Anthracene Photodimerisation

Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C₁₄H₁₀, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes, as a scintillator to detect high energy particles, as production of pharmaceutical drugs. Anthracene is colorless but exhibits a blue (400–500 nm peak) fluorescence under ultraviolet radiation.

History and etymology

Crude anthracene (with a melting point of only 180°) was discovered in 1832 by Jean-Baptiste Dumas and Auguste Laurent who crystalized it from a fraction of coal tar later known as "anthracene oil". Since their (inaccurate) measurements showed the proportions of carbon and hydrogen of it to be the same as in naphthalene, Laurent called it ''paranaphtaline'' in his 1835 publication of the discovery, which is translated to English as paranaphthalene. Two years later, however, he decided to rename the compound to its modern name derived from because after discovering other polyaromatic hydrocarbons he decided it was only one of isomers of naphthalene. This notion was disproved in 1850s and 1860s.

Occurrence and production

Anthracene, as many other polycyclic aromatic hydrocarbons, is generated during combustion processes. Most human exposure is through tobacco smoke or ingestion of charred food.

The mineral form of anthracene is called freitalite and is related to a coal deposit. Coal tar, which contains around 1.5% anthracene, remains a major industrial source of this material. Common impurities are phenanthrene and carbazole.

A classic laboratory method for the preparation of anthracene is by cyclodehydration of o-methyl- or o-methylene-substituted diarylketones in the so-called Elbs reaction, for example from ''o''-tolyl phenyl ketone.

Reactions

Reduction

Reduction of anthracene with alkali metals yields the deeply colored radical anion salts M⁺ nthracenesup>− (M = Li, Na, K). Hydrogenation gives 9,10- dihydroanthracene, preserving the aromaticity of the two flanking rings.

Cycloadditions

In any solvent except water, anthracene photodimerizes by the action of UV light:
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The dimer, called dianthracene (or sometimes paranthracene), is connected by a pair of new carbon-carbon bonds, the result of the +4cycloaddition. It reverts to anthracene thermally or with UV irradiation below 300 nm. Substituted anthracene derivatives behave similarly. The reaction is affected by the presence of oxygen.

Anthracene also reacts with dienophile singlet oxygen in a +2cycloaddition (Diels–Alder reaction):
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With electrophiles

Chemical oxidation occurs readily, giving anthraquinone, C₁₄H₈O₂ (below), for example using hydrogen peroxide and vanadyl acetylacetonate.
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Electrophilic substitution of anthracene occurs at the 9 position. For example, formylation affords 9-anthracenecarboxaldehyde. Substitution at other positions is effected indirectly, for example starting with anthroquinone. Bromination of anthracene gives 9,10-dibromoanthracene.

Uses

Anthracene proper has application as an organic semiconductor and chemical feedstock for various preservatives and dyes.

Electronics

Anthracene is a wide band-gap organic semiconductor, with an emission spectrum peaking between 400 nm and 440 nm. Organic field-effect transistors have been constructed from it. In particle physics, it is used as a scintillator to detect high-energy photons, electrons, or alpha particles. Plastics, such as polyvinyltoluene, can be doped with anthracene to produce an approximately water-equivalent scintillator in radiation therapy dosimetry.

Anthracene is commonly used as a UV tracer in conformal coatings applied to printed wiring boards. The anthracene tracer allows the conformal coating to be inspected under UV light.

It is also used in wood preservatives, insecticides, and coating materials.

Derivatives

A variety of anthracene derivatives find specialized uses. Industrially, anthracene is converted mainly to anthraquinone, a precursor to dyes.Collin, Gerd; Höke, Hartmut and Talbiersky, Jörg (2006) "Anthracene" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim. Derivatives having a hydroxyl group are 1-hydroxyanthracene and 2-hydroxyanthracene, homologous to phenol and naphthols, and hydroxyanthracene (also called anthrol, and anthracenol) are pharmacologically active.
Anthracene may also be found with multiple hydroxyl groups, as in 9,10-dihydroxyanthracene.

Some anthracene derivatives are used as pharmaceutical drugs, including bisantrene, trazitiline, and benzoctamine.

Toxicology

Many investigations indicate that anthracene is noncarcinogenic: "consistently negative findings in numerous in vitro and in vivo genotoxicity tests". Early experiments suggested otherwise because crude samples were contaminated with other polycyclic aromatic hydrocarbons. Furthermore, it is readily biodegraded in soil. It is especially susceptible to degradation in the presence of light. The International Agency for Research on Cancer (IARC) classifies anthracene as IARC group 2B, possibly carcinogenic to humans.

See also

* 9,10-Dithioanthracene, derivative with two thiol groups added to the central ring
* Phenanthrene
* Acridine
* Phenazine
* Tetracene

References

Cited sources

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External links

*

IARC – Monograph 32


* ttps://web.archive.org/web/20110122105255/http://echa.europa.eu/home_en.asp European Chemicals Agency – ECHA*

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Organic semiconductors
Phosphors and scintillators
IARC Group 2B carcinogens

Ionising radiation detectors
Acenes
PBT substances
Polycyclic aromatic hydrocarbons