Five-membered Monoheterocyclic Compounds (continued): The Indigo Group

The indigo group includes, in addition to indigotin and its derivatives, a variety or structurally related compounds, several of which, like the prototype are commercially important vat dyes. They are reducible by virtue of the carbonyl groups to dihydroxy compounds; these form water-soluble salts from solutions of which textile fibers can be impregnated with the reduced or “leuco” compound (generally colorless or much less highly colored than the parent), which is readily oxidized to reproduce the dye deposited in the textile material. The methods that are or have been applied technically to the manufacture of indigo depend for the main part on the production of indoxyl or indoxylic acid (indolin-3-one-2-carboxylic acid) with subsequent oxidation to indigotin by air or an oxidizing agent such as ferric chloride. The various derivatives of indigotin include N-derivatives, derivatives of the carbonyl function, homologs such as 5,5'-dimethylindigo, halogenoindigos, dehydroindigo, leuco-indigo (indigo-white) and polycyclic indigos. The position isomeride, indirubin, is present in small quantities in synthetic indigotin, and in larger and varying amounts in the natural dye; it is unlikely that it contributes significantly to the tinctorial properties of the coloring matter. Isoindigo or isoindigotin, produced by heating 3-diazo-oxindole, is the third possible bisindole indigo.