228 



NATURE 



\7zdy 7, 1 88 1 



Indigo, as is well known, is a colouring matter which has 

 attracted attention from very early times. Cloth dyed with 

 indigo has been found in the old Egyptian tombs. The method 

 of preparing and using this colour is accurately described by 

 both Pliny and Dioscorides, and the early inhabitants of these 

 islands were well acquainted with indigo, which they obtained 

 from the European indigo plant, Isatis tincloria, the woad plant, 

 or pastel. With this they dyed their garments and painted their 

 skins. After the discovery of the passage to India by the Cape 

 of Good Hope, the eastern indigo, derived from various species 

 of Indigo/era, gradually displaced woad as containing more of 

 the colouring matter. But this was not accomplished without great 

 opposition from the European growers of woad ; and severe enact- 

 ments were promulgated against the introduction of the foreign 

 colouring matter, an edict condemning to death persons "who 

 used that pernicious drug called devil's food," being issued by 

 Henry the Fourth of France. The chief source of Indian indigo 

 is the Indigofcra tincloria, an herbaceous plant raised from seed 

 which is sown in either spring or autumn. The plant grows with 

 a single stalk to a height of about three feet six inches, and about 

 the thickness of a finger. It is usually cut for the first time in 

 June or July, and a second or even a third cutting obtained later 

 in the year. The value of the crop depends on the number of 

 leaves which the plant puts forth, as it is in the leaves that the 

 colouring principle is chiefiy contained. Both the prepar.ition 

 of the colouring matter from the plant, and its employment as a 

 dyeing agent, are carried on at the present day exactly as they 

 have been for ages past. The description of the procei-ses given 

 by Dioscorides and Pliny tally e.xactly with the crude mode of 

 manufacture carried on in Bengal at the present day. 



Dioscorides says : — " Indigo used in dyeing is a purple- 

 coloured froth formed at the top of the boiler ; this is collected 

 and dried by the manuhcturer ; that possessing a blue tint and 

 being brittle is esteemed the most." 



The identity of the blue colouring matter of woad and that of 

 the Bengal plant was proved by Hellot, and by Planer and 

 Trommsdorff at the end of the last century. These latter 

 chemists showed that the blue colour of the woad can be sub- 

 limed, and thus obtained in the pure state, a fact which was first 

 mentioned in the case of indigo by O'Brien in 1789, in his 

 treatise on calico printing. Indigo thus purified is termed indi- 

 gotin. It has been analysed by various chemists, who ascertained 

 that its composition may be most simply expressed by the 

 formula CjHsNO. 



Concerning the origin of indigo in the leaves of the Indigo/era, 

 various and contradictory views have been held. Some have 

 supposed that blue indigo exists ready formed in the plant ; 

 others, that white indigo is present, which on exposure to air is 

 converted into indigo-blue. Schunck has, however, proved 

 beyond doubt that the woad plant (Isatis tincloria), the Indi- 

 gofcra tinctoria of ludia, and the Chinese and Japanese indigo 

 plant (Polyoonum iincloriuvi) contain neither indigo-blue nor 

 white indigo ready formed. By careful treatment the leaves of 

 all these indigo-yielding plants can be shown to contain a colour- 

 less principle termed indican, and that this easily decomposes, 

 yielding a sugar-like body and indigo-blue. That white indigo 

 is not present in the leaves is proved by the fact that this com- 

 pound requires an alkali to be present in order to bring it into 

 solution, whereas the sap of plants is always acid. The decom- 

 position is represented by Schunck as follows :— 



C„«H3iN0j, 



2H„0 = CsHjNO -f sC^HioOc. 

 Indigotiii. Indiglucin. 



