Sept. 14. 1876] 



NATURE 



433 



formation of a vegetable product artificially, but the process 

 used by Graebe and Liebermann was of little practical value 

 on account of the difficulty and expense of working it. 



Having previously worked on anthracene derivatives, it oc- 

 curred to me to make some experiments on this subject, which 

 resulted in the discovery of a process by which the colouring- 

 matter could be economically produced on the large scale : 

 Messrs. Caro, Graebe, and Liebermann at about the same time 

 obtained similar results in Germany ; this was in 1869. Further 

 investigation during the same year yielded me a new process, by 

 which " dichloranthracene " could be used in place of the more 

 costly product anthraquinone, which was required by the origi- 

 nal processes. I mention this as most of the artificial alizarin 

 used in this country up to the end of 1873, and a good deal 

 since, has been prepared by this new process. 



It was observed that when commercial artificial alizarin pre- 

 pared from anthraquinone, but more especially from dichloran- 

 thracene, v/as used for dyeing, the colours produced differed 

 from those dyed with madder or pure alizarin, and many per- 

 sons therefore concluded that the artificial colouring-matter was 

 not alizarin at all. This question, however, was set at rest by 

 separating out the pure artificial alizarin from the commercial 

 product and comparing it with the natural alizarin, when it was 

 found to produce exactly the same colours on mordanted fabrics, 

 to have the same composition, to give the same reactions with 

 reagents, and to yield the same products on oxidation. 



But whilst examining into this subject it was found that a 

 second colouring-matter was present in the commercial product, 

 and in somewhat large quantities, especially when dichloranthra- 

 cene had been employed in its preparation, and to this was due 

 the difference in shade of colour referred to. 



This substance, when investigated, was found to have the 

 same composition as " purpurin," also a colouring-matter found 

 in madder, but of very little value on account of the looseness 

 and dulness of some of the colours it produces. This new sub- 

 stance, being derived from anthracene, was named anthrapur- 

 purin ; unlike its isomer purpurin, however, it is of great value 

 as a colouring-matier. I do not think I shall be going beyond 

 the results of experience if I say it is of as great importance as 

 alizarin itself ; with alumina mordants it produces reds of a 

 more scarlet or fiery hue than those from alizarin. In fact, so 

 fine are the colours produced that, with ordinary alumina- 

 mordants on unoiled cotton, it gives results nearly equal to 

 Turkey-red produced with madder or garancine, and I believe 

 the rapid success of artificial alizarin was greatly due to its pre- 

 sence. Most of that consumed at first was for Turkey-red 

 dyeing, and the colours were so clear and brilliant that it was 

 mostly used in combination with madder or garancin, to brighten 

 the colours produced by these natural products. 



The purple colours anthrapurpurin produces with iron mor- 

 dants are bluer in shade than those of alizarin, and the blacks 

 are very intense. Its application is practically the same as 

 alizarin, so that they can be used in combination. 



As noted just now, the commercial product called "artificial 

 alizarin " first supplied to the consumer was always a mixture of 

 alizarin and anthrapurpurin, and various mixtures of these two 

 colouring-matters are still sent into the market ; but owing to 

 the investigations that have been made, and the study and atten- 

 tion that have been given to it by manufacturers, nearly pure 

 alizarin and anthrapurpurin are also sent into the market ; the 

 first being known as " blue shade alizarin," and the second as 

 red or "scarlet alizarin." 



The formation of anthrapurpurin in the manufacture of alizarin 

 may to some extent be said to have arisen from a want of know- 

 ledge oi the true conditions required for the production of 

 the latter. 



It is now well known that alizarin is a dioxyanthraquinone, or, 

 in other words, anthraquinone, in which two atoms of hydrogen 

 are replaced by hydroxyl. 



C14H8O2 Ci4H6(HO),03 



Anthraquinone. Alizarin. 



If we want to introduce hydroxyl into a compound, there are 

 several processes which can be used, but I will only refer to 

 those connected with the history of this colouring matter. 



