Structure & Bonding in Benzene

Question 1

What is the kekule model for benzene?

Answer 1

Hexagonal ring with alternating double bonds

Question 2

What is the delocalised model of benzene?

Answer 2

Hexagonal ring with a circle in the centre

Question 3

What are the properties of benzene expected if kekule was right?

Answer 3

Undergo addition reactions (3 x C–C)
To react with halogens without a catalyst, like alkenes (localised 𝛑 electrons able to polarise X2)
Irregular hexagonal ring with alternating bond lengths (C-C longer than C–C)
Enthalpy change of hydrogenation should be 3x more exothermic than for cyclohexane (-129x3 = -360KJmol-1)

Question 4

What was the experimental evidence that led to the updated model of benzene?

Answer 4

• It undergoes substitution reactions and not addition
• When benzene reacts it requires a catalyst = the e- density in benzene ring is unable to polarise halogen molecules.
• X-ray diffraction shows all C-C bonds are “intermediate in length” between C-C and C–C = Regular hexagonal shape
• Enthalpy change of hydrogenation is less exothermic than predicted from cyclohexane

Question 5

Describe the bonding in the delocalised model of benzene

Answer 5

Each C atom forms 3 σ (to 2 C and H) and also has an e- in p orbital.
The p orbitals overlap forming 𝛑 bonds (they are shared between 2 atoms).
The electrons in the 𝛑 bonds become delocalised (no longer shared), leading to a 𝛑-electron cloud above and below the ring