1881.] on IndigOj and its Artificial Production. 587 



4. Wliicli on oxiilation gives isatiii, 



CcH^^T "^0. (Q. E. D.) 



Q\Y 



This process, the feasibility of wLicli had also been foreseen by 

 Kekule, is however not available as a practical scheme for various 

 reasons. 



Process No. 3. — This may be called the manufacturing process, and 

 -was also proposed by Bacyer. It starts from cinnamic acid, a substance 

 contained in gum benzoin, balsam of Peru, and some few other 

 aromatic bodies. These sources arc, however, far too expensive to 

 render this acid thus obtained available for manufacturing purposes. 

 But Bertagnini, in 1856, had obtained cinnamic acid artificially from 

 oil of bitter almonds, and other processes for the same purpose have 

 since been carried out. Of these, that most likely to be widely 

 adopted is the following practical modification by Dr. Caro of Mr. 

 Perkin's beautiful synthesis of cinnamic acid : — 



1. C6H5CH3 + 4C1 = CgH^CHCl^ + 2HC1. 



Toluene. Benzylene dichloride. 



2. C6H5CHCI2 + 2CH3 . CO . . Na = CgHsCHUCH . CO . OH. + 2Nacl. 



Benzylene Sodium acetate. Cinnamic acid, 



dichloride. 



But why did Baeyer select this nine-carbon acid from which to 

 prepare indigo ? For this he had several reasons. In the first place, 

 it had long been known that all indigo compounds when heated with 

 zinc-dust yield indol, CgH^N, a body which stands therefore to indigo 

 in the same relation as anthracene to alizarin, and Baeyer and 

 Emmerling had so long ago as 18G9 prepared this indol from ortho- 

 nitro-cinnamic acid thus : — 



C«H6(N02)C02H = CgH^N + 0^ + CO^. 



Secondly, the ortho-nitro-cinnamic acid required (for we must 

 remember that indigo is an ortho-compound and also contains 

 nitrogen) can be readily prepared from cinnamic acid, and this itself 

 again can be obtained on a largo scale. Thirdly, this acid readily 

 parts with one atom of carbon, and thus renders possible its conversion 

 into eight-carbon indigo. 



The next steps in the process are (3) the formation of ortho- 

 nitro-cinnamic acid, (4) the conversion of this into its dibromide, (5) 

 the separation from this of the two molecules of hydrobromic 



