# Anthraquinone

Anthraquinone, also called anthracenedione or dioxoanthracene, is an aromatic organic compound with formula C $_{14}$H $_8$O $_2$. Several isomers exist but these terms usually refer to 9,10-anthraquinone (IUPAC: 9,10-dioxoanthracene) wherein the keto groups are located on the central ring. It is used as a digester additive to wood pulp for papermaking. Many anthraquinone derivatives are generated by organisms or synthesised industrially for use as dyes, pharmaceuticals, and catalysts. Anthraquinone 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.

## Synthesis

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 AlCl$_3$. 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.

## Reactions

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.

## Applications
### Digester additive in papermaking

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). The anthraquinone is reduced to 9,10-dihydroxyanthracene which then can react with lignin. The lignin is degraded and becomes more watersoluble and thereby more easy to wash away from the pulp, while the anthraquinone is regenerated. This process gives an increase in yield of pulp, typically 1–3% and a reduction in kappa number.

### Hydrogen Peroxide Production

2-Alkyl-9,10-Anthroquinones are used as a catalyst in the Anthraquinone Process for the production of hydrogen peroxide. This process is the dominant industrial method of hydrogen peroxide production.

### Niche uses

9,10-anthraquinone is used as a bird repellant on seeds, and as a gas generator in satellite balloons. It has also been mixed with lanolin and used as a wool spray to protect sheep flocks against kea attacks in New Zealand.

## Other isomers

Several other isomers of anthraquinone exist, including the 1,2-, 1,4-, and 2,6-anthraquinones. They are of minor importance compared to 9,10-anthraquinone.

## Safety

Anthraquinone has no recorded LD$_{50}$, probably because it is so insoluble in water.
In terms of metabolism of substituted anthraquinones, the enzyme encoded by the gene UGT1A8 has glucuronidase activity with many substrates including anthraquinones.