<unk>  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 π system. Due to its aromaticity,<unk> '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. ::Resonance contributors of<unk>  *<unk>  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: ::Furan Diels–Alder reaction with ethyl (E)-3-nitroacrylate :Diels-Alder reaction of<unk>  with arynes provides corresponding derivatives of dihydronaphthalenes, which are useful intermediates in synthesis of other polycyclic aromatic compounds. ::Reaction of<unk>  with benzyne * Hydrogenation of<unk> s sequentially affords dihydrofurans and tetrahydrofurans. * In the Achmatowicz reaction,<unk> s are converted to dihydropyran compounds. * Pyrrole can be prepared industrially by reacting<unk>  and ammonia in the presence of solid acid catalysts, such as SiO2 and Al2O3.
Furan
Lime 
Naphthalene
Mycin
Gluten
Spirolactone
Nitrobenzene
Palate