Indigo Chloro

Chloro-triaquo-diammino-cobaltic Sulphate, [Co(NH3)2(H30)3 Ci]S04.H.,0, is obtained by decomposing an aqueous solution of acid dichloro-diaquo-diammino-cobaltic sulphate, [Co(b IIg)2(H20)2Cl2]S04H, with a small quantity of sulphuric acid and evaporating the solution in vacuo. It separates in indigo-coloured crystals and is soluble in water, giving a blue solution. 

Incubate the filters with 5-bromo-4-chloro-3-indolylphosphate (BCIP) and nitroblue tetrazolium (NBT). Indoxyl generated from BCIP by the action of alkaline phosphatase condenses to form indigo (blue). Indigo then reacts with NBT to form insoluble diformazan (purple). 

Chloronitrotoluene and 2,6-chlorotoluidine are technically important in other respects also. The former compound is the starting material for the prepara-oitn of 4,4 -dichloroindigo, which yields, on further chlorination or bromination, the very greenish 4,5,4, 5 -tetrahalogenindigos (e.g., brilliant indigo 4G). 2,6-Chloro-toluidine, as fast scarlet TR base, is used in generating ice colors (naphthol AS). The isomeric 4-chloro-2-toluidine (fast red KB base), prepared by reduction of the by-product 4-chloro-2-nitrotoluene, is used for the same purpose. 

Other well-known indoles that have various natural sources are skatole (3-methylindole) (2), serotonin (3), L-tryptophan (4), tryptamine (5), the plant growth hormones 3-indoleacetic acid (6) and 4-chloro-3-indoleacetic acid (7) [19], the mushroom hallucinogen psilocin (8), and the indole-derived ancient dyes indigo (9) [20] and Tyrian Purple (10) [19] (Scheme 1). 

Interestingly, in the case of supramolecular complex consisting of dihexadecyl sulfosuccinate 20 and [Pt(en)2][PtCl2(en)2]" ", indigo-colored dispersion was obtained at room temperature, whereas the color disappeared upon heating the solution to 60 C. As the CT transition requires the existence of chloro-bridged extended coordination structure, the observed thermochromism indicates disruption of the one-dimensional complex structures at 60 C. The color reappeared reversibly... 

Important derivatives of indigo are the chloro and bromo derivatives. Vat Blue 5, which is tetrabromoindigo, is similar to Vat Blue 1 in shade but is considerably brighter. It is made by bromination of indigo in acetic acid. 

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