Formula.1

(a) Leuco-Malachite Green.

37.8 Gms. (3/10 mol.) dimethylaniline, 24 gms. (2/10 mol.) 30 % hydrochloric acid, and 10.6 gms. (1/10 mol.) benzaldehyde are placed in a 300 c.c. bolthead and the mixture heated up for 12 hours with a reflux-condenser. To prevent the oxidation of too much aldehyde, the end of the condenser is closed with a plug of cottonwool. It is necessary to stir vigorously during the whole reaction. At the end of this time the benzaldehyde will have disappeared almost completely. 12 Gms. anhydrous sodium carbonate are added, and the excess of dimethylaniline is driven off with steam, and may be readily recovered. The residual leuco base of Malachite Green is separated from the water after cooling, powdered, and again washed. The yield of dry product is about 24 gms.

37.8 gms. Dimethylaniline.

24 gms.

30%

Hc1.

10.6 gms. Benzaldehyde.

12 gms. Na2Co3.

The formula of Malachite Green is given here in accordance with the scheme put forward in Helvetica Chimica Acta, 1918, part 3.

(b) Oxidation of Leuco Base to Colour.

Reaction:

Oxidation is effected in dilute aqueous solution with the exactly calculated quantity of lead peroxide (see analytical section). On the laboratory scale it is possible to obtain a lead peroxide paste of exact composition, without resorting to analysis, by dissolving a weighed portion of lead nitrate in water and treating with calcium hypochlorite solution until all the lead is precipitated.1 The precipitated peroxide is then washed with plenty of water and used as a moist paste.

16.5 Gms. (1/20 mol.) pure leuco base are dissolved in 300 c.cs. water and 20 gms. strong hydrochloric acid, and the solution made up to 400 c.cs. at o° with ice. The liquid is well agitated and to it is added quickly a peroxide paste from exactly 1 /20th molecule lead nitrate (=15.5 gms.). After 2 hours a solution of 25 gms. Glauber salt is added, the lead being precipitated as the insoluble sulphate, which is removed by filtration. The colour base is now precipitated by means of 15 gms. anhydrous sodium carbonate, and is filtered off; usually it comes out in a resinous form. The yield of dry product is about 16 gms., or almost 100 % of theory.

16.5 gms. Leuco base. 300 c.cs. H2O. 20 gms.

30% Hc1. + Ice. 11.9 gms. PbO2.

25 gms. Na2So4, or 10 gms. 66° Be. H2So4. IS gms. Na2Co3.

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(c) Crystallization of Malachite Green.

It is not easy to effect the crystallization in the laboratory as large quantities are required for the production of fine crystals. 120 Gms. base (or, better, several times this amount) are dissolved in 72 gms. crystallized oxalic acid and 300 gms. distilled water, and the impurities filtered from the boiling liquid. A very concentrated solution of 7 gms. ammonium oxalate is then added to the hot liquid, and the mixture allowed to stand away from draughts. The best plan is to place the vessel containing the solution inside another larger vessel filled with hot water, so as to allow the former to cool down slowly. The temperature is now allowed to cool during the course of a day to 700, when the fine crystals are filtered off. A further quantity of less pure dye separates out from the mother-liquor on cooling, and forms the Malachite Green II. of commerce. The yield from one part of leuco base may be up to 1.45 parts oxalate or 145 % by weight of the initial material.

120 gms. "Base."

72 gms. cryst.

Oxalic acid. 300 c.cs. H2O.

1 Calcium hypochlorite is very soluble, like calcium chloride; the insoluble residue, after treating with water, consists of lime and chalk. All heating must be avoided.

Notes on Works Technique and Practice. - Malachite Green is still a very important product, and serves for dyeing tin-weighted silk, wool, and paper. Mixed with other dyes, it yields pure mixed shades which are very cheap, but possesses only moderate fastness. It is also used for printing on silk and cotton, but for these purposes its fastness is not adequate to modern demands, so that its use is diminishing.

The condensation is effected nowadays only with mineral acid, the old zinc chloride method having long been given up; Doebner's method, also, starting from berizotrichloride, is no longer utilized. The condensation is effected by means of hydrochloric or sulphuric acid. Hydrochloric acid effects the condensation more quickly, but requires the use of enamelled apparatus, whilst the sulphuric acid condensation may be carried out in homogeneously lead-lined vessels. It is important not to use too much acid or else the condensation goes to a certain extent in another direction, a benzydrol of the following formula being produced as a by-product:

7 gms. (Nh4)2.C2O4

which is, of course, incapable of giving the required dye by oxidation. Various fractions are obtained when working on the large scale, as different customers demand products of varying appearance. The oxalate of Malachite Green has the formula

2 X C23H24N2+3 X C2H2O4.

The crystallization may occupy several days when working with quantities of from one to six cubic metres, and crystals are frequently obtained of considerable beauty. The addition of ammonium oxalate to start the crystallization is reminiscent of similar relationships in alkaloid chemistry and is a purely empirical discovery. For further details see under Xylene Blue.

Xylene Blue VS (Sandoz).

Xylene Blue belongs to the so-called Patent Blue class, which includes sulphonated triphenylmethane dyes which are fast to alkalis. These products all have the common factor that the sul-phonic group is in the ortho position to the methane carbon atom. The general formula is therefore: