The Preparation of lndanthrene Scarlet GG

A Vat Dye Experiment for the Introductory Organic Chemistry Laboratory

Wilson 6. Lutz Manchester College. N. Manchester. IN 46962

Experiments involving the preparation of dyes are often included in undergraduate organic laboratory courses. Stu- dents are usuallvim~ressed bv the visual effect obtained when reactants ohittie or no coior give rise to highly colored substances. The obvious utility of a dye provides added relevance. Typical of such experiments are preparations of Congo red, methyl orange, para red, and Martius yellow. Congored is not only adirect dye for cotton but is also useful as an indicator.

The preparation of Congo red has become less attractive in recent years with the recognition of the carcinogenic na- ture of henzidine. an intermediate reauired for the svnthesis of the dye. In searching for a replacement experiment, we have found that the vat dve Indanthrene Scarlet GG can easily be synthesized and applied to cloth within a single 2-h laboratory period. This dye was first prepared by the Ger- man chemists W. Eckert and H. Greune in 1924.' They found that when 1 mol of naphthalene-1,4,5,8-tetracarboxy- lir acid and 2 mol of o-phenylenediamine are heated togeth- er, a mixture of dyes 1 and 2 is obtained.

The mixture is known as Indanthrene Scarlet GG. I t is aho possible to separate the dyes to give the pure isomers 1 and 2, which are known as Indanthrene Brilliant Orange GR and Indanthrene Bordeaux RR, respectively.

Although the reaction of the acid and amine has been - carried out in a number of solvents including nitrobenzene, acetic acid. and dilute sulfuric acid. we have found that 85% phosphoric acid is particularly effective. The reaction pro- ceeds rapidly a t about 190 "C, the temperature of boiling ethylene glycol. The use of a boiling liquid bath illustrates one way to obtain a precisely controlled reaction tempera- ture. ~ d d i t i o n of waier to the cooled reaction mixtuie af- fords the dye asadeep red precipitate. Workup by filtration is too rlow to he practical. The product is easily isolated and

washed. however. with the aid of a centrifuee. In addition t o - the foregoing experimental techniques, the experiment illus- trates the formation of amide bonds and also how relativelv complex ring systems can often he assembled with surprising ease.

The ''vat" is prepared by boiling the crude dye with an alkaline solution of sodium dithionite. The solid dissolves to give adichroic red fluorescent solution that is green or red by transmitted light, depending on the concentration and/or thickness of the solution. Cotton acquires a greenish black color when boiled in the vat. Exposure of the vatted cloth to oxygen of the air or to dissolved oxygen in tap water results in a striking color change to orangered. The high quality of the dye is attested by the fact that samples of dyed cloth, placed in a sunny location for 3 years have shown no evi- dence of fading.

A heating bath is prepared by plaring 2.5 ml. of ethylene glycol and one or two boiling stones in an I R - X 150-mm horoarlicate g1a.s test tube. The reamon vessel ia a 12- X 1M.mm horosilicate alasv test tube having a sufficiently wide rim that it can rest on the i m of the outer tube. In the inner (reaction) tube are placed a finely ground mixture of 50 mg (0.16 mmol) of naphthalene-1,4,5,8-tetra- carhoxylie acid2 and 50 mg (0.46 mmol) of o-phenylenediamine followed by 0.2 mL (5 drops) of 85% phosphoric acid. The glycol bath is heated by means of a microburner, the flame of which is adjusted so that the ring of boiling vapor rises about halfway up the inner tube. Heating and stirring (glass rod) are continuedfor 15 min. Water is stirred into the cooled reaction mixture until the liquid Level is about 1 cm from the top of the tube. The mixture is centri- fuged for about 2 min, the clear yellow supernate discarded, and the process repeated.3The moist solid is transferred to a 125-mL Erlen- meyer flask with the aid of 15 mL of 1 M KOH. Sodium dithionite, 1.0 g, is added and the mixture heated to boiling. When the solid has completely dissolved, an 8- X 10-em piece of clean cotton cloth is placed in the solution. Boiling and stirring are continued for at least 5 min. The cloth is removed from the flask, rinsed thoroughly with tap water, and exposed to the air. Oxidation to the final deep or- angered color requires 5-10 min.

No toxicity problems have arisen during more than 3 years' exw- rience with this experiment. o-Phenylenediamine is, however, now listed as cancer susaeet aeent irritant. Prudence dictates that ~ ~ ~ ~~~ ~~ .~ rbin eonlort u i rh ih~substonees m e d i n this experim<mr beouozd- ed. The use of protective glover ir therefore recommended.

A copy of thisexperiment as used inour laboratory will hsent on request.

Presented at the 10lst Annual Meeting of the Indiana Academy of Science, Bloomington, IN, November 1985.

' Venkataraman, S. The Chemistry of Synfhetic Dyes; Academic: New York, 1952; Vol. 2. p 1191.

Available from Aldrich Chemlcal Company, Milwaukee, WI. If the solid dye is desired, washes with dil. HCI, dil. NH40H, and

water can be carried out followed by drying the centrifuged solid in a currem of dry air. Typically, 62 mg (94%) of the mixed dyes is obtained.

Volume 67 Number 1 January 1990 71