547.758

ON INDIGO. I. THE NITRATION OF INDIGO BY

J. VAN ALPHEN.

When indigo was nitrated with absolute nitric acid, either directly or dissolved in sulphuric acid or acetic acid only decomposition products, containing nitrated isatins, could be obtained. but when indigo was nitrated in acetic anhydride, well defined products could be prepared. These were derivatives of 2-acetoxy-3 : 3’diketc+diindolinyl-2 : 2’ into which one, two or three nitro groups were introduced, depending on the amount of nitric acid used. On heating. these compounds split off a molecule of acetic acid and mono-, di-, or tri-nitroindigos remained behind. An analogous result was obtained, when indigo was nitrated in butyric anhydride. Indigo could also be nitrated in acetyl chloride but 5-chloroisatin was then obtained.

Introduction 1 ) .

I t has already been known for a long time, that v:hen indigo is nitrated it is decomposed 2 ) and according to the circumstances, isatin, nitrosalicylic acid or picric acid is obtained. Only one patent 3 ) is known to the author in which it is stated, that it is possible to nitrate indigo without breaking the bond between the two indole nuclei. This patent of the Bad. Anilin und Soda Fabr. states, that when indigo is nitrated with the exclusion of water, nitrated indigos are formed and. as an example. nitration in strong sulphuric acid is given. Nitrated indigos are formed. but no particulars about them are mentioned, so that is would appear that no well defined products could be isolated. When dehydroindigo acetate ( 2 : 2’-diacetoxy-3 : 3’-diketo-diindolinyl- 2 : 2 ’ ) :

l ) A review of the various formulae proposed for indigo is given in the Chem. Weekblad 35, 435 (1938). It is also proposed in chis article to consider this ccnn- pound as a resonance hybrid.

2 ) B e i 1 s t e i n s Handbuch der org. Chemie 4. Aufl., XXIV, 421. 3, B.A.S.F., D. R. P. 242149, Friedlaender, Fortschr. d. Teerf. 10, 397 (1910-

1912). LVlI 28

838 J . van Alphen.

is suspended in acetic anhydride and stirred for three hours with nitric acid, dinitro-dehydroindigo acetate is formed as a yellow powder, which dissolves in hot trichlorobenzene with a brown, cherry- red colour and at the same time looses acetic acid. From the intense violet blue solution in strong sulphuric acid water precipitates dinitro- indigo. As this dinitroindigo shows the same properties as 5 : 5’- dinitroindigod) the formula of the yellow compound is given by the editors of B e i 1 s t e i n as 5 : 5’-dinitro-2 : 2’-diacetoxy-3 : 3’-diketo- diindolinyl-2 : 2’:

0 CHx CH7 0

It is also stated, that a yellow solution is formed, when indigo is treated with acetic anhydride and nitric acid. After some hours there separates from this solution a compound with the same properties as that obtained from dehydroindigo acetate.

The nitration of indigo by means of absolute nitric acid under various conditions has now been investigated by the author. It has shown that when indigo is nitrated with absolute nitric acid alone or with mixtures of nitric acid and acetic acid, resinous products are obtained, which become sticky, when dried at room temperatures; they are soluble in alcohol and give a blue indophenine reaction when treated with strong sulphuric acid and benzene with a trace of thiophen. T h u s they contain derivatives of isatin but not of indigo. The same result was obtained when indigo was nitrated with a mixture of concentrated sulphuric acid and nitric acid. When indigo was dissolved in 30 % oleum and then nitrated, sometimes very small quantities of more solid brown products could be isolated but

4) A. B a e y e r . Ber. 12, 1316 (1879). 5) B e i 1 s t e i n s Handbuch der org. Chemie. 4. Aufl., Erg. Band XXIII/XXV,

520.

On indigo. 1. 839

the experiments could not always be reproduced, the yield was always very small and the greyish brown products did not seem to be indigo derivatives, hence the experiments were not continued.

In acetic anhydride, however, indigo could be nitrated easily and in good yield and derivatives of 2-acetoxy-3 : 3’-diketo-diindolinyI- 2 : 2‘ were formed:

0 CH? 0

H H

with one, two or three nitro groups, depending on the amount of nitric acid used in the nitration. O n heating, these compounds lose a molecule of acetic acid and mono-, di-. or tri-nitroindigo remains behind. The same result is the case, when the compounds are recrystallised from nitrobenzene:

H H H H + CH3COOH Thus, contrary to what is stated in the B.A.S.F. patent not a diacetoxy- but a monoacetoxy-derivative was obtained.

