Furan aromaticity

Furan has a cyclic compound, and its ring contains five members, fourcarbon atoms and one oxygen atom.

All of the four carbon atoms are sp² hybridized. Among the three sp² obitals in each carbonatom, two of them form two σ bonds with two neigbouring carbon atoms, and therest one forms one σ bond with a hydrogen atom. The unpaired electron in the unhybrized 2p orbital of each carbon atom isπ electron.

The oxygen atom is also sp² hybridized. Among the three sp² orbitals in the oxygen atom, two of them form two σbonds with the two neigbouring carbon atoms, and the rest one is a lone pair ofelectrons. The two unpaired electrons inthe unhybridized 2p obital are π electrons.

Conclusively, there is one π electron in each of the four carbon atoms and two πelectrons in the oxygen atom. In thefuran ring, there are totally six π electrons and fulfill the rule of (4n + 2) πelectrons for aromaticity. Therefore,furan is aromatic.

The delocalization of the 6π electrons in furan molecule can be shown in thediagram below :
http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Furan-2D-full.svg/300px-Furan-2D-full.svg.png

2012-12-11 23:24:01 補充：
To explain chemical bonding using hybridization, concentrate on orbitals but NOT electrons. The hybrized orbitals are used to form sigma bonds and lone pairs, and the unhybrized p obitals are called pi orbitals which contains pi electron(s) (if any).

2012-12-11 23:27:54 補充：
O atom is sp² hybridized and one of the 2p orbitals are not hybridized. Two sp² orbitals contains unpaired electrons to form sigma bonds with 2 neighbouring C atoms, and one sp² orbital contains two electrons forms a lone pair. The unhybridized 2p orbital contains two pi electrons.

2012-12-13 00:59:59 補充：
Hybridization is a theory to explain the phenomenon. Only sp² hybridization of O atom (1 sp² oribital is a lone pair) can explain the aromaticity of furan.

O atom contributes 2 pi electrons for delocalization, but there is no formal pi bond formed.

2012-12-18 17:17:35 補充：
The oxygen atom in furan molecule obeys the octet rule.

2012-12-18 17:18:37 補充：
Oxygen does not have low-energy valence d orbital, and thus it cannot expand its octet.