6.1 - Aromatic Compounds, Carbonyls and Acids

Question 1

Describe the bonding in benzene (C6H6) using Kekulé’s model and the delocalised model.

Answer 1

Kekulé’s model. A ring of 6 carbon atoms joined by alternate single and double bonds. Benzene is not very reactive (will not react with bromine under standard conditions) so two forms of the molecule in rapid equilibrium were suggested. It was hypothesised that the bromine molecule could not be attracted to the double bond before the structure changed.
Delocalised model. Carbon atoms are arranged in a planar hexagonal ring. 120°, trigonal planar around each carbon. 1 sigma bond between each carbon. The p-orbitals overlap to form a delocalised pi system. The delocalised pi system is made up of two ring-shaped electron clouds (one above and one below the plane of the six carbon atoms). To achieve maximum overlap the molecule must be planar.