414 



SCIENCE PROGRESS 



were once more encouraged by the appearance of two new 

 syntheses of indigo discovered by Prof. Heumann, the one 

 starting from naphthalene, the other from benzene. 



Although somewhat complicated, it is worth while noting 

 the lines taken by each method : 



(i) From Naphthalene. 



'CO /\ /CO 



/ 



(0 



O 



(2) 



Naphthalene. 



(4) 



\/\CO 



Phthalic anhydride. 



NH 



NH 



(3) 



\/\co 



Phthalimide. 



CH 2 .COOH 

 \COOH 



Phenyl-giycme-o.-carboxylic acid. 



/\ /NH 



(5) 

 fusion with 



potash, 

 > 



\co 



NH 5 



\/\COOH 



Anthranilic acid. 



NH 

 CH 2 



Indoxyl. 



c 



<;o 



INDIGO. 



NH 



CO 



(2) From Benzene. 

 NH 2 



(1) 



Benzene. 



/\/ 



NH 



(2) 



*CH, . COOH 



.(3) . 

 fusion with 



potash, 



Aniline. 



Phenyl-glycine. 



(4) 



Indoxyl -> indigo. 



Unfortunately, however, both the syntheses had serious 

 drawbacks. By the first method the yields obtained in all 

 the processes, from phthalic anhydride on, were excellent, 

 particularly in the stage (5), which is the most difficult to 

 carry out, but there was no cheap method of obtaining phthalic 

 anhydride except by oxidising naphthalene with fuming sul- 

 phuric acid, and here the yield was so poor that it was 

 almost impracticable. 



In the second method, from benzene via aniline, excellent 

 yields were obtained in each stage of the operations until (3), 

 the fusion with potash ; but here also the yield was so minute, 

 only amounting to quite a few per cent, of the theoretical 

 yield of indigo, that the process had to be abandoned. 



