anthraquinone dyes

File:Alizarin.svg]]

One of the most important anthraquinone dyes of herbal origin is alizarin, which is extracted from the dyer's madder (Rubia tinctorum). Alizarin is the eponym for a number of structurally related dyes that use alizarin dyes (sometimes synonymous with anthraquinone dyes). It was the first natural dye for which an industrial synthesis was developed as early as 1869.

Anthraquinone dyes include red insect dyes derived from scale insects such as carminic acid, kermesic acid, and laccaic acids. The colorant carmine with the main component carminic acid is used, for example, as an approved food colorant E 120.{{cite journal |last1=Cooksey |first1=C. J. |title=The red insect dyes: carminic, kermesic and laccaic acids and their derivatives |journal=Biotechnic & Histochemistry |date=17 February 2019 |volume=94 |issue=2 |pages=100–107 |doi=10.1080/10520295.2018.1511065 |url=https://doi.org/10.1080/10520295.2018.1511065 |access-date=28 March 2022 |issn=1052-0295}} The traditional methods for carmine production are labour, land, and insect-intensive. Because demand for red dyes is predicted to increase, researchers are exploring metabolic engineering approaches for the production of synthetic carminic acid.{{cite journal |last1=Miller |first1=Brittney J. |title=Cochineal, a red dye from bugs, moves to the lab |journal=Knowable Magazine |date=25 March 2022 |doi=10.1146/knowable-032522-1 |doi-access=free |url=https://knowablemagazine.org/article/technology/2022/cochineal-red-dye-bugs-moves-lab |access-date=28 March 2022}}{{cite journal |last1=Seo |first1=Seung-Oh |last2=Jin |first2=Yong-Su |title=Next-Generation Genetic and Fermentation Technologies for Safe and Sustainable Production of Food Ingredients: Colors and Flavorings |journal=Annual Review of Food Science and Technology |date=25 March 2022 |volume=13 |issue=1 |pages=463–488 |doi=10.1146/annurev-food-052720-012228 |url=https://doi.org/10.1146/annurev-food-052720-012228 |access-date=28 March 2022 |issn=1941-1413}}

The synthesis of most anthraquinone dyes is based on anthraquinone sulfonic acid (2) or nitroanthraquinone (3), which is obtained by sulfonation or nitration of anthraquinone (1).

:File:Synthesis 1-Aminoanthraquinone.svg

:Synthesis of 1-aminoanthraquinone

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 (4) is thus accessible by reaction of anthraquinone sulfonic acid with ammonia or by reduction of nitroanthraquinone.{{citation |editor-first=Klaus |editor-last=Hunger |publisher=WILEY-VCH Verlag|publication-place=Weinheim|pages=200 ff.|isbn=978-3-662-01950-4|date=2003 |title=Industrial Dyes: Chemistry, Properties, Applications |url={{Google books|uAzS4Hk2TgwC|plainurl=yes}}

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An important intermediate product for many acid anthraquinone dyes is bromamic acid (1-amino-4-bromoanthraquinone-2-sulfonic acid) (6), which can be obtained from 1-aminoanthraquinone (4) by sulfonation with chlorosulfonic acid and subsequent bromination.

:File:Synthesis Bromaminic Acid.svg

:Synthesis of bromamic acid

By replacing the bromine substituent with an aliphatic or aromatic amine, vibrant blue dyes are obtained.{{citation|surname1=Heinz-Gerhard Franck, Jürgen W. Stadelhofer|publisher=Springer Verlag|publication-place=Berlin, Heidelberg|pages=365 ff.|isbn=978-3-662-07876-1|date=1978 |language=de |title=Industrielle Aromatenchemie: Rohstoffe · Verfahren · Produkte |url={{Google books|sAuzBgAAQBAJ|plainurl=yes}}

}} For example, bromamic acid can be condensed with 3-(2-hydroxyethylsulfonyl)-aniline (7) to form the vibrant blue dye (8) (oxysulfone blue), from which the reactive dye C.I. Reactive Blue 19 is obtained after esterification with sulfuric acid.

:File:Synthesis Reactive Blue 19.svg

:Synthesis of C.I. Reactive Blue 19

Reactive Blue 19 is one of the oldest and still the most important reactive dyes,{{Cite patent | country =DE| number =4422160 | title = Verfahren zur Herstellung von C.I. Reactive Blue 19 | fdate = 1994-06-24 | gdate =1996-04-01 | invent1 =Andreas Von Der Eltz | assign1 = Hoechst AG}} patented in 1949.{{Cite patent | country =DE| number = 965902 | title = Verfahren zum Fixieren wasserloeslicher organischer Verbindungen auf Unterlagen faseriger Struktur | fdate =1949-07-19 | gdate =1957-09-19 | invent1 =Johannes Heyna, Willy Schumacher | assign1 = Hoechst AG}}

The first anthraquinone-based synthetic vat dye was indanthrone (C.I. Vat Blue 4) - the synthesis of which was developed by René Bohn in 1901:

:File:Synthesis Indanthrone.svg

:Synthesis of indanthrone

By dimerization of 2-aminoanthraquinone (1) 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.{{citation |first=Heinrich |last=Zollinger |edition=3rd |publisher=WILEY-VCH Verlag|publication-place=Weinheim|page=289|isbn=3-906390-23-3|date=2003 |title=Color Chemistry: Syntheses, Properties, and Applications of Organic Dyes and Pigments}}