So readily does this change from indican to indigo take place, 

 that bruising the leaf or exposing it to great cold is sufficient to 

 produce a blue stain. Even after mere immersion in cold alcohol 

 or ether, when the chlorophyll has been removed the leaves 

 appear blue, and this has been taken to show the preexistenceof 

 indigo in the plant. But these appearances are deceptive, for 

 Schunck has proved that if boiling alcohol or ether be used, the 

 whole of the colour-producing body as well as the chlorophyll is 

 removed, the leaves retaining only a faint yellow tinge, whilst 

 the alcoholic extract contains no indigo blue, but on adding 

 an acid to this liquid the indican is decomposed and indigo-blue 

 is formed. 



What now was the first step gained in our knowledge concern- 

 ing the constitution of indigo, of which the simplest formula is 



CgllsNO? 



Step No i. — This was made so long ago as 1840, when 

 Fritsche proved that aniline, CeH^NHj, can be obtained from 



indigo. The name for this now well known substance is indeed 

 derived from the Portuguese "anil," a w rd used to designate the 

 blue colour from indigo. This result of Fritsche's is of great im- 

 portance, as showing that indigo is built up from the well-known 

 benzene ring CjHg, the skeleton of all the .aromatic compounds, 

 and moreover that it contains an amido group. 



Step No. 2 was also made by Fritsclie in the following year, 

 when, by boilir.g indigo with soda and maganese dioxide, he 

 obtained ortho-amido-benzoic acid, or, as he then termed it, 

 anthranilic acid. The following is the reaction which here 

 occurs : — 



C.H^NO 4- O -f- 2H„0 = C.H5NH.Pi,!+ CHjOj.^ 

 indigo. Ortho-.imido-benzoic acid. 



What light does this fact shed upon the constitution of 

 indigo? It shows (i) that one of the eight atoms of carbon in 

 indigo can be readily separated from the rest ; (2) that the 

 carboxyl and the amido-group are in' neighbouring positions 

 in the benzene ring, viz. i and 2. For we have three isomeric 

 acids of the above composition. 



CO.,H 



Para-position, 

 CO„H 



NHj 



Step No. 3. — The next advance of importance in this some- 

 what complicated matter is the discovery by Erdmann and 

 Laurent independently, that indigo on oxidation yields a 

 crystalline body, which, however, possesses no colouring power, 

 to which they gave the name of isatin. 



CgHsNO -I- O = CgHsNOo. 



Indigo. Isatin. 



Step No. 4. — The reverse of this action, viz. the reduction of 

 isatin to indigo, was accomplished by Baeyer and Emmerling in 

 1S70 and 1S78, by acting. with phosphorus pentachloride on 

 isatin, and by the reducing action of ammonium sulphide on the 

 chloride thus formed. 



Understanding now something of the structure and of the 

 relationships of the body which we wish to build up, let us see 

 how this edifice has, in fact, been reared. Three processes have 

 been successfully employed for carrying out this object. But of 

 these three only one is of practical importance. 



For the sake of completeness, let us, however, consider all 

 three processes, although Nos. I and 2 are at present beyond the 

 pale of practical schemes. 



These three processes have certain points in common. (l) 

 They all proceed from some compound containing the benzene 

 nucleus. (2) They all start from compounds containing a 

 nitrogen atom. {3) They all commence with an ortho-compound. 



They differ from one another ; inasmuch as process No. i starts 

 from a compound containing seven atoms of carbon (instead of 

 eight), and to this, therefore one more atom must be added; 

 process No. 2, on the other hand, starts from a body which 

 contains exactly the right number (eight) of carbon atoms ; whilst 

 No. 3 commences with a compound in which nine atoms of 

 carbon are contained, and from which, therefore, one atom has 

 to be abstracted before indigo can be reached. 



Process No. i (Kekule — Claissen andShadwell). — So long ago 

 as 1869 Kekule predicted the constitution of isatin, and gave to 

 it the formula which we now know that it possesses, viz. 

 CO 



/ \ 

 CcHj CO. 



\ / 



NH 



Following up this view, Claissen and Shadwell, two of Kekule's 

 ^ Bottinger, Dent. Chem. Ges. 1S77, i. 269. 



