B.—CHEMISTRY 47 
All the indigotin syntheses can give II just as well as I, and II can yield 
isatin on oxidation. ‘The recent researches of E. Hope prove that some 
of the products of the action of benzoyl chloride on indigotin possess the 
skeleton of II, but of course this may arise from an intramolecular 
rearrangement. The chief argument against II is derived from a con- 
sideration of the numerous classes of indigoid dyestuffs which are easily 
formulated on the model of I. 
The industrial analogues of indigo are its substituted derivatives, the 
thioindigos (e.g. V) and similar indigoid dyes, and indanthrone (VI). 
The first-mentioned classes were made in imitation of the indigotin 
(V) (VI) 
structure, but in the case of indanthrone flattery was unconscious. 
Both indigotin, and indanthrone contain the chromophoric quinone 
group —CO—C = C—CO— and closely situated auxochromic —NH— 
groups. 
Graebe and Liebermann’s recognition of the constitution of alizarin 
(madder) (VII) led, as in the case of indigo, to the industrial synthesis of 
the colouring matter itself and of numerous derivatives and analogues. 
ave OH Ai pa ? 
ON “ 
Bm pon Ole 
CO 
(VI) (VHT) 
At the present time we recognise in retrospect that the most important 
outcome of the work on madder was the attention focussed on the study: 
of anthraquinone and its derivatives. It may seem a far cry from the 
adjective natural dyestuff to the modern vat dyestuff Caledon Jade 
Green (VIII), but the descent is in the direct line—alizarin, Alizarin 
Blue, benzanthrone, dibenzanthrone, Jade Green. Technical analogues 
of the anthocyanidins are to be found in the phthaleins, pyronines and 
rhodamines, and some more or less close dyestuff analogy can be found to 
correspond with most of the series of natural colouring matters. 
