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 dyealizarin 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.[13]
History and etymology
Crude anthracene (with a melting point of only 180°) was discovered in 1832 by Jean-Baptiste Dumas and Auguste Laurent[14] 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,[15] which is translated to English as paranaphthalene.[14] Two years later, however, he decided to rename the compound to its modern name derived from Ancient Greek: ἄνθραξ, romanized: anthrax, lit. 'coal' because after discovering other polyaromatic hydrocarbons he decided it was only one of isomers of naphthalene.[16] This notion was disproved in 1850s and 1860s.[17][18]
Occurrence and production
Coal tar, which contains around 1.5% anthracene, remains a major source of this material. Common impurities are phenanthrene and carbazole. The mineral form of anthracene is called freitalite and is related to a coal deposit.[19] 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.[20]
Reactions
Reduction
Reduction of anthracene with alkali metals yields the deeply colored radical anion salts M+[anthracene]− (M = Li, Na, K). Hydrogenation gives 9,10-dihydroanthracene, preserving the aromaticity of the two flanking rings.[21]
Cycloadditions
In any solvent except water,[22] anthracene photodimerizes by the action of UV light:
The dimer, called dianthracene (or sometimes paranthracene), is connected by a pair of new carbon-carbon bonds, the result of the [4+4] cycloaddition. 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.[23][24]
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.[26] Bromination of anthracene gives 9,10-dibromoanthracene.[27]
Uses
Anthracene is converted mainly to anthraquinone, a precursor to dyes.[28]
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.[29]
Fluorescence of anthracene under UV light
Derivatives
False-color AFM image of anthracene diradical, where hydrogen atoms are removed at carbons 9 and 10
A variety of anthracene derivatives find specialized uses. Derivatives having a hydroxyl group are 1-hydroxyanthracene and 2-hydroxyanthracene, homologous to phenol and naphthols, and hydroxyanthracene (also called anthrol, and anthracenol)[30][31] are pharmacologically active.
Anthracene may also be found with multiple hydroxyl groups, as in 9,10-dihydroxyanthracene.
More recently, crystalline anthracene was found to be a useful wide band-gap semiconductor in devices such as organic field-effect transistors and Scintillators for detecting high-energy subatomic particles.[32]
Occurrence
Anthracene, as many other polycyclic aromatic hydrocarbons, is generated during combustion processes. Exposure to humans happens mainly through tobacco smoke and ingestion of food contaminated with combustion products.
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.[28] The International Agency for Research on Cancer (IARC) classifies anthracene as IARC group 2B, possibly carcinogenic to humans.[33]
^ abcAnthracene in Linstrom, Peter J.; Mallard, William G. (eds.); NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD)
^Rickborn, Bruce (1998). "The Retro– <SCP>D</SCP> iels– <SCP>A</SCP> lder Reaction Part <SCP>I</SCP> . <SCP>C</SCP> <SCP>C</SCP> Dienophiles". Organic Reactions. pp. 1–393. doi:10.1002/0471264180.or052.01. ISBN978-0-471-26418-7.
^Bouas-Laurent, Henri; Desvergne, Jean-Pierre; Castellan, Alain; Lapouyade, Rene (2000). "Photodimerization of anthracenes in fluid solution: Structural aspects". Chemical Society Reviews. 29: 43–55. doi:10.1039/a801821i.
^Charleton, Kimberly D. M.; Prokopchuk, Ernest M. (2011). "Coordination Complexes as Catalysts: The Oxidation of Anthracene by Hydrogen Peroxide in the Presence of VO(acac)2". Journal of Chemical Education. 88 (8): 1155–1157. Bibcode:2011JChEd..88.1155C. doi:10.1021/ed100843a.