The diacetoxy-derivative could be prepared by the nitration of dehydroindigo acetate in acetic anhydride and under the same circum- stances that indigo furnished a dinitro-derivative. here a trinitro- derivative was obtained, which on heating partly decomposed into trinitroindigo and acetic acid:

but as two hydrogen atoms are lacking for this reaction, a large part of the product formed a resinous mass.

Experiments with a view to introducing four nitro groups into the indigo molecule failed, for when the trinitro-derivative of mono- or di-acetoxy-3 : 3’-diketo-diindolinyl-2 : 2’ was dissolved in absolute

I. van Afphen. -

840

nitric acid it could be precipitated unchanged from the solution and when the solution was kept for some time or gently heated, the indigo derivative was oxidised.

As the nitration in acetic anhydride gave interesting results, an attempt was made to nitrate indigo in acetyl chloride, but under these circumstances indigo was quantitatively oxidised and chlorinated to give 5-chloroisatin.

0

A. 1.

Nitration of indigo in acetic anhydride. 5'-Nitro-2-acetoxy-3 : 3'-diketo-diindoliny1-2 : 2'.

H H

Absolute nitric acid ( 5 cm3) was added drop by drop to 10 cm3 of acetic anhydride cooled to -10' ( i f the temperature rises too high. a violent reaction occurs) and this mixture was added very slowly to a suspension of 5 g of indigo in 50 cm3 of acetic anhydride also kept at -loo. At a particular moment a large part of the indigo had disappeared and a brown solution had formed: this was filtered as quickly as possible through a glass filter and decomposed with a large amount of fine ice. At first, the mixture was frequently shaken and then it was kept overnight. A yellow-brown solid had formed. This was repeatedly extracted with boiling methyl alcohol and with absolute ether and then dried at 120'.

Analysis: 0.1512 g subst.: 14.9 cm' N (16", 758 mm). 0.1500 g subst. (from a second preparation): 15.0 cma N (16". 767.8 mm). C,,H,,O,N,. Calc. N 11.44; found N 11.40, 11.73.

The mononitro-compound could be dissolved in boiling acetic anhydride, filtered from some dinitro-compound and precipated unchanged by the addition of water.

Analysis: 0.1302 g subst.: 13.0 un' N (16.5", 767.4 mm). C,,H,,O,N,. Calc. N 11.44: found N 11.68.

On indigo. I . 84 1

The mononitro-derivate is a greenish yellow compound, which dissolves with a blue colour in strong sulphuric acid. Its formula is founded on the fact, that on heating it is decomposed into mononitro- indigo and acetic acid. This occurs at about 200-250° but is best carried out by boiling the compound with nitrobenzene. Acetic acid escapes and is characterised by its smell, its acid reaction, the formation of ethyl acetate and the fact that with aniline no acetanilide is formed, thus showing the absence of acetic anhydride. From the intense, cherry-red solution in nitrobenzene. 5-nitro-indigo

H H

crystallises on cooling and if necessary adding light petroleum. Analysis:

0.1314 g subst.: 16.1 cm3 N (13". 751.3 mm). CieHeOaNs. Calc. N 13.61; found N 14.25.

This mononitroindigo is a bluish violet compound, which does not melt below 300°, and dissolves with a blue colour in strong sulphuric acid.

That the nitro group is introduced in the para-position with respect to the imino group is proved by the fact that when the nitro-2-acetoxy- 3 : 3'-diketo-diindoliny1-2 : 2' is dissolved in acetic anhydride and some nitric acid is added, the dinitro-compound described under 2 separates after some time.

That the nitro group stands in the 5'- and not in the 5-position in the acetoxy derivative is assumed: further only two nitro groups can be introduced and therefore the acetoxy group must have some hindering effect.

2. 5 : 5'-Dinitro-2-acetoxy-3 : 3'-diketo-diindoliny1-2 : 2'. 0 CH3 0

02N/ /v 1 cy.-:/qp \ A N / \ N N

H H When indigo is nitrated as described under 1 but a mixture of

10 cm3 absolute nitric acid and 20 cm3 of acetic anhydride is now

842 J . van Alphen.

added, the indigo dissolves completely but after a short time a yellow precipitate appears, which is filtered off after two days, washed with boiling methyl alcohol and with absolute ether and dried at 120'. This compound is not soluble in acetic anhydride but dissolves with a greenish yellow colour in strong sulphuric acid.