The first process which I will refer to has been used by che- 

 mists for a long period. It consists in first replacing the hydro- 

 gen by bromine, and then treating the resulting body with 

 potassic or other metallic hydrate ; and according as one, two, 

 or more atoms of hydrogen have been replaced by the bromine, 



so on its removal by the metal of the metallic hydrate, a com- 

 pound containing a corresponding number of atoms of hydrogen 

 replaced by hydroxl is obtained. 



Graebe and Liebermann acted upon this principle in their ex- 

 periments on the artificial formation of alizarin ; and as it was 

 necessary to replace two atoms of hydrogen in anthraquinone, 

 they first of all prepared a dibrominated derivative, called dibro- 

 manth raquinone, 



CuHgBrgOs, 

 By decomposing this with potassic hydrate at a high tempera- 

 ture, they obtained a violet-coloured product, which, when 

 acidified to remove the alkali, gave a yellow precipitate of 

 alizarin, 



Ci4H6(HO)202. 



The second process I wish to speak of for the replacement 

 of hydrogen by hydroxyl in a compoimd is by converting it into 

 a sulpho-acid (usually by means of sulphuric acid), and subse- 

 quently decomposing this with potassic or other hydrate ; and 

 according as a mono- or disulpho-acid is employed, it yields on 

 decomposition a compound with one or two atoms of hydrogen 

 replaced by hydroxyl. 



The discovery of sulpho- acids of anthraquinone, and their use in 

 place of the brominated derivative originally employed by Graebe 

 and Liebermann, constituted the great improvement in the manu- 

 facture of alizarin already referred to. 



From what has just been stated, it was naturally supposed 

 that a disulpho-acid of anthraquinone would be required to pro- 

 duce alizarin ; and this was believed to be the case for some 

 time ; but further experiments have proved it to be a mistake, 

 and shown that the monosulpho-acid is required to produce 

 alizarin, the disulpho-acid yielding anthrapurpurin. 



But how are we to explain this apparent anomaly ? It would 

 take up too much time to enter into a discussion respecting the 

 constitution of the sulpho-acids of anthraquinone in reference to 

 the position of the HSO3 groups. I will therefore confine my 

 remarks to their decomposition. 



Monosulphoanthraquinonic acid, 



Ci4H,(HS03)02, 



when heated strongly with caustic alkali, as potassic or sodic 

 hydrate, decomposes in the ordinary way, and we get "monoxy- 

 anthraquinone, " 



Ci4H,(HO)02, 

 which is a yellow body possessing no dyeing properties. On 

 further treating this, however, with caustic alkali it changes, 

 being oxidised, and yields alizarin, 



Ci4H6(HO)202. 



Disulphoanthraquinonic acid, 



Ci4H«(HS03)02, 

 when subjected to the influence of caustic alkali, at first changes 

 into an intermediate acid, 



CnH6{HO)(HS03)02, 

 and then into a dioxyanthraquinone, 



Ci4H6(HO)202, 



now known as " isoanthraflavic acid " — a substance having the 

 same composition as alizarin, but being only an isomer of that 

 body, and possessing no affinity for mordants ; like monoxy- 

 anthraquinone, however, when further heated with alkali, it 

 becomes oxidised and yields a colouring-matter, which is 

 " anthrapurpurin," 



Ci4H5(HO)30,. 



Looking at these reactions, it appears rather remarkable that 

 Graebe and Liebermann should have succeeded in preparing 

 alizarin from dibromanthraquinone. It can only be explained 

 on the assumption that the hydrogen atoms replaced in the 

 disulpho-acid are different in position to those replaced in the 

 dibromanthraquinone ; and of course it is possible that a disul- 

 pho-acid isomeric with that now known may be discovered that 

 will yield alizarin as a first product on treatment with alkali. 



In the reaction which takes place when monoxyanthraquinone 

 or isoanthraflavic acid become oxidised and change into alizarin 

 and anthrapurpurin, nascent hydrogen is formed ; and this causes 

 a reverse action to take place, ordinary anthraquinone or its 

 hydrogen derivative, being formed, and a loss of colouring- 

 matter resulting. A small amount of potassic chlorate is now 

 used with the caustic alkali, just sufficient to overcome the 



