INDIGO 247 



formula C 16 H 10 N 2 2 . Careful study of its related products 

 leads to the following structural formula for indigotin which 

 was first prepared artificially by von Baeyer in 1878 : 



xoc\ xco\ 



6 4 



It is to von Baeyer and his pupils that we owe the know- 

 ledge of the structure of indigo, which has rendered possible 

 its commercial production on the large scale from raw 

 material, such as naphthalene, found in coal-tar. 



Indigo does not exist as such in the indigo plant. Schunck 

 in 1855 showed that the plant contained a glucoside which he 

 termed indican. Schunck regarded this as a compound of 

 indigo with sugar. Kecent investigations of Hoogewerff and 

 Termeulen, which have been confirmed and extended by 

 Perkin and Bloxam, have shown that indican is a glucoside, 

 not of indigo itself, but of a substance which was originally 

 discovered by Von Baeyer, known as indoxyl (see note 

 p. 255). which yields indigo in contact with oxygen. 



In the steeping process described above the indican is 

 fermented, yielding indoxyl and glucose ; in the subsequent 

 beating operation the indoxyl is oxidised. The equations 

 representing these changes, supposing them to be complete, 

 are as follows : 



C 14 H 1? 6 N + H 2 = QgHjON + C^O. 



Indican Indoxyl Dextrose 



H2 



Indoxyl 



- CH /COx r-r //co 



t6H4 



Indigo 



Perkin and Bloxam's researches were concerned with the 

 exact study of these chemical changes. Before this was 

 possible, accurate methods of analysis had to be devised in 



