Bromine indigo

Tetrabromindigo is prepared by brominating indigo in glacial acetic acid. It has a brilliant, bathochromically shifted shade. 

Brominated indigo with a lower degree of bromination is also commercially available. 

From the researches of Baeyer [51] and Bloem it is shown that on brominating acetylortho-amidoacetophenone, the bromine atom enters in the methyl group. If the benzene chain is substituted brominated indigos are formed but if only the benzene chain contains bromine, indigo cannot be produced. In these reactions indoxyl appears to be formed as an intermediate product in the reaction. 

Figure 50.1 Tyrian purple or royal purple (bromine Indigo).

Odier bromine-containing dye intemiediates include 6-bromo-2,4-dinitroaniline [1817-73-8] C H BrN O, and 5,7-dibromoisatin [6374-91-0] CgH2Bi2N02. Bromodinitroaniline is used in making a2o dyes and dibromoisatin is used in making Alizarin Indigos B, 3R, and G. 

Another ancient dye is the deep blue indigo [482-89-3], the presence of two bromine atoms at positions gives the dye Tyrian purple [19201 -53-7] once laboriously extracted from certain sea shells and worn by Roman emperors. 

It yields a characteristic reaction with bromine. If a few drops are, dissolved in 3 c.c. of glacial acetic acid and a little bromine vapour allowed to pass down the tube, a fine crimson colour forms which rapidly extends to the whole of the liquid and soon changes to violet and then to indigo blue with phosphoric acid, the acetic acid solution gives a rose madder colour at the junction of the liquids, and when the liquids are mixed, the colour changes to crimson and then slowly to violet. Baker and Smith consider that the sesquiterpene contains one double linkage. Semmler considers that it is a mixture of at least two bodies, one a bicyclic and the other a tricyclic sesquiterpene. 

H), followed by bromine-lithium exchange using 2 equivalents of tert-butyllithium to give the desired intermediate. This intermediate readily picked up carbon monoxide and work-up of the reaction mixture gave indigo (Fig. 17) (ref. 31). 

Conversely a suspension of finely divided indigo in chloroform can be converted into a beautiful reddish-brown solution of dehydroindigo by adding first a little calcium hydroxide and then bromine drop by drop. 

Centuries before the element bromine was discovered, one of its organic compounds, Tyrian purple, was used as a rich costly dye prepared from a white juice secreted by the Mediterranean mollusk, the straight-spined Murex (M. brandaris Linne) (91,166). Strabo described the Tyrian dye works in his Geography, and the product was mentioned frequently in the Bible (Ezek. 27, 7, 16) (92). In 1909 H. Friedlander of Vienna discovered that this royal dye from Murex brandaris is identical with the 6 6 dibrom indigo which F. Sachs of Berlin and his collaborators had prepared only five years previously from p-bromo-o-nitrobenzalde-hyde (93, 94, 95). 

Direct bromination of indigo results in bromine derivatives, which have a certain value as colorants [32],... 

Rosin Dissolve 2 g of sample in 10 mL of dehydrated alcohol, and add slowly, with shaking, 50 mL of solvent hexane. Transfer the solution to a separator, wash with two 50-mL portions of water, and discard the washings. Filter the solvent layer, evaporate it to dryness, and add 2 mL of 1 2 (v/v) liquefied phenohmethylene chloride to its residue. Stir, and transfer a portion of the mixture to a cavity of a color-reaction plate. Fill an adjacent cavity with 1 4 (v/v) bromine methylene chloride, and cover both cavities with an inverted watch glass. No purple or deep indigo blue color appears in or above the liquid containing the sample residue. 

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. 

Further, the reaction is of practical value in the preparation of o-amido-benzoic acid, used in the manufacture of artificial indigo. If, as above, bromine and caustic potash are allowed to act on phthalimide, there is first formed, by the addition of water, an add-amide ... 

The o-nitrodnnamic add from which indigo is synthetically prepared is of technical importance. If dnnamic add, or better, an ester of it, is nitrated, a mixture of the o- and p-nitroderivatives is obtained which can be separated into its constituents. If bromine is allowed to act on the o-nitrocinnamic add, there is obtained ... 

---