2 - The Structure Of Benzene

Question 0

What was Kekulé’s proposed equilibrium structure?

Answer 0

The double and single bonds constantly switching places.

Question 1

Why was Kekulé’s theory flawed?

Answer 1

The double bonds in benzene should act the same as those in alkenes. Thereby they should decolourise bromine water.

Question 2

What were the issues with carbon’s bond length?

Answer 2

Kekulé’s structure was symmetrical, which didn’t work as single and double bonds are different lengths. In reality there was an average bond length of 0.139nm (compared to 0.153 for C-C, and 0.134nm for C=C).

Question 3

What was the issue with the enthalpy change of combustion?

Answer 3

Alkene + Hydrogen - A way of testing enthalpy of combustion. Cyclohexane has 1 double bond, compared to the 3 in benzene, and an enthalpy value of -120KJmol-1. The experimental value was actually -208KJMol-1

Question 4

What is the delocalisation energy of benzene?

Answer 4

360-208=152KJ mol-1 - this is known as the delocalisation energy for benzene.