the night working out the consequences of the hypothesis. Let us learn to dream, gentlemen, and then perhaps we shall learn the truth but let us beware of making our dreams public before they have been approved by the waking mind.”( ). One hundred years later, on the occasion of Kekulé’s centenary celebrations a group of compounds having polybenzenoid structures have been named as Kekulenes. FRIEDRICH AUGUST KEKULÉ (7th September –13th July ) However, benzene was found to form only one ortho disubstituted product.
This problem was overcome by Kekul é by suggesting the concept of oscillating nature of double bonds in benzene as given below. Even with this modification, Kekul é structure of benzene fails to explain unusual stability and preference to substitution reactions than addition reactions, which could later on be explained by resonance. Resonance and stability of benzene According to Valence Bond Theory, the concept of oscillating double bonds in benzene is now explained by resonance. Benzene is a hybrid of various resonating structures.
The two structures, A and B given by Kekulé are the main contributing structures. The hybrid structure is represented by inserting a circle or a dotted circle in the hexagon as shown in (C). The circle represents the six electrons which are delocalised between the six carbon atoms of the benzene ring. (A) (B) (C) The orbital overlapping gives us better picture about the structure of benzene.
All the six carbon atoms in benzene are sp hybridized. Two sp hybrid orbitals of each carbon atom overlap with sp hybrid orbitals of adjacent carbon atoms to form six C—C sigma bonds which are in the hexagonal plane. The remaining sp hybrid orbital of each carbon atom overlaps with s orbital of a hydrogen atom to form six C—H sigma bonds. Each carbon atom is now left with one unhybridised p orbital perpendicular to the plane of the ring as shown below: The unhybridised p orbital of carbon atoms are close enough to form a π bond by lateral overlap.
There are two equal possibilities of forming three π bonds by