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Concept
Anthraquinone
Summary
Anthraquinone, also called anthracenedione or dioxoanthracene, is an aromatic organic compound with formula C14H8O2. Isomers include various quinone derivatives. The term anthraquinone however refers to the isomer, 9,10-anthraquinone (IUPAC: 9,10-dioxoanthracene) wherein the keto groups are located on the central ring. It is a building block of many dyes and is used in bleaching pulp for papermaking. It is a yellow, highly crystalline solid, poorly soluble in water but soluble in hot organic solvents. It is almost completely insoluble in ethanol near room temperature but 2.25 g will dissolve in 100 g of boiling ethanol. It is found in nature as the rare mineral hoelite.
There are several current industrial methods to produce 9,10-anthraquinone:
The oxidation of anthracene. Chromium(VI) is the typical oxidant.
The Friedel-Crafts reaction of benzene and phthalic anhydride in presence of AlCl3. o-Benzoylbenzoic acid is an intermediate. This reaction is useful for producing substituted anthraquinones.
The Diels-Alder reaction of naphthoquinone and butadiene followed by oxidative dehydrogenation.
The acid-catalyzed dimerization of styrene to give a 1,3-diphenylbutene, which then can be transformed to the anthraquinone. This process was pioneered by BASF.
It also arises via the Rickert-Alder reaction, a retro-Diels-Alder reaction.
Hydrogenation gives dihydroanthraquinone (anthrahydroquinone). Reduction with copper gives anthrone. Sulfonation with sulfuric acid gives anthroquinone-1-sulfonic acid, which reacts with sodium chlorate to give 1-chloroanthaquinone.
anthraquinones
9,10-Anthraquinone is used as a digester additive in production of paper pulp by alkaline processes, like the kraft, the alkaline sulfite or the Soda-AQ processes. The anthraquinone is a redox catalyst. The reaction mechanism may involve single electron transfer (SET). The anthraquinone oxidizes the reducing end of polysaccharides in the pulp, i.e., cellulose and hemicellulose, and thereby protecting it from alkaline degradation (peeling).
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