Furan

Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon atoms and one oxygen. Chemical compounds containing such rings are also referred to as furans.

Furan is a colorless, flammable, highly volatile liquid with a boiling point close to room temperature. It is soluble in common organic solvents, including alcohol, ether, and acetone, and is slightly soluble in water.^[3] It is toxic and may be carcinogenic in humans. Furan is used as a starting point to other specialty chemicals.^[4]

History

The name "furan" comes from the Latin furfur, which means bran.^[5] The first furan derivative to be described was 2-furoic acid, by Carl Wilhelm Scheele in 1780. Another important derivative, furfural, was reported by Johann Wolfgang Döbereiner in 1831 and characterised nine years later by John Stenhouse. Furan itself was first prepared by Heinrich Limpricht in 1870, although he called it "tetraphenol" (as if it were a four-carbon analog to phenol, C₆H₆O).^[6]^[7]

Production

Industrially, furan is manufactured by the palladium-catalyzed decarbonylation of furfural, or by the copper-catalyzed oxidation of 1,3-butadiene:^[4]

In the laboratory, furan can be obtained from furfural by oxidation to 2-furoic acid, followed by decarboxylation.^[8] It can also be prepared directly by thermal decomposition of pentose-containing materials, and cellulosic solids, especially pine wood.

Synthesis of furans

The Feist–Benary synthesis is a classic way to synthesize furans, although many syntheses have been developed.^[9] One of the simplest synthesis methods for furans is the reaction of 1,4-diketones with phosphorus pentoxide (P₂O₅) in the Paal–Knorr synthesis. The thiophene formation reaction of 1,4-diketones with Lawesson's reagent also forms furans as side products. Many routes exist for the synthesis of substituted furans.^[10]

Chemistry

Furan is aromatic because one of the lone pairs of electrons on the oxygen atom is delocalized into the ring, creating a 4n + 2 aromatic system (see Hückel's rule) similar to benzene. Because of the aromaticity, the molecule is flat and lacks discrete double bonds. The other lone pair of electrons of the oxygen atom extends in the plane of the flat ring system. The sp² hybridization is to allow one of the lone pairs of oxygen to reside in a p orbital and thus allow it to interact within the π system.

Due to its aromaticity, furan's behavior is quite dissimilar to that of the more typical heterocyclic ethers such as tetrahydrofuran.

It is considerably more reactive than benzene in electrophilic substitution reactions, due to the electron-donating effects of the oxygen heteroatom. Examination of the resonance contributors shows the increased electron density of the ring, leading to increased rates of electrophilic substitution.^[11]

Furan serves as a diene in Diels–Alder reactions with electron-deficient dienophiles such as ethyl (E)-3-nitroacrylate.^[12] The reaction product is a mixture of isomers with preference for the endo isomer:

Diels-Alder reaction of furan with arynes provides corresponding derivatives of dihydronaphthalenes, which are useful intermediates in synthesis of other polycyclic aromatic compounds.^[13]

Hydrogenation of furans sequentially affords dihydrofurans and tetrahydrofurans.
In the Achmatowicz reaction, furans are converted to dihydropyran compounds.

Safety

Furan is found in heat-treated commercial foods and is produced through thermal degradation of natural food constituents.^[14]^[15] It can be found in roasted coffee, instant coffee, and processed baby foods.^[15]^[16]^[17] Research has indicated that coffee made in espresso makers, and, above all, coffee made from capsules, contains more furan than that made in traditional drip coffee makers, although the levels are still within safe health limits.^[18]

Exposure to furan at doses about 2000 times the projected level of human exposure from foods increases the risk of hepatocellular tumors in rats and mice and bile duct tumors in rats.^[19] Furan is therefore listed as a possible human carcinogen.^[19]

