Bromindigo
bromine-substituted indigo, vat dyes of bright tints from red-violet to greenish-blue. The color depends on the number of halogen atoms and their positions in the molecule. One of the dyes of the bromindigo class, 6,6′-dibromindigo, a dye of red-violet color,
was found to be identical to “antique purple,” which has been used for dyeing cloth and thread since 1400-1500 B.C. and was of great value. In antiquity, purple was extracted from the vesicles of the purple snail Murex brandaris (12,000 vesicles yielded 1.4 g of dye).
Upon bromination of indigo in concentrated sulfuric acid or glacial acetic acid, the blue dye 5,5′, 7,7′ - tetrabromindigo, also called bromindigo, is formed. The latter surpasses indigo in brightness and the resistance of its color to the action of chlorine; it is widely used for the dyeing of cotton and viscose fabrics.
REFERENCE
Kogan, I. M. Khimiia krasitelei, 3rd ed. Moscow, 1956.L. S. POVAROV