Anthraquinone dyes are an abundant group of dyes comprising a anthraquinone unit as the shared structural element. Anthraquinone itself is colourless, but red to blue dyes are obtained by introducing electron donor groups such as hydroxy or amino groups in the 1-, 4-, 5- or 8-position. Anthraquinone dyestuffs are structurally related to indigo dyestuffs and are classified together with these in the group of carbonyl dyes. Members of this dye group can be found in natural dyes as well as in synthetic dyes. Anthraquinone dyestuffs are represented in mordant and vat, but also in reactive and disperse dyes. They are characterized by very good light fastness.
One of the most important anthraquinone dyes of herbal origin is alizarin, which is extracted from the dyer's madder. Alizarin is the eponym for a number of structurally related dyes that use alizarin dyes. It was the first natural dye for which an industrial synthesis was developed as early as 1869. Anthraquinone dyes include insect dyes derived from scale insects such as carminic acid, kermesic acid and laccainic acids. The colorant carmine with the main component carminic acid is used, for example, as an approved food colorant E 120.
Synthetic anthraquinone dyes
The synthesis of most anthraquinone dyes is based on anthraquinone sulfonic acid or nitroanthraquinone, which is obtained by sulfonation or nitration of anthraquinone. Sulfonation in α position is reversible and both the sulfonic acid groups and the nitro groups can be relatively easily replaced by amino, alkylamino, hydroxy and alkoxy groups. Aminoanthraquinone is thus accessible by reaction of anthraquinone sulfonic acid with ammonia or by reduction of nitroanthraquinone. An important intermediate product for many acid anthraquinone dyes is bromamic acid , which can be obtained from 1-aminoanthraquinone by sulfonation with chlorosulfonic acid and subsequent bromination. By replacing the bromine substituent with an aliphatic or aromatic amine, vibrant blue dyes are obtained. For example, bromamic acid can be condensed with 3--aniline to form the vibrant blue dye , from which the reactive dye C.I. Reactive Blue 19 is obtained after esterification with sulfuric acid. Reactive Blue 19 is one of the oldest and still the most important reactive dyes, patented in 1949. The first anthraquinone-based synthetic vat dye was indanthrone - the synthesis of which was developed by René Bohn in 1901: By dimerization of 2-aminoanthraquinone under strongly alkaline conditions at 220-235 °C, intermediate stage 3 is obtained in two steps, which is cyclized intramolecularly and oxidized to indanthrone 5.
