July 7, 1881] 



NATURE 



229 



pupils, succeeded in preparing isatin, and, therefore, now indigo, 

 from ortlio-nitro-benzoic acid. 



Tlie following are the steps in the ascent : 



1. Ortho-nitro-benzoic acid acted on by phosphorus penta- 



chloride yields the chloride CgHjiNOojCOCl. 



2. This latter heated with silver cyanide yields the uitril C^Hj 



(N0.2)C0.CN. 

 •3 On healinc this with caustic p tash it yields ortho-nitro- 



phenylglfoxylic acid, CjH^fNOaJCO.COsH. 

 4 This is con%'erted by nascent hydrogen into the amido-com- 



pound C6H4(NH.,)CO.CO„H. 

 1; And this loses water and yields isatin, CjHjNH.CO.CO. 

 ^ (Q. E. D.) 



The reasons why this process will not work on a large scale 

 are patent to all those who have had even bowing acquaintance 

 with such unpleasant and costly bodies as phosphorus penta- 

 chloride or cyanogen. 



Prccess No. 2. — Baeyer's (1878) synthesis from ortho-nitro- 

 phenylacetic acid. 



This acid can be obtained synthetically from toluol, and it is 

 first converted into the amido-acid, which, like several ortho 

 compounds, loses water, and is converted into a body called 

 oxindol, from which isatin, and therefore indigo, can be obtained. 

 The precise steps to be followed are : — 



1. Ortho-amido-phenylacetic acid yields o.xindol : 



CH.,CO.,H CH., 



CeH, = QH, CO + H„0. 



\ \ / 



NH„ NH 



2. This on treatment with nitrous acid yields nitrosoxindol : 



C(NO) 



/ \ 



CeH. CO. 



\ / 



NH 



3. This again with nascent[hydrogen gives amidoxindol : 



CH(NH„) 



/ \ 



CfiH, CO. 



\ / 



NH 



4. Which on oxidation gives isatin, 



CO 

 / \ 

 CjHj CO. (Q.E.D.) 



\ / 

 CH 



This process, the feasibility of which had also been foreseen by 

 Kekule, is however not available as a practical scheme for various 

 reasons. 



Process A^o. 3. — This maybe called the manufncturing proce s, 

 and was also proposed by Baeyer. It starts from cinnamic acid, 

 a substance contained in gum benzoin, balsam of Pent, and some 

 few other aromatic bodies. These sources are, however, far too 

 expensive to render this acid thus obtained available for manu- 

 facturing purposes. But Bertagnini, in 1856, had obtained 

 cinnamic acid artificially from oil of bitter almonds, and other 

 processes for the same purpose have since been carried out. Of 

 these, that most likely to be mdely adopted is the following 

 practical modification by Dr. Caro of Mr. Perkin's beautiful 

 synthesis of cinnamic acid : — 



1. CeHjCHj + 4CI = CeHjCHCL + 2HCI. 



Toluene. Benzj'lene dichloride. 



2. CsHsCHCU + 2CH3.CO.O.Na = 



Benzj'lene Sodium acetate, 



dichloride. 



CeHsCHrCH.CO.OH + 2NaCl. 

 Cinnamic acid. 



But why did Baeyer select this nine carbon acid from which 

 to pre]iare indigo ? For this he had several reasons. In the first 

 place, it had long been known that all indigo compounds when 

 heated with zinc dust yield indol, CgHyN, a body which stands 

 therefore to indigo in the same relation as anthracene to alizarin. 



and Baeyer and Emmerling had so long ago as 1869 prepared 

 this indol from ortho-nitro-cinnamic acid thus : 



CgHs(NO„)CO„H =CsH-N + O., + COo. 



Secondly, the ortho-nitro-cinnamic acid required (for we must 

 remember that indigo is an ortho-compound and also contains 

 nitrogen) can be readily prepared from cinnamic acid, and this itself 

 again can be obtained on a large scale. Thirdly, this acid readily 

 parts with one atom of carbon, and thus renders possible its 

 conversion into eight-carbon indigo. 



