The delocalised model of benzene

Question 1

Write the properties of the delocalised model of benzene.

Answer 1

• Benzene is a cyclic hydrocarbon with the molecular formula C6H6.
• The six carbon atoms are arranged in a planar hexagonal ring. Each carbon atom is bonded to two other carbon atoms and one hydrogen atoms.
• The shape around each carbon atom is trigonal planar with a bond angle of 120*.
• Each carbon atom has four outer shell electrons. Three of these electrons bond to two other carbon atoms and one hydrogen atom. The three bonds in this plane are called sigma bonds. This leaves a fourth outer shell electron in a 2p orbital above and below the plane of the carbon atoms.
• The electron in a p-orbital of a carbon atom overlaps with the electrons in the p-orbitals of the carbon atoms on either side. This results in a ring of electron density above and below the plane of the carbon atoms.
• This overlap produces a system of pi-bonds which spread over all six carbon atoms. The p electrons are no longer held between just two carbon atoms. The p electrons are now spread over the whole ring and are said to be delocalised.

Question 2

Define the term “ Delocalised electrons “

Answer 2

Delocalised electrons are shared between more than two atoms.

Question 3

What does the increased stability of benzene mean?

Answer 3

This increased stability means that benzene struggles to take part in addition reactions.

Question 4

Give examples of typical alkene reactions that benzene does not take part in.

Answer 4

Under normal conditions, benzene does not:

• Decolourise bromine water
• react with strong acids such as HCl
• react with the halogens chlorine, bromine or iodine

Question 5

Why does benzene not take place in many addition reactions?

Answer 5

In an addition reaction. electrons from the delocalised system would need to bond to the atom or group of atoms being added. This would result in the product being less stable than benzene, and the reaction would not then be energetically favourable. Addition reactions would disrupt the delocalisation of the ring structure.

Question 6

Explain why instead of addition reactions, benzene takes place in substitution reactions?

Answer 6

One of benzene’s hydrogen atoms is replaced by another atom or group of atoms. The organic product formed retains the delocalisation, and hence the stability, of the benzene ring.