Analysis: 0.1384 g subst.: 16.4 cm' N (14'. 750.7 mm). 0.1514 g subst.: 17.1 u n a N (12". 770 mm). C,,H,,O,N,. 13.59; found N 13.69, 13.58.

When this compound is heated it does not melt but a t about 200-250° loses a molecule of acetic acid and is transformed into 5 : 5'-dinitroindigo

H N without further decomposition. The same transformation can be brought about by dissolving the compound in strong sulphuric acid and diluting this solution with water and also by treating the acetoxy- compound with a solution of caustic potash.

I t is best obtained, however, by recrystallisation of the compound from boiling nitrobenzene. The acetic acid boils off and from the intense red solution a product crystallises with all the properties of 5 : 5'-dinitroindigo 6 ) . It is a dark red-violet compound, which does not melt below 300' and explodes at higher temperatures. It dissolves with a violet colour in strong sulphuric acid and with a cherry-red colour in nitrobenzene.

Analysis: 0.1396 g subst.: 19.4 cms N (17". 764.1 mm). C,,H,O,N,. Calc. N 15.91; found N 16.13.

3. 5 : 5' : 7-Trinitro-2-acetoxy-3 : 3'-diketo-diindolinyl-Z : 2'. 0 CH3 0

H -

A. B a e y e r , Ber. 12, 1316 (1879).

On indigo. I . 843

When more nitric acid was used for the nitration of indigo than for the experiments described under 2, it was possible to obtain products with a higher content of nitrogen. Thus when a mixture of 20 cm3 of absolute nitric acid and 40 cm3 of acetic anhydride was used, the nitrogen content of the product obtained was 14.75 o/c, but the purest product was obtained by slowly dissolving 5 g of indigo in a mixture of 20 cm3 of absolute nitric acid and 20 cm3 of acetic anhydride cooled down to -10' and adding 5 cm3 of acetic acid. After two days, a yellow precipitate had formed, which was extracted with boiling ethyl alcohol and absolute ether and dried a t 120'.

Analysis: 0.1468 g subst.: 18.8 cms N (loo, 754.8 mm). CieHiiOioNe. Calc. N 15.31: found N 15.08.

A product with the same properties could be obtained by dissolving one g of the dinitrornonoacetoxy-derivative in 10 cm3 of absolute nitric acid cooled to -10' and slowly adding acetic acid to the solution.

Analysis: 0.1480 g subst.: 19.6 cms N (17". 742.3 mm). C,,H,,O,,N,. Calc. N 15.31: found N 14.91.

This compound dissolves with an orange yellow colour in strong sulphuric acid. When it is heated alone, or with nitrobenzene, it splits off acetic acid and 5 : 5' : 7-trinitroindigo

is formed. It can be recrystallised from boiling nitrobenzene. in which solvent it dissolves with a cherry-red colour and separates in the form of brown-violet needles. which do not melt below 300' and dissolve with a light reddish violet colour in strong sulphuric acid.

Analysis: 0.1216 g subst.: 19.0 tins N (18". 765.8 mm). C,,H,O,N,. Calc. N 17.63; found N 18.07.

I t is assumed that the third nitro group has entered the 7'-position since on bromination, bromine atoms enter the indigo nucleus first in the 5- and then in the 7-position7). The acetoxy group in

') V i c t. M e y e r. Lehrbuch der org. Chem. 11. 3, 312 (1914).

844 J. van Alphen.

5 : 5’ : 7’-trinitro-2-acetoxy-3 : 3’-diketo-diindoliny1-2 : 2’ is responsible for the fact, that only three nitro groups can be introduced and therefore it is probable that the third nitro .group is introduced in the other half of the molecule in the 7’-position.

When this trinitroacetoxy-compound is dissolved in absolute nitric acid, it is recovered unchanged, when water or acetic acid is added to the solution after a short time, but when it remains for a longer time in solution, it is oxidised.

B. 5 : 5‘-7- Trinitro-2 : 2’-diacetoxy-3 : 3’-diketo-diindoliny1-2 : 2’.

Nitration of dehydro-indigoacetate in acetic anhydride.