The next steps in the process are (3) the formation of ortho- 

 nitro-cinnamic acid, (4) the conversion of this into its dibromide, 

 (5) the separation from this of the two molecules of hydrobromic 

 acid, giving rise to ortho-nitro-phenyl-propiolic acid, and (6), and 

 lastly, the conversion of this latter into indigo by heating its 

 alkalme solution with grape sugar, xanthate of soda, or other 

 reducing agent. These reactions are thus represented : — 



3. CgHsCHZCHCOOH C6Hj(N0„)CHZCH.C00H. 



Cinnamic acid yields Ortho-nitro-cinnamic acid. 



In this process the para acid is also obtained, and as this is 

 useless for the manufacture of indigo, it has to be removed. This 

 is effected by converting the acids into their ethyl ethers, which, 

 possessing different degrees of solubility, can be readily separated 

 from one another : — 



4. This is next converted into the dibromide 



C6Hj(N0„)CHBr.CHBrC00H. 



5. And by careful treatment with caustic soda this yields ortho- 

 nitro-phenvl-propiolic acid, thus : — 



CJI,lNO.,)CHBrCHBrCOOH -f 2NaOH = 



' C6Hi(N0.)C.,.C00H + 2NaBr -f 2H2O. 



6. n[C6H4(NOo)C„.COOH -f H„ = CgH^NO -i- C0„ -h H„0]. 



Ortho-nitro-phenyl-propiolic acic. Indigotin. 



(Q.E.D.) 

 The last of these reactions is in reality not so simple as the 

 equation indicates. For only about 40 per cent, of indigo is 

 obtained, whereas according to theory 68 per cent, should result. 

 Indeed although, as we have seen, indigo can be prei)ared by 

 these three methods, chemists are as yet in doubt as to its mole- 

 cular weight, the probability being that the molecule of indigo 

 contains twice 16 atoms of carbon, or has the formula 4(C8l I5NO) 

 or CsjHjjNjOj. Still it must be remembered that according to 

 Sommaruga the vapour density of indigo is 9'45, a number 

 corresponding to Ihe simpler formula Ci6n,„N„0„. 



The artificial pr< duction of indigo may even now be said 

 to be within measurable distance of commercial success, for 

 the ortho-nitro-phenyl-propiolic acid, the colourless substance 

 which on treatment with a reducing agent yields indigo-blue, is 

 already in the hands of the Manchester calico printers, and is 

 furnished by the Baden Company for alkali and aniline colours 

 at the price of ds. per lb. for a paste containing 25 per cent, of 

 the dry acid. 



With regard to the nature of the competition between the arti- 

 ficial and the natural colouring matters it is necessary to say a 

 few words. In the first ]jlace, the present price at which the 

 manufacturers are able to sell their propiolic acid is 50^. per 

 kilo. But 100 parts of this can only yield, according to theory, 

 68'58 parts of indigo-blue, so that the price of the artificial 

 (being "iy. per kilo.) is more than twice that of the pure natural 

 colour. Hence competition with the natural dye-stuff is not 

 to be thought of until the makers can reduce the price of dry 

 propiolic acid to zos. per kilo., and also obtain a theoretical 

 yield from their acid. This may, or it may not, be some day ac 

 complished, but at present it will not pay to produce indigo from 

 nitro-phenyl-propiolic acid. Nevertheless a large field lies open 

 in the immediate future for turning Baeyer's discovery to practical 

 account. It is well known that a great loss of colouring matter 

 occurs in all the processes now in use for either dyeing or print- 

 ing with indigo. It has already been stated that a large per- 

 centage of indigo is lost in the "cold vats" in the sediment. 

 Another porti >n is washed off and wasted after the numerous 

 dippings, w hilst in order to produce a pattern much indigo must 

 be desuoyed I efore it has entered into the fibre of the cloth. 

 Moreover, the '> c ■ of the piece is uselessly loaded with colour. 

 In the proce-^ses if printing w ith indigo the losses are as great, 

 or even greater, md, in addition, such consideralilr difficulties 

 are met with, that only a few firms (Potter, Grafton in Man- 

 chester, and Schlie er in Elberfeld) have been succe sful in this 



