified. The yield of pure gallamine blue is about 40 grams. The alcoholic mother liquor yields a gray, nigrosine-like dye which gives very fast gray tints on cotton (with chromium acetate); this dye is called methylene gray.
Gallamine blue, which is quite insoluble in water and hence can- not be used as such, can be converted to soluble forms in various ways.
One part of gallamine blue is heated to 50°C. on a water bath with 6 parts of sodium bisulfite solution (25 per cent S 02) until the evolution of sulfur dioxide ceases (about 1 hour). The mixture is then heated for 1 to 3 days at 85° until the mixture has become gray green.
The resulting product is the sulfonic acid of the leuco compound (per- haps complex sulfonate) which, with chromium acetate, gives brilliant and fast marine blue shades on wool. It can also be used in cotton print- ing, but is not as important for this use as the related dye, modern violet.
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Reduction of gallamine blue with hydrogen sulfide produces a leuco compound, modern violet, which gives extremely pure and fast chromi- um lakes on cotton.
To the clear solution of 50 grams of gallamine blue in about 40 grams of 30 per cent sodium hydroxide and 400 cc. water, 50 grams of crystalline sodium sulfide is added. During the course of 1 hour at 60° C , the mixture is slowly acidified by the addition of about 100 grams of con- centrated hydrochloric acid until a permanent acid reaction to Congo red is produced. The blue color almost disappears, and the nearly color- less solution is filtered to remove sulfur. The filtrate is treated with 150 grams of salt to precipitate the leuco compound which is filtered off, washed with a small amount of saturated salt solution, and pressed out well. The product should be dried in vacuo at 60° since it is easily re- oxidized. The yield is about 55 grams.
Technical Observations. Industrial preparations are carried out in enameled iron vessels with lead-tube reflux condensers. The preparation starting with 40 kilo- grams of gallamide requires about 12 hours.
Modern violet must be powdered in the cold because its high oxidizability may cause spontaneous ignition. This may be caused, in some cases, by the presence of finely divided sulfur.
The oxazines are excellent printing colors. In addition to dimethylaniline deriva- tives, those from diethylaniline are also prepared. The latter give very pure green- ish tints. If gallic acid is used in place of gallamide, gallocyanine is formed. This dye was first discovered by H. Kochlin, who, in trying to fix nitrosodimethylaniline to cotton by means of tannin and tartar emetic, obtained blue dyes which he recog- nized as oxazines. Gallocyanine cannot be prepared satisfactorily in ethyl alcohol solution. Methyl alcohol gives better results, but is an undesirable solvent because of its toxicity and volatility. Many more complicated oxazines are known but these cannot be described here.
It should be mentioned in passing that the first oxazine to attain technical im- portance was Meldola blue (naphthol blue, Bengal blue), prepared from nitroso- dimethylaniline hydrochloride and 0-naphthol. It is a very fast dye but does not give pure shades. Furthermore, its dust irritates the mucuous membranes so severe- ly that many persons cannot work with it. Despite these disadvantages, however, Meldola blue is still rather widely used.
OXAZINE AND THIAZINE DYES 311 Methylene Blue from Dimethylaniline
The formation of methylene blue is interesting both scientifically and technically, and should be reviewed briefly before the actual pro- cedure is described.
(a) Nitrosodimethylaniline is prepared from dimethylaniline by treament with nitrite in acid solution. The nitroso compound is then reduced to form p-aminodimethylaniline.
(b) The p-aminodimethylaniline is oxidized in acid solution with another molecule of dimethylaniline, and simultaneously a thiosulfonic acid group is introduced. This step is accomplished by oxidizing in the presence of thiosulfuric acid in statu nascendi.
Thiosulfonic acid of p-amino- Thiosulfonic acid of dimethylaniline Bindschedler green
(c) The thiosulfonic acid is then oxidized further and undergoes ring closure to form methylene blue.
Methylene blue (a) p-Aminodimethylaniline
A cooled solution of 24.2 grams (0.2 mole) of pure dimethylaniline in 75 grams of 30 per cent hydrochloric acid is mixed with 150 grams of ice,
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and a 20 per cent solution containing 14.7 grams of sodium nitrite is added over a period of 1 hour. Nitrosation is complete in 4 hours. To the solution are added 110 grams of 30 per cent hydrochloric acid and 200 grams of ice, and then, over a 15-minute period with thorough mechani- cal stirring, 35 grams of high quality zinc dust. The temperature may rise to 25°C. without causing damage. The solution, which becomes colorless and neutral, is filtered and the residue of zinc dust is washed with a small amount of water.
(b) Thiosulfonic Acid of Bindschedler Green
This oxidation must be carried out in the presence of a zinc chloride solution which has no reducing action. A suitable solution can be pre- pared by dissolving sheet zinc in concentrated hydrochloric acid. (In the industry, the technical zinc chloride liquor is treated with enough sodium bichromate to destroy the reducing action. Frequently, this requires 100 to 250 grams of bichromate for 100 kilograms of the liquor.) The thiosulfuric acid is supplied by aluminum thiosulfate which is so highly dissociated that it reacts as free thiosulfuric acid.
