Furan is a heterocyclic organic compound, consisting of a five-membered aromatic ring with four carbon Atom and one oxygen atom. 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 Solvent, including ethanol, diethyl ether, and acetone, and is slightly soluble in water.[ DHHS (NIOSH) Publication No. 2016–171, p. 2, Accessed Nov 2019] It is toxic and may be carcinogenic in humans. Furan is used as a starting point for other speciality chemicals.
History
The name "furan" comes from the
Latin furfur, which means
bran (
furfural is produced from 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" (as if it were a four-carbon analog to
phenol, C
6H
5OH).
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 2-furoic acid, followed by decarboxylation.
Synthesis of furans
The Feist–Benary synthesis is a classic way to synthesize furans. The reaction involves alkylation of 1,3-
with α-bromoketones followed by dehydration of an intermediate hydroxy
dihydrofuran.
The other traditional route involve the reaction of
ketone with phosphorus pentoxide (P
2O
5) in the Paal–Knorr synthesis.
Many routes exist for the synthesis of substituted furans.
File:Ranitidine Structural Formulae.png|The drug ranitidine, also known as Zantac.
File:Rosefuran-2D-skeletal.png|Rosefuran, an aroma compound found in rose oil.[
File:Furfural.svg|Furfural, derived from sugars, is the major source of furans
File:MethanofuranFeb2011.png|methanofuran is a cofactor in methanogenesis.
]
Structure and bonding
Furan has
aromatic character because one of the
of
on the oxygen atom is delocalized into the ring, creating a aromatic system (see Hückel's rule). The aromaticity is modest relative to that for
benzene and related heterocycles
thiophene and
pyrrole. The
resonance energy of benzene,
pyrrole,
thiophene, and furan are, respectively, . Thus, these heterocycles, especially furan, are far less aromatic than benzene, as is manifested in the lability of these rings.
The molecule is flat but the C=C groups attached to oxygen retain significant
double bond character. The other lone pair of electrons of the oxygen atom extends in the plane of the flat ring system.
Examination of the resonance contributors shows the increased electron density of the ring relative to benzene, leading to increased rates of electrophilic substitution.
- :
Reactivity
Because of its partial aromatic character, furan's behavior is intermediate between that of an enol ether and an aromatic ring. It is dissimilar vs
diethyl ether such as
tetrahydrofuran.
Like enol ethers, 2,5-disubstituted furans are susceptible to hydrolysis to reversibly give 1,4-diketones.
Furan serves as a diene in Diels–Alder reactions with electron-deficient such as ethyl ( E)-3-nitroacrylate.
- :
Diels-Alder reaction of furan with arynes provides corresponding derivatives of dihydronaphthalenes, which are useful intermediates in synthesis of other polycyclic aromatic compounds.
- :
-
It is considerably more reactive than benzene in electrophilic substitution reactions, due to the electron-donating effects of the oxygen heteroatom. It reacts with bromine at 0 °C to give 2-bromofuran.
Safety
Furan is found in heat-treated commercial foods and is produced through thermal degradation of natural food constituents.
It can be found in roasted
coffee, instant coffee, and processed
.
[ ]
Exposure to furan at doses about 2,000 times the projected level of human exposure from foods increases the risk of liver cancer in rats and mice and bile duct tumors in rats.[
]
See also
External links
