Furan, also known as furane and furfuran, is a heterocyclic organic compound. It is typically derived by the thermal decomposition of pentose-containing materials, cellulosic solids especially pine-wood. Furan is a colorless, flammable, highly volatile liquid with a boiling point close to room temperature. It is toxic and may be carcinogenic. Catalytic hydrogenation (see redox) of furan with a palladium catalyst gives tetrahydrofuran.
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
Synthesis and isolation
Furan is aromatic
because one of the lone pairs
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.
- Furan serves as a diene in Diels-Alder reactions with electron-deficient dienophiles such as ethyl (E)-3-nitroacrylate. The reaction product is a mixture of isomers with preference for the endo isomer: