Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring
with four carbon atoms and one oxygen. The class of 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, but is slightly soluble in water. It is toxic
and may be carcinogenic in humans. Furan is used as a starting point to other specialty
chemicals.
History The name furan comes from the Latin furfur,
which means bran. 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.
Production Industrially, furan is manufactured by the
palladium-catalyzed decarbonylation of furfural, or by the copper-catalyzed oxidation of 1,3-butadiene:
In the laboratory, furan can be obtained from furfural by oxidation to furan-2-carboxylic
acid, followed by decarboxylation. It can also be prepared directly by thermal decomposition
of pentose-containing materials, 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. One of the simplest synthesis
methods for furans is the reaction of 1,4-diketones with phosphorus pentoxide 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.
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
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 pi-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.
Furan serves as a diene in Diels-Alder reactions with electron-deficient dienophiles such as
ethyl-3-nitroacrylate. 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.
Hydrogenation of furans affords sequentially dihydrofurans and tetrahydrofurans.
In the Achmatowicz reaction, furans converted to dihydropyran compounds.
Safety Furan is found in heat-treated commercial
foods and it is produced through thermal degradation of natural food constituents. Notably, it
can be found in roasted coffee, instant coffee, and processed baby foods. 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. Furan is
therefore listed as a possible human carcinogen. See also
BS 4994 – Furan resin as thermoset FRP for chemical process plant equipments
Furanoflavonoid Furanose
Furantetracarboxylic acid Simple aromatic rings
References
External links Recent synthetic methods