Indirubin. / Constituents of Natural Indigo. (Natural Indigo.)(CHAPTER XV. Indole Group.)

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

In addition to indigotin, natural indigo contains varying proportions of indirubin, indigo brown, indigo gluten, and mineral matter. Indigo yellow or kaempferol is also present as a rule when the I. arrecta has been employed for the manufacture.

Indirubin.

The identity of the natural indirubin or indigo red with the artificial product prepared according to Baeyer's method (loc. cit.), about which there was formerly some controversy, appears now to be fully established - (Marchlewski and Radcliffe, J. Soc. Chem. Ind., 1898, 17, 434). Bloxam at one time (Chem. Soc. Trans., 1905, 87, 979) considered that a red substance other than indirubin was present in some quantity in natural indigo, whereas Bergtheil (Report of the Indigo Research Station, Sirsiah, 1906) has stated that "decisively there is more than one red body in most commercial indigos". The investigation of numerous samples of the dyestuff by Perkin and Bloxam (Chem. Soc. Trans., 1907, 91, 279, and 1910, 97, 1461) indicate, however, that this is not the case. Whereas certain varieties of natural indigo, notably Java and Coventry process indigos, contain notable amounts of indirubin, it is probable that a trace occurs in all samples of the natural dyestuff. That the indirubin originates from the indican existing in the leaves of the various species of Indigofera, and is due to no second constituent of the plant, is now certain, and its production is to be explained in all cases as due to the condensation of isatin with indoxyl. Thus it has been shown by Thomas, Bloxam, and Perkin (loc. cit.) that indigo containing indirubin can be readily produced from indican by a repetition of the factory method, and again, isatin itself has been isolated from natural indigo rich in indirubin (Perkin, Chem. Soc. Proc., 1907, 23, 30). The formation of the isatin is favoured by special circumstances such as the oxidation of the indoxyl by air in the presence of alkali or acid, and may also be affected to some extent by temperature. That indoxyl can be converted into isatin without an intermediate formation of indigotin has been shown by the Badische Anilin und Soda-Fabrik (D.R.P., 107719, 1898), and it has been found by Perkin (Chem. Soc. Trans., 1909, 85, 847) that indoxylic acid, on long standing in the presence of moist air, is converted chiefly into indirubin, although some quantity of indigotin together with a substance, probably indigo brown, and traces of isatin are simultaneously produced. Again, it has been pointed out (Thomas, Bloxam, and Perkin) that the indican present in air-dried leaves of the indigo plant slowly disappears, and, according to Perkin (private communication), this is accompanied in most cases by a development in the leaf of considerable quantities of indirubin. It appears probable that this so-called "secondary" oxidation of the indoxyl proceeds according to the following scheme: [KUVA PUUTTUU] and may also be indirectly the cause of the production of indigo brown. The following are the results of analyses illustrating the percentages of indirubin and indigotin in certain indigos (Bloxam and Perkin):

Java Indigo. New Process with hot water.

Sample.	Total Colouring Matter.	Indigotin.	Indirubin.
1.	75,20	67,76	7,43
2.	73,60	63,86	9,51
6.	62,91	57,35	5,01

Java Indigo. New Process with cold water.

Sample.	Total Colouring Matter.	Indigotin.	Indirubin.
7.	72,88	69,23	3,06
8.	71,02	66,35	4,04
9.	58,30	55,61	2,15

Java Indigo. Old Process without Chemicals.

Sample.	Total Colouring Matter.	Indigotin.	Indirubin.
13.	74,96	72,89	1,74
15.	69,54	38,28	0,99

Coventry Process indigo.

Sample.	Total Colouring Matter.	Indigotin.	Indirubin.
	61,76	56,63	5,23

Finally, Bloxam and Perkin refer to an abnormal sample of laboratory indigo prepared from pure indican, which contained 88,9 per cent, of colouring matter, and of this 25,83 per cent, consisted of indirubin.

Though indirubin was at one time considered to be a valuable constituent of natural indigo (cf. Rawson and Knecht, J. Soc. Dyers, 1888, 4; Hummel, ibid.; and Bergtheil, Report Indigo Research Station, Sirsiah, 1907, 7), it is now known that such is not the case. Fasal (Mitt. K. Tech. Gew.-Mus. Wien, 1895, 11, 307) found that the shade of colour given by an indirubin vat became bluer from day to day, and that this was due to the formation of indoxyl by the further reduction of the leuco-indirubin. More recently Perkin (Chem. Soc. Proc., 1909, 25, 127) has shown that in addition to indoxyl, oxindole is simultaneously produced, and this is in harmony withrthe formula assigned by Baeyer (loc. cit.) to this substance [KUVA PUUTTUU]

In vat dyeing, therefore, indirubin may thus produce not more than one-half its weight of indigotin. Matthews (J. Soc. Chem. Ind., 1902, 21, 22), again, points out that indirubin requires for reduction a much stronger reagent than indigotin, and as a result, in practice, the greater part of this dyestuff is not attacked, but settles to the bottom of the vat.

On the other hand, indirubin disulphonic acid is, according to Fasal (loc. cit.) and also to Rawson and Knecht (loc. cit.) a useful dyestuff, and gives colours much faster to light than indigotin disulphonic acid, which is present as sodium salt in the "indigo extract" of commerce.