547.758 ON INDIGO, 11.

The ozonisation of indigo BY

J. VAN ALPHEN.

When indigo is suspended in dry chloroform and treated with ozone an ozonide is formed, which can. be decomposed with water when it furnishes isatin. The same result is obtained when dry ethyl acetate is used but when indigo is ozonised in wet ethyl acetate a quantitative yield of isatinic anhydride is obtained.

Introduction. The formula of indigo

0 0

/Vc '\ , A2 \/\ ' I c = c I / \/\N ' ~% \N/\/

H H given by v o n B a e y e r 1) assumes that the two indole halves of the molecule are connected by a double bound between two carbon atoms. Now no direct proof of the presence of this ethene bond is known, for it does not react by addition with the common reagents such as bromine, chlorine or hydrogen and though it would require the existence of two geometrical isomers only one form of indigo is found 2 ) . Thus it can be well understood why formulae have been proposed for indigo in which only a single bond is present between the two halves of the molecule 3 ) .

I t is all the more remarkable therefore, that bhe reaction between indigo and the most powerful reagent on the double bond, ozone, has

l ) A. B a e y e r, Ber. 16, 2203 (1883) ). 2, For these difficulties see J. v a n A I p h e n. Chem. Weekblad 35, 435 ( 1938).

Sometimes it is found that indigo adds on another compound and it is then assumed that this compound adds on to the ethene bond; J. v a n A 1 p h e n, this Journal 57. 837 (1938).

3 ) ,H. K i n g. J. SOC. Chem. Ind. 4, 135 (1925): J. v a n A 1 p h e n . Chem. Week- blad 35, 435 (1938).

912 1. van Alphen.

never been investigated, although it has been known for a long time that ozone decolorises indigo4) and indigo has been used even for the quantitative titration of ozone 5 ) .

I t has now been shown that when indigo is suspended in dry chloroform and treated with ozone, i t completely disappears and an insoluble, brownish yellow ozonide is formed, which could be decom- posed with water, isatin being obtained:

H H H this reaction proves without doubt the presence of the ethene bond in indigo.

When the ozonisation was extended for a longer time, products were obtained which contained more ozone, decomposed at a lower tem- perature and did not furnish an isolable quantity of isatin with water. When indigo was suspended in dry ethyl acetate and ozonised, it dissolved with an orange yellow colour but the oronide was not precipitated. On treating this solution with water, a good yield of isatin was obtained. When indigo was suspended in wet ethyl acetate however, and treated with a very large excess of ozone it was com- pletely transformed into isatinic anhydride.

I ! V \ / O

H Probably isatin, formed by the fission of the ozonide, was oxidised

for a quantity of isatinic anhydride could be abtained also when isatin was treated in the same way as indigo. Dry indigo is also attacked by dry ozone but only slowly and the resulting product gives isatin when treated with water.

When indigo is treated with ozone in aqueous suspension it decomposes but only impure isatin can be isolated from the product.

4 ) J. E r d m a n n and A. W a g n e r find that ozone decolorises indigo and A. H o u z e a u and A. and P. T h k n a r d find the same for indigo sulphonic acid: E. F o n r o b e r t . “Das Ozon” 1912, p. 128. M. N e n c k i , Ber. 8, 727 (1875) finds that indigo suspended in water is attacked by ozone; A. H o u z e a u . Ber. 5, 827 (1872) finds that hydrogen peroxide is formed.

J ) A. and P. T h e n a r d , Ber. 5, 828 (1872).

On indigo 11. 913 .~

A. The theoretical quantity of ozonised oxygen (75 mg ozone/], velocity

10 llhour) was passed through a suspension of 2 g of indigo in 50 cm3 of dry chloroform. Most of the indigo remained unchanged but when the orange-yellow filtrate was shaken with water a precipitate formed which consisted of isatin and when the chloroform was evaporated, more isatin was obtained. In this and in the following experiments the isatin was always characterised by its melting point, the mixed melting point with an authentic speci,men and by the blue indophenin reaction when treated with strong sulphuric acid and benzene containing a trace of thiophen.

When the same experiment was repeated but ozonised oxygen was passed through till the blue indigo had disappeared (after about five hours ) , a greyish yellow powder had formed: this was washed with dry ether and dried in vacuo over strong sulphuric acid. It decomposed below looo with a strong evolution of gases and when treated with water no isatin could be detected. It contained only 6.23 % N and consisted therefore of one mol of indigo with more than one molecule of ozone.

Therefore the experiment was again carried out but now oxygen containing only 10 mg/l was passed through. Again after about five hours the indigo had disappeared and a yellow-brown precipitate had formed. This precipitate was washed with dry ether and dried in a vacuum over strong sulphuric acid.

Ozonisation of indigo in dry chloroform.

Analysis: 0.1816 g subst.: 12.7 cms N (14", 766.8 mm). 0.1830 g subst. ( of i i second preparation): 12.8 cmS N ( l l ' , 765.8 mm). C,,H,,O,N,. Calc. N 9.00; found N 8.26. 8.33.

TBhe nitrogen content is somewhat low and therefore it is prohable that the benzene nucleus is attacked to some extent,

This ozonide does not dissolve in indifferent solvents such as benzene, ether, chloroform or light petroleum. It decomposes with evolution of gases when heated to 100-I4O0. When it is introduced into a small quantity of boiling water, it dissolves with an orange colour and isatin separates from the solution on cooling. Isatin is also formed when i t is moistened or kept in the air.

B. When indigo ( 2 g ) is suspended in dry ethyl acetate (100 cm3)

and the theoretical quantity of ozone is passed through (15 or 75 mg ozone/]. velocity 10 ]/hour). most of the indigo is unchanged. T h e

Ozonisation of indigo in ethyl acetate.

914 1. van Alphen. On indigo 11. -

orange filtrate is shaken with water and on evaporation a small quantity of isatin can be isolated.

When 5 g of indigo are suspended in 200 cm3 of ethyl acetate (not specially dried) and an excess of ozone is passed through (75 mg orone/l, velocity 10 ]/hour) it disappears completely (after about five hours). A small quantity of a yellow crystalline precipitate is formed and the rest is obtained by evaporation of the ethyl acetate. It consists of isatinic anhydride as is proved by its melting point (about 230' with decomposition) and its mixed melting point with an authentic specimen: it can be purified by recristallisation from water or ethyl alcohol.

When isatin is treated in the same way, isatinic anhydride is also formed: it can be separated from a small quantity of unchanged isatin by extraction with boiling water in which isatin is much more soluble.

C. When 2 g of indigo are suspended in 50 cmx of water and ozonised

oxygen (75 mg/1, velocity 10 ]/hour) is passed through, for five hours, part of the indigo has dissolved to an orange solution. When this solution is evaporated, a semi-solid mass is obtained which gives the blue indophenin reaction but it is not possible to isolate pure isatin from this product.

When dry indigo is exposed for four hours to a current of ozonised oxygen (15 mg/1, velocity 10 ]/hour) it looses its blue colour and turns greenish. When this product is boiled with water, it proves that most of the indigo has remained unchanged but pure isatin crystallises from the filtrate.

Oronisation of indigo under different conditions.

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

(Received M a y 31st 1938).