Benzene is a cyclic hydrocarbon with a chemical formula C6H6, that is, each carbon atom in benzene is arranged in a six-membered ring and is bonded to only one hydrogen atom. According to molecular orbital theory, benzene ring involves the formation of three delocalized π – orbitals spanning all six carbon atoms, while the valence bond theory describes two stable resonance structures for the ring.
Resonance: The oscillating double bonds in the benzene ring are explained with the help of resonance structures as per valence bond theory. All the carbon atoms in the benzene ring are sp2 hybridized. One of the twosp2 hybridized orbital of one atom overlaps with sp2 orbital of adjacent carbon atom forming six C-C sigma bonds. Other left sp2 hybridized orbitals combine with s orbital of hydrogen to form six C-H sigma bonds. Remaining unhybridized p orbitals of carbon atoms form π bonds with adjacent carbon atoms by lateral overlap. This explains an equal possibility for the formation of C1 –C2, C3 – C4, C5 – C6 π bonds or C2 – C3, C4 – C5, C6-C1 π bonds. The hybrid structure is represented by inserting a circle in the ring as shown below in the figure. Hence, it explains the formation of two resonance structures proposed by Kekule.
Aromaticity of benzene: Benzene is an aromatic compound, as the C-C bonds formed in the ring are not exactly single or double, rather they are of intermediate length. Aromatic compounds are divided into two categories: benzenoids (one containing benzene ring) and non-benzenoids (those not containing benzene ring), provided they follow Huckel rule. According to Huckel rule, for a ring to be aromatic it should have following property:
- Complete delocalization of the π electrons in the ring
- Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .)
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