References

↑ Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 392. ISBN 978-0-85404-182-4. doi:10.1039/9781849733069-FP001.
↑ Webster's Online Dictionary
↑ Jakubke, Hans Dieter; Jeschkeit, Hans (1994). Concise Encyclopedia of Chemistry. Walter de Gruyter. pp. 1–1201. ISBN 0-89925-457-8.
1 2 Hoydonckx, H. E.; Van Rhijn, W. M.; Van Rhijn, W.; De Vos, D. E.; Jacobs, P. A. (2005), "Furfural and Derivatives", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a12_119.pub2
↑ Senning, Alexander (2006). Elsevier's Dictionary of Chemoetymology. Elsevier. ISBN 0-444-52239-5.
↑ Limpricht, H. (1870). "Ueber das Tetraphenol C₄H₄O". Berichte der deutschen chemischen Gesellschaft. 3 (1): 90–91. doi:10.1002/cber.18700030129.
↑ Rodd, Ernest Harry (1971). Chemistry of Carbon Compounds: A Modern Comprehensive Treatise. Elsevier.
↑ Wilson, W. C. (1941). "Furan". Org. Synth. ; Coll. Vol., 1, p. 274
↑ Hou, X. L.; Cheung, H. Y.; Hon, T. Y.; Kwan, P. L.; Lo, T. H.; Tong, S. Y.; Wong, H. N. (1998). "Regioselective syntheses of substituted furans". Tetrahedron. 54 (10): 1955–2020. doi:10.1016/S0040-4020(97)10303-9.
↑ Katritzky, Alan R. (2003). "Synthesis of 2,4-disubstituted furans and 4,6-diaryl-substituted 2,3-benzo-1,3a,6a-triazapentalenes". Arkivoc. 2004 (2): 109. doi:10.3998/ark.5550190.0005.208.
↑ Bruice, Paula Y. (2007). Organic Chemistry (5th ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 0-13-196316-3.
↑ Masesane, I.; Batsanov, A.; Howard, J.; Modal, R.; Steel, P. (2006). "The oxanorbornene approach to 3-hydroxy, 3,4-dihydroxy and 3,4,5-trihydroxy derivatives of 2-aminocyclohexanecarboxylic acid". Beilstein Journal of Organic Chemistry. 2 (9): 9. PMC 1524792 . PMID 16674802. doi:10.1186/1860-5397-2-9.
↑ Filatov, M. A.; Baluschev, S.; Ilieva, I. Z.; Enkelmann, V.; Miteva, T.; Landfester, K.; Aleshchenkov, S. E.; Cheprakov, A. V. (2012). "Tetraaryltetraanthra[2,3]porphyrins: Synthesis, Structure, and Optical Properties". J. Org. Chem. 77 (24): 11119–11131. doi:10.1021/jo302135q.
↑ Anese, M.; Manzocco, L.; Calligaris, S.; Nicoli, M. C. (2013). "Industrially Applicable Strategies for Mitigating Acrylamide, Furan and 5-Hydroxymethylfurfural in Food". Journal of Agricultural and Food Chemistry. 61 (43): 130528102950009. PMID 23627283. doi:10.1021/jf305085r.
1 2 Moro, S.; Chipman, J. K.; Wegener, J. W.; Hamberger, C.; Dekant, W.; Mally, A. (2012). "Furan in heat-treated foods: Formation, exposure, toxicity, and aspects of risk assessment". Molecular Nutrition & Food Research. 56 (8): 1197–1211. PMID 22641279. doi:10.1002/mnfr.201200093.
↑ European Food Safety Authority (2011). "Update on furan levels in food from monitoring years 2004–2010 and exposure assessment". EFSA Journal. 9 (9): 2347. doi:10.2903/j.efsa.2011.2347.
↑ Waizenegger, J.; Winkler, G.; Kuballa, T.; Ruge, W.; Kersting, M.; Alexy, U.; Lachenmeier, D. W. (2012). "Analysis and risk assessment of furan in coffee products targeted to adolescents". Food Additives & Contaminants: Part A. 29 (1): 19–28. PMID 22035212. doi:10.1080/19440049.2011.617012.
↑ "Espresso makers: Coffee in capsules contains more furan than the rest". Science Daily. April 14, 2011.
1 2 Bakhiya, N.; Appel, K. E. (2010). "Toxicity and carcinogenicity of furan in human diet". Archives of Toxicology. 84 (7): 563–578. PMID 20237914. doi:10.1007/s00204-010-0531-y.

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