>“C<

Dehydroindigo acetate or 2 : 2’-diacetoxy-3 : 3’-diketo-diindolinyI- 2 : 2’ was prepared according to the method of L. K a 1 b 8 ) . 5 g of this compound were suspended in 50 cm3 of acetic anhydride cooled to -10’ and then a mixture of 10 cm3 of absolute nitric acid and 20 cm3 of acetic anhydride was slowly added. As far as could be seen, nothing happened. Therefore the mixture was kept for three days at room temperatures. The precipitate that first had a green colour, was now clear yellow. It was extracted with boiling ethyl alcohol and with absolute ether and dried at 110’.

Analysis: 0.1826 g subst.: 22.4 cms N (18”, 775.2). C,,HIsO,,N,. Calc. N 13.33; found N 14.09.

This compound dissolves with a green colour in strong sulphuric acid and when heated, it melts partly with violent decomposition at about 200O. When 2 g are boiled with nitrobenzene, acetic acid escapes and about 150 mg of 5 : 5’ : 7-trinitroindigo:

Analysis: 0.1240 g subst.: 18.5 cms N (15.5”. 778.7 mm). C,,H,O,N,. Calc. N 17.63; found N 11.78.

can be obtained but most of the compound resinifies and is precipitated

”) L. K a 1 b, Ber. 42, 3651 (1909).

On indigo. 1. 845

as a sticky mass on the addition of absolute ethyl alcohol to the nitro- benzene solution.

When the trinitrodiacetoxy-derivative is dissolved in absolute nitric acid and precipitated immediately with acetic acid, the nitrogen content becomes somewhat higher ( 14.39 o/o ) but when the nitric acid solution is kept for some time. the compound is completely oxidised and nothing is Precipitated on the addition of acetic acid or water.

C. 5 : St-Dinitro-2-n-butyroxy-3 : 3'-diketo-diindoliny1-2 : 2'.

Nitration of indigo in n-butyric anhydride.

0

H H

Indigo ( 2 9 ) was suspended in 20 cm3 of n-butyric anhydride and cooled to -10'. Then a mixture of 4 cm3 of absolute nitric acid and 8 cm3 of n-butyric anhydride was added. The indigo dissolved and after standing for two days at room temperatures a yellow precipitate had formed, which was extracted with boiling ethyl alcohol and with absolute ether and dried a t 120'.

Analysis: 0.1494 g subst.: 16.9 cmS N (17", 775 mm). C,,H,,O,N,. Calc. N 12.75: found N 13.32.

This compound dissolves with a greenish-yellow colour in strong sulphuric acid and when heated to about 250° it loses n-butyric acid (identified by its odour and the odour of its ethyl ester) and dinitro- indigo remains behind (identified by its colour and the colour of its solution in strong sulphuric acid).

D. 5-Chloroisatin.

Nitration of indigo in acetyl chloride.

0

N Indigo ( 5 9 ) was suspended in 50 cm3 of acetyl chloride at -loo and a mixture of 10 cm3 of absolute nitric acid and 20 cm3 of acetyl

846 1. van Alphen, On indigo. I.

chloride was slowly added. (When the nitric acid and acetyl chloride were mixed, there was a violent reaction). The indigo dissolved, and after standing for a day, brown crystals had formed, which were recrystallised from ethyl alcohol and consisted of 5-chloroisatin, for they melted at 2 4 6 O 9 ) , contained halogen. and gave a blue colour when mixed with strong sulphuric acid and benzene containing a trace of thiophen.

Analysis: 0.1340 g subst.: 9.2 cms (N (18", 768.2 mm). CsHrOzNCI. Calc. N 7.68; found N 7.97.

E. The following nitrations all gave resinous products with no definite

melting point, which dissolved in ethyl alcohol and separated from the solvent in the form of syrupy masses. They also dissolved in ether and with a brown yellow colour in strong sulphuric acid and as the latter solution gave blue colours with benzene. containing a trace of thiophen, they probably consisted partly of nitro-isatins.

Indigo was dissolved in absolute nitric acid at -loo (very violent reaction) and the solution was poured on to ice or slowly diluted with ice-cold water. It was nitrated with a mixture of strong sulphuric acid and absolute nitric acid or dissolved in 7 "/o oleum and nitrated with nitric acid. Further, it was nitrated in a suspension in acetic acid with an excess of nitric acid or with the calculated amount of nitric acid and also in carbon tetrachloride but all these experiments failed to give indigo derivatives.

Nitration of indigo under various conditions.

L e i d e n. Organic Chemical Laboratory of the University.

(Received April 28th 1938).

9) B e i 1 s t e i n , Handbuch der org. Chemie. 4. Aufl., XXI, p. 45.