34 
and Nencki, who confirmed this observation in 1875, was 
able to convert indol into indigo tin by the action of ozonised 
air. But the yield is only very small, as the colouring 
matter readily undergoes further oxidation. 
However, the synthesis of indigo was thus completed, 
because indol can he built up from its elements; but 
chemists were not satisfied with it, the method being neither 
a practical one nor giving any clue as to the chemical consti- 
tution of indigo. 
In the next year Baeyer and Caro found a very simple 
and elegant method for preparing indol ; they obtained it 
by passing the vapour of ethyl-aniline through a red-hot 
tube : 
H 
CH 2 , 
c 6 h 5 -n 
CH 2 - CHo 
= C 6 H 4 
N 
CH + 2H 2 
Baeyer succeeded, in 1878, in obtaining oxindol from 
phenylacetic acid C 6 H 5 .CH 2 .C0 2 H, which can be prepared 
synthetically by different methods, and may be easily 
obtained from toluol. By treating the acid with nitric acid 
it is converted in the orthonitro-compound, which is easily 
reduced to the corresponding amido-acid. But this, like 
several other ortho-compounds, readily loses water and 
yields oxindol : 
CH 2 .CO.OH 
CH 2s 
= c 6 h 4 v ^>co + h 2 o 
This compound, as Baeyer and Knop had already found, 
is converted by the action of nitrous acid into nitros- 
oxindol. On treating this with nascent hydrogen it is 
transformed into amidoxindol, and this yields on oxidation 
isatin, the constitution of these bodies being expressed by 
the following formulae : 
Nitrosoxindol. Amidoxindol. Isatin. 
CH(NO) CH(NH S ) CO 
c 6 h 4 < >co c 6 h 4 < >co CoH 4 < >co 
:n! 1 .Ml NH 
