58 REPORT — 1876. 



The formation of antlirapurpurin in tlie manufacture of alizarin may to some 

 extent te said to have arisen from a want of knowledge of 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. 



CuH,0, C,,H3(HO),o/ 



^^-■^ ^ ^., ' 



Anthraquinbne. Alizarin. 



If we want to introduce hydroxyl into a compoimd, 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 chemists for a long 

 period. It consists in first replacing the hydrogen 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 two, or more atoms of hydrogen have been replaced by the bromine, so on 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 hydi-ate, a compound containing a corre- 

 sponding number of atoms of hydrogen replaced by hydroxyl is obtained. 



Graebe and Liebermann acted upon this principle in their experiments 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 dibromanthraquinone, 



C,,H,Br,0,. 



By decomposing this with potassic hydrate at a high temperature, they obtained a 

 violet-coloured product, which, when acidified to remove the alkali, gave a yellow 

 precipitate of alizarin, 



C,,He(HO),0,. 



The second process I wish to speak of for the replacement of hydrogen by hydroxvl 

 in a compound is by converting it into a sulpho-acid (usually by means of sulphunc 

 acid), and subsequently 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 hydrogeji 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 manufacture 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 produce 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 requhed 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, 



C,,H,(IIS03)0„ 



when heated strongly with caustic alkali, as potassic or sodie hydi-ate, decomposes 

 in the ordinary way, and we get "monoxyanthraquinone," 



0,JI,(HO)0„ 



which is a yellow body possessing no dyeing properties. On further treating this, 

 however, with caustic alkali it changes, being oxidized, and yields alizarin, 



Disulphoanthraquinonic acid, 



C,,H,(HS03)0„ 



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

 acid, 



C,.H,(H0)(HS03)0„ 