Before the preparation of methylene blue is started, solutions of the necessary reagents are prepared so that the materials can be added rapidly and at the correct temperature.
Solution I. 38 grams of pure aluminum sulfate in 60 cc. water.
Solution II. 52.5 grams of crystalline sodium thiosulfate in 50 cc. water.
Solution III. 57 grams of sodium bichromate in 90 cc. solution.
Solution IV. 20 grams of dimethylaniline in 27 grams of concentrated hydro- chloric acid.
Solution V. 25 grams of very finely ground manganese dioxide slurried with 30 cc. water.
The clear, neutral solution of p-aminodimethylaniline is made acid by the addition of 4 grams of concentrated sulfuric acid, and 100 grams of a 50 per cent, nonreducing solution of zinc chloride is added.
The beaker is placed on a felt pad, and a tube is arranged for blow- ing in steam. With thorough stirring, Solution I is added at room tem- perature, followed by Solution II, and 2 seconds later by one-third of Solution III (equivalent to 19 grams of sodium bichromate). The tem- perature of the mixture is raised to 40°C. in the course of 1 minute by the introduction of dry steam, and then Solution IV is added followed by the remainder of Solution III. The mixture is then heated rapidly to 70°. The solution becomes dark greenish blue in color due to the forma- tion of the thiosulfonic acid of Bindschedler green. When the tempera- ture reaches 70°, the slurry (V) is added and the temperature is raised to 85°.
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The manganese dioxide is added to convert the sulfurous acid formed in the ring closure reaction to dithionate which is harmless Equally good results can be obtained by using 40 grams of copper sulfate which is converted in the reaction to insoluble CU2O.
The solution at 85° has a lustrous bronzy appearance as the dye precipitates from the concentrated zinc chloride solution. After 30 min- utes, the mixture is allowed to cool to 50°C, and 70 grams of concen- trated sulfuric acid is added to dissolve the manganese salts, aluminum hydroxide, and chromium oxide. The dye is filtered off at 20° and washed with a small volume of 10 per cent salt solution. This crude product is dissolved in 1 liter water at 100°, the solution is filtered, and the dye is reprecipitated by the addition of 50 grams of ordinary 50 per cent zinc chloride solution and 150 grams of salt. The zinc chloride double salt of the dye separates completely in 24 hours in the form of beautiful bronzy red crystals which are filtered off, washed with 10 per cent salt solution, and dried at 50° (no higher). The yield of pure, concentrated dye is about 44 grams.
The method described above was worked out by Bernthsen and Ulnch, who also lecommended the use of aluminum thiosulfate The practice of adding man- ganese dioxide or copper sulfate is generally followed. Relatively small quantities of dye are prepared in one run because rapid heating is important The final dye is filtered oft, using filters such as the one shown in Figure 27, page 151, and then placed m small bags and centrifuged
Methylene blue is highly valued because it gives pure shades and is inexpen- sive. It is widely used for coloring tanned cotton The zinc-free dye is used for dis- charging on silk. To produce the zinc-free dye, ordinary methylene blue is dissolved in water, and the zinc is precipitated by adding soda, leaving the easily soluble dye base in solution. The base is then precipitated from the filtered solution by add- ing salt. In large scale preparations, the crystallization requires several days, and is promoted by cooling, using lead pipes through which cold water is circulated.
Methylene green, the nitro derivative of methylene blue, is an inter- esting dye. The nitration is effected in the same way as that of tropaeo- line, using the crude zinc chloride double salt without further treat- ment.
The moist, crude methylene blue, as obtained in the above prepara- tion, is mixed with 50 cc. water and 20 grams of 62 per cent nitric acid
(40° Be); and 5 grams of sodium nitrite, dissolved in the minimum amount of water, is added at 25°C. The temperature is then raised to 50°
with continuous stirring, and held at this point for 2 hours. The mix- ture is then diluted with 200 grams of saturated salt solution and filtered after 12 hours. The crude dye is redissolved in 1 liter water at 60°
(not higher), the solution is filtered, and the dye is reprecipitated by the addition of 150 grams of salt and 50 grams of 50 per cent zinc chloride
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solution. The product is filtered off after 12 hours and dried at 45° until it can just be powdered. At this point, the dye still contains about 20 per cent of water, but it cannot be dried further without causing the color strength to decrease and part of the product to become insoluble.
The yield is about 38 grams of concentrated material.
Methylene blue and methylene green are diluted with dextrin since the use of salts greatly decreases their solubility. Methylene green is used chiefly for pro- ducing black shades on silk, in combination with logwood-iron mordants and also with tin phosphate. The shades produced in this way are the most brilliant and fast blacks for silks.
When diethylaniline is used in place of dimethylaniline, the pure greenish thiazine blue is formed. (Monoethyl-o-toluidine gives thionine blue.) Thiazine blue gives very pure blue shades on silk, but its importance is reduced by the availabil- ity of more fast alizarin dyes. The nonalkylated methylene blue, diaminophenazthio- nium chloride or Lauth violet, is used in moderate amounts for producing pure vio- let shades. It is still prepared by the old method involving simultaneous oxidation of p-phenylenediamine and hydrogen sulfide with iron chloride.