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.
The history of the determination of the constitution of indigotin and of the many syntheses which have been devised for its preparation, leading as they have done to the successful manufacture of the artificial product, constitutes without doubt one of the most interesting chapters in the annals of synthetical organic chemistry. This has been dealt with so fully in other manuals that a brief resume of the main features of the subject will only be given here, and to avoid detail the main reactions are only expressed by constitutional formulæ.
Whereas early work had proved the benzenoid character of indigotin, by the production from it of aniline, anthranilic acid, picric acid and nitrosalicylic acid and isatin, the commencement of a systematic attack on the problem of its structure first dates from the work of Baeyer and Knop (Annalen, 1865, 141, 1).
That isatin, C8H5NO2, was simply related to indigotin, at that time expressed as C8H5NO, appeared probable, and with the hope of reconverting isatin into the latter, its behaviour on reduction was studied by these chemists. The results obtained, though unsuccessful at first in their immediate object, proved to be of considerable importance, and indeed form the basis from which much of our present knowledge of the subject has been derived.
When reduced isatin gives dioxindol (i), oxindol (2), and these substances are now known to respectively consist of the inner anhydrides of α-amino-phenylglycollic (3) and ο-amino-phenylacetic acids (4) [KUVA PUUTTUU]
By further reduction indole is obtained, and to this, which was subsequently synthesised by Baeyer and Emmerling (Ber., 1869, 2, 680), by fusing 0-nitro-cinnamic acid with potash and iron filings the formula [KUVA PUUTTUU] was assigned (Ber., 1870, 3, 517).
The same chemists again by heating isatin with phosphorus oxychloride and acetyl chloride under pressure obtained indigotin.
In 1879 Baeyer and Sinda (Ber., 1878, 11, 584) converted oxindole into isatin according to the following scheme: [KUVA PUUTTUU] and such a series of reactions formed the coping-stone of the first artificial synthesis of indigotin.
Isatin is the inner anhydride of ο-amino-phenylglyoxylic acid (isatinic acid) and such a constitution was predicted for it by Kekule in 1869 (Ber., 2, 748). Isatin, which possesses acid properties and is capable of forming metallic compounds, may exist as pointed out by Baeyer in two modifications. These are known as pseudo-isatin (lactamisatin) and isatin (lactimisatin).
A synthesis of isatin from 0-nitro-benzoyl chloride was announced by Claisen and Shadwell in 1879 (Ber., 12, 350), and the reactions involved may be expressed by the following formulæ [KUVA PUUTTUU]
The fact that indole can be prepared from 0-nitro-cinnamic acid (loc. cit.) and that indole is closely related to indigotin, as indeed was shown by Nencki (Ber., 1875, 8, 727), who prepared indigotin by the action of osonised air upon an aqueous suspension of indole, led Baeyer to experiment on the synthesis of indigo from this same acid (Ber., 1880, 13, 254). This object he eventually accomplished by the two methods given below [KUVA PUUTTUU]
The former method is exceptionally interesting, in that it provided the basis for the first attempt to manufacture indigo on a commercial scale, and though this was hardly successful, the 0-nitrophenylpropiolic acid obtained by this method was of some service to the dyeing industry, as a means for obtaining indigo prints on calico.
Baeyer, again, in 1882 (Ber., 15, 50) announced a further synthesis employing ο-nitro-phenylpropiolic acid which was important in connection with the constitution of indigotin. When boiled with water ο-nitro-phenylpropiolic acid yields ο-nitro-phenylacetylene and from the copper compound of this by oxidation with ferricyanide, dinitro-diphenylacetylene is obtained. With fuming sulphuric acid this forms diisatogen, a compound which on reduction gives indigotin.
ο-Nitro-phenylpropiolic acid, on the other hand, by the action of sulphuric acid (Baeyer, Ber., 1881, 17, 1741) is transformed into its isomer isatogenic acid. Reducing agents convert this into ethyl indoxylate which by heating gives indoxyl and this latter when oxidised readily passes into indigotin.
Indoxyl reacts with aldehydes and ketones to form the so-called indogenides. Thus with benzaldehyde the indogenide of benzaldehyde (benzylidene pseudo-indoxyl) is produced (Baeyer, Ber., 16, 2188).
In a similar way indoxyl condenses with isatin to form indirubin a colouring matter present in natural indigo (loc. cit.), and which is to be regarded as the indogenide of isatin.
Baeyer in 1883 reviewing the facts here enumerated was enabled to deduce the following constitution of indigotin - which is now accepted as correct. The main arguments he employed in support of this formula are as follows:
1. Indigotin contains two imido groups.
2. As a result of its formation from diphenylacetylene the carbon atoms of indigotin must be arranged in a similar manner to those present in this substance -
C6H5.C.C.C.C.C6H5
3. Indigotin is only formed from compounds in which the carbon atoms adjacent to the benzene ring are united with oxygen.
4. The properties of indigotin point to the fact that it is closely related to indirubin.
As a result indigotin is to be regarded as the a-indogenide of pseudo-isatin, indirubin itself being the β-indogenide. Owing, however, to the lack of activity of the α-oxygen atom in isatin, indigotin cannot, like indirubin, be directly prepared from indoxyl and isatin.
In 1882 (Ber., 15, 2856) Baeyer and Drewsen synthesised indigotin by the action of acetone on ο-nitro-benzaldehyde in the presence of alkali.
When the acetone is replaced by acetaldehyde ο-nitro-phenyl-lactic aldehyde is obtained, whereas with pyroracemic acid ο-nitrocinnamyl- formic acid is produced.
These compounds under the influence of alkali are transformed into indigotin.
Heumann in 1890 (Ber., 23, 2043) devised the synthesis of indigotin from phenylglycocoll (phenylglycine). This on fusion with alkali is transformed into indoxyl which passes readily by oxidation into indigotin.
The yield by this method is extremely small, but this can be improved by employing in the place of phenylglycine, phenylglycine ο-carboxylic acid (Heumann, ibid., 3431).
This important reaction forms the basis of the first economical synthesis of indigo, the large scale manufacturing operations of which were perfected by the Badische Anilin und Soda Fabrik in 1897. For the preparation of phenylglycine ο-carboxylic acid, naphthalene is employed as the starting-point, and the procedure involved will be evident from the following formula: [kuva puuttuu]
An improved method for the production of phenylglycine ocarboxylic acid from anthranilic acid has subsequently been adopted, the reagents employed being formaldehyde, bisulphite, and potassium cyanide.
Phenylglycine can be prepared directly from aniline by the same method.
More recently it has been recognised that the unsatisfactory yield of indigo by the original process of Heumann is due to the presence of water in the alkali fusion. By the replacement of the sodium hydroxide with sodium amide the destructive action of the water is avoided and the fusion can be successfully carried out at a lower temperature. The manufacture of indigo by such a method has been more recently adopted by the firm of Meister Lucius & Brüning at Höchst.
Interesting is also the fact that by treatment with fuming sulphuric acid phenylglycine is converted into indigotin disulphonic acid.
Of other indigo syntheses that of Sandmeyer, at one time employed on the manufacturing scale, is of importance. The startingpoint in this method is thio-carbanilide obtained by the action of carbon disulphide on aniline. This compound by the action of potassium cyanide and lead carbonate forms hydrocyano-carbodiphenylimide, which on treatment with ammonium sulphide gives the thio-amide. The latter by heating with sulphuric acid is converted into isatin anilide and from this by reduction with sulphuretted hydrogen in acid solution thio-isatin is obtained. By the action of dilute alkalis thio-isatin readily passes into indigotin.
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