Furan has aromatic character 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). The aromaticity is modest relative to that for benzene and related heterocycles thiophene and pyrrole. The resonance energies of benzene, pyrrole, thiophene, and furan are, respectively, 152, 88, 121, and 67 kJ/mol (36, 21, 29, and 16 kcal/mol). Thus, these heterocycles, especially furan, are far less aromatic than benzene, as is manifested in the lability of these rings.[13] 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, leading to increased rates of electrophilic substitution.[14]
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 ethers such as tetrahydrofuran.
Like enol ethers, 2,5-disubstituted furans are susceptible to hydrolysis to reversibly give 1,4-diketones.
Diels-Alder reaction of furan with arynes provides corresponding derivatives of dihydronaphthalenes, which are useful intermediates in synthesis of other polycyclic aromatic compounds.[16]
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.
Furan is found in heat-treated commercial foods and is produced through thermal degradation of natural food constituents.[18][19] It can be found in roasted coffee, instant coffee, and processed baby foods.[19][20][21] Research has indicated that coffee made in espressomakers and coffee made from capsules contain more furan than that made in traditional drip coffee makers, although the levels are still within safe health limits.[22]
Exposure to furan at doses about 2,000 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.[23] Furan is therefore listed as a possible human carcinogen.[23]
See also
BS 4994 – Furan resin as thermoset FRP for chemical process plant equipments