Scientific American 4, 24.7.1869
By Dr. Reimann.
(For the Scientific American.)
A method of artificially producing the coloring matter of the madder plant, known under the name of "alizarin," has been recently discovered by Messrs. Graebe and Liebermann, of Berlin.
The extraction of alizarin from the madder root was effected some time ago by Mr. Kopp, whose process was as follows: He heated the powdered madder with a watery solution of sulphurous acid (SO2), which extracted two coloring matters, the alizarin and purpurin, with other substances.
When this solution was heated to 122° Fah. a precipitate, consisting of almost pure alizarin, was deposited. On filtering the solution and heating it again to 212° Fah. the sulphurous acid was all expellet, and the solution consequeutly lost its ability to retain the second coloring matter or purpurin, which was therefore precipitated.
The two coloring matters just mentioned are prepared on a large scale in the manufactory of Mssrs. Schaaf & Lauth, Wasselonne, France, and supplied to cotton manufacturers for dyeing and printing.
The coloring matters obtained from coal tar have the property of removing all other coloring matters except indigo, cochineal, and madder. Strange to say, the chief coloring matter of the last is now produced from coal tar itself.
On distilling coal tar, volatile and fixed oils are obtained which boil at from 86° to 572° Fah. Among the latter is found a hydrocarbon, called "anthracene," tho formula of which is C28H10. From this substance was obtained alizarin, previously only found in the root of the madder.
The method of producing alizarin was first made known by the publication of the French patent, the particulars of which will be here narrated, with additional observations on the manner in which the formation of the alizarin can be effected.
In the introductory remarks respecting the patent, it is stated that alizarin is the chief coloring matter of the madder that dyers, and especially calico printers employ to dye their goods rose, pink, violet, brown, and black, according to the kind of mordant made use of in the operation of printing. It is also asserted that the prepared madder, called goraneen (fleurs de garance), is consumed to the extent of thousands of tuns yearly. In modern times also the pure coloring matters were extracted from the madder and used in dyeing and printing operations.
The process discovered by Messrs. Graebe and Liebermann, of Berlin, consists in the production of Alizarin without the employment of madder, and in an entirely new way. The process presants three different stages.
In the first place the hydrocarbon called anthracene, already alluded to, is employed as the raw material.
For some time past the antrarcne, or parapraphthalin, has been obtained from the destructive distillation of coal tar. This anthraecne must be transformed in the first stage of the process into a substance containing more oxygen than itself; namely, oxanthrecene, or anthrachinon. This substance has been alrealy obtained by other chemists by heating anthracene with nitric acid, and then purifying the product by distillation, when it presents itself in the form of yellow needles. Messrs. Graebe and Liebermann, however, effected the transformation of anthracene into anthrachinon in the three following ways:
In the first process the anthracene is treated with a solution of bichromate of potash, and then treated with sulphuric acid, until all the chromic acid is reduced to peroxide of chromium.
As regards the relative quantities of the two substances made use of, one part of anthracene is treated with two parts of bichromate, and the necessary amount of sulphuric acid afterward added.
Any other chromate may be employed instead of the bichromate of potash.
The oxanthracene, or anthrachinon, thus obtained appears as a solid, insoluble brown mass.
In the second process, two parts of bichromate of potash are heated with one part of anthracene, and about fifty parts glacial acetic acid, until all the chromic acid is reduced. When cold, the anthrachinon is found in the same form as in the preceding process. Some of the anthrachinon which remains dissolved in the acetic acid, may be obtained by distilling the latter.
In the third operation a mixture of anthracene and glacial acetic acid is heated to 176° Fah., and heated with nitric acid, which must be added a drop at a time.
The relative quantities of the two substances employed are one part of anthracene to one part of nitric acid. In this process the anthracene is converted into a substance richer in oxygen than itself, the formula for anthracene being C28H8O2.
ln the second stage of the operation the oxanthracene is heated with bromine, when two equivalents of hydrogen are replaced by two equivalents of the latter, and a substance is formed which has tho composition C28H6O2 · B22.
To obtain this substance the anthracene is placed in a sealed tuba, with two equivalents of bromine, and heated ten hours at a temperature of 176° to 266° Fah. The hydrobromic acid formed during the operation can be removed by leading the gases into a solution of some alkali. A crystalline mass is found in the tube, which has to be purified by recryatallizetion. This substance has the composition C28H6O2 · B22.
It may be obtained likewise in the following way: Eight equivalents of bromine are allowed to act upon one equivalent of anthracene (C28H10), when a substance in formed having the composition C28H8B28, and which appears iu the tube as a crystalline mass.
On treating this substance with an alcoholic solution of potash it is transformed into an anthracene in which four equivalents of hydrogen are simply replaced by four equivalents of bromine. The formula therefore of this new substance is as follows: C28H6B24
When this last is treated with any oxidizing agents a new substance is formed, two equivalents of bromine replacing two equivalents of oxygen, so that the new substance has the composition C28H6O2· B24. This is the so-called bibromide of anthrachinon.
By employing chlorine instead of bromine an analogous substance is formed, in which two equivalents of chlorine have replaced two equivalents of hydrogen. This is the bichloride of anthrachinon, as its discoverers call it. Its formula is C28H6O2· Cl2. This substance is treated with a concentrated solution of an alkali, and heated to a temperature of from 356° to 500° Fah., when the liquid becomes blue. When the formation of this blue substance is completed the liquid is allowed to become cold, and the substance is then extracted by water. This is a salt, in which an organic acid is united to potash.
On heating the bichloranthrachinon (C28H6O2· Cl2) with an alkali, the chlorine is replaced by hydrogen, and six equivalents of water are at the same time added to the composition of the new substance (C28H6O2· Cl2)+ 2 KO HO 2 2O = KCl + (C28H8O8)
The substance C28H8O8 is still in combination with the alkali, but can be separated from the latter by adding an acid to the alkaline solution, when a yellow precipitate results.
This yellow precipitate is found to be pure alizarin, and can bo employed in dyeing and calico printing.
Experiments were made in the manufactory of Messrs. Liebermann, of Berlin, to test the efficacy of this artificial alizarin, and the results were extremely satisfactory. It is to be regretted, however, that this interesting discovery of Messrs. Graebe and Liebermann has hitherto been turned to little practical account.
In the first place the anthracene, or raw material from which the alizarin is extracted, is very difficult to obtain. The coal oils contain only very minute quantities of anthracene. But then the same method which was employed to produce aniline may be chosen. But even the aniline was found in such small quantities in the tar that the employment of aniline would be far less extensive than it is in the present day if this were its only source.
The labors of Zinin, Béchamp, Hitscherlich, and others, taught us to prepare aniline from benzole, a substance abounding in the elements of tar.
The same synthetical method must be employed to obtain anthracene. Messrs. Limpericht and Berthelot showed that anthracene can be prepared synthetically.
To prepare aniline toluen, a hydrocarbon consisting of C14H8 is heated with chlorine and then decomposed at 392° Fah. by the vapor of water, or the token is conducted through a tube heated to redness [2 C14H8] (toluen)= C28H10+6H (anthracen.)
The possibility of forming anthracene synthetically is shown in our laboratories. Why, then, should it not be used to produce alizarin on a large scale?
It is, of course, necessary that the new preparation or artificial alizarin should be cheaper than the alizarin extracted from the root of the madder. One considerable obstacle to this is, that alizarin obtained from madder is already prepared on a large scale in France.
But, after all, the new discovery must be considered as an important step in the art of manufacturing colors, and it is sincerely to be, hoped that the new method of preparing artificial alizarin will soon be cheap enough to allow of its general employment in dyeing and printing.