584 



Professor H. E. Boscoe 



[May 27, 



fact being ascertained, the next step was tlie further process of 

 clothing the hydro-carbon by adding four atoms of oxygen and sub- 

 tracting the two atoms of hydrogen present in excess, and this was 

 soon successfully accomplished, so that now, as we know, artificial 

 alizarin has excluded the natural colouring matter altogether. 



^14^10 



Anthracei 



Ci,He02(0H), 



Alizarin. 



What now was the first step gained in our knowledge concerning 

 the constitution of indigo, of which the simplest formula is CgHgNO ? 



Step No. 1. — This was made so long ago as 1840, when Fritsche 

 proved that aniline, C6H5NH2, can be obtained from indigo. The 

 name for this now well-known substance is indeed derived from the 

 Portuguese " anil," a word used to designate the blue colour from 

 indigo. This result of Fritsche's is of great importance, as showing 

 that indigo is built up from the well-known benzene ring CgHg, the 

 skeleton of all the aromatic compounds, and moreover that it contains 

 an amido-group. 



Step No. 2 was also made by Fritsche in the following year, 

 when, by boiling indigo with soda and manganese dioxide, he obtained 

 ortho-amido-benzoic acid, or, as he then termed it, anthranilic acid. 

 The following is the reaction which here occurs : — 



CsH^NO + + 2H20 = C,H5NHA + CHoO 



Indigo. 



Ortho-amido-benzoic acid. 



What light does this fact shed upon the constitution of indigo ? It 

 shows (1) that one of the eight atoms of carbon in indigo can be 

 readily separated from the rest ; (2) that the carboxyl and the amido- 



i 



Ortho-position, 

 CO^H 



Meta-position. 

 CO^H 



2]NH, 



Para-position. 

 CO^H 



/ 



NHfi 



group are in neighbouring positions in the benzene ring, viz. 1 and 2. 

 For we have three isomeric acids of the above composition. 



Bottinger, Deut. Chem. Ges. 1877, i. 269. 



