Exam questions- CD

Question 1

Explain in terms of reactions of benzene why the delocalised model of benzene is preferred over the Kekule model:

Answer 1

• Benzene undergoes substitution more easily than addition
• Benzene does not readily undergo addition
• Because it has no double bonds
• 1 electron from each carbon atom is delocalised, forming a charge cloud, above and below the plane of the molecule

Question 2

Describe how the bonding in benzene accounts for the fact that all the C-C bond lengths are equal:

Answer 2

1 electron from each carbon is delocalised

So no double bonds

Question 3

Explain in terms of delocalisation and energy levels why a dye is yellow and benzene is colourless

Answer 3

• Electrons move to a higher energy level
• Absorb electromagnetic radiation deltaE=hv
• Yellow dye has more delocalisation so energy levels will be closer together + deltaE smaller
• Energy absorbed is at lower visible frequencies
• Complementary colour is transmitted

Question 4

Give the reagents + conditions for the laboratory conversion of benzene to nitrobenzene

Answer 4

• Concentrated nitric acid + sulphuric acids

- Keep temperature below 55 degrees