Furan

Furan

Names
IUPAC name Furan
Systematic IUPAC name Oxole 5-Oxacyclopenta-1,3-diene 5-Oxacyclo-1,3-pentadiene 1,4-Epoxybuta-1,3-diene 1,4-Epoxy-1,3-butadiene
Other names Furfuran Furane (misspelling)[1] Divinylene oxide
Identifiers
CAS Registry Number	110-00-9 Y
ChEBI	CHEBI:35559 Y
ChEMBL	ChEMBL278980 Y
ChemSpider	7738 Y
InChI InChI=1S/C4H4O/c1-2-4-5-3-1/h1-4H Y Key: YLQBMQCUIZJEEH-UHFFFAOYSA-N Y InChI=1/C4H4O/c1-2-4-5-3-1/h1-4H Key: YLQBMQCUIZJEEH-UHFFFAOYAC
Jmol-3D images	Image
KEGG	C14275 Y
PubChem	8029
SMILES c1ccoc1
Properties
Molecular formula	C4H4O
Molar mass	68.07 g·mol−1
Appearance	Colorless, volatile liquid
Density	0.936 g/mL
Melting point	−85.6 °C (−122.1 °F; 187.6 K)
Boiling point	31.3 °C (88.3 °F; 304.4 K)
Hazards
MSDS	Pennakem
R-phrases	R26/27/28, R45
S-phrases	S16, S37, S45, S28
NFPA 704	4 3 1
Flash point	−69 °C (−92 °F; 204 K)
Autoignition temperature	390 °C (734 °F; 663 K)
Explosive limits	Lower:2.3%, upper:14.3% @ 20 °C
LD50 (Median lethal dose)	> 2 g/kg (rat)
Related compounds
Related heterocycles	Pyrrole Thiophene
Related compounds	Tetrahydrofuran (THF) 2,5-Dimethylfuran Benzofuran Dibenzofuran
Structure
Point group	C2v
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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.^[2] It is toxic and may be carcinogenic in humans. Furan is used as a starting point to other specialty chemicals.^[3]

History

The name furan comes from the Latin furfur, which means bran.^[4] 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.^[5][6]

Production

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

In the laboratory, furan can be obtained from furfural by oxidation to furan-2-carboxylic acid, followed by decarboxylation.^[7] 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.^[8] 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.^[9]

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 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.^[10]

• Furan serves as a diene in Diels-Alder reactions with electron-deficient dienophiles such as ethyl (E)-3-nitroacrylate.^[11] 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.^[12]

• 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.^[13][14] Notably, it can be found in roasted coffee, instant coffee, and processed baby foods.^[15][14][16] 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.^[17] Furan is therefore listed as a possible human carcinogen.^[17]

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