25.1 Introducing Benzene

Question 1

What is the formula of benzene?

Answer 1

C₆H₆

Question 2

Draw & label the Kekulé model of benzene:

Answer 2

Question 2

Draw the delocalised model for benzene:

Answer 2

Question 3

What is the difference between the Kekulé & delocalised model of benzene?

Answer 3

• In delocalised model, electrons are delocalised in a ring.
• In Kekulé model, electrons are localised in pi-bonds

Question 4

Describe & explain the evidence which disproves the accuracy of the Kekulé model:

Answer 4

• LACK OF REACTIVITY
  
  • Since benzene contains double bonds, it should undergo electrophilic addition with bromine & decolourise bromine water.
  • However, benzene does not decolourise bromine water.
• BOND LENGTH
  
  • Every carbon-carbon bond in benzene are longer than C＝C but shorter than C―C
  • but in Kekulé model, alternating single & double bonds suggests that bond length alternates.
• HYDROGENATION ENTHALPIES
  
  • Since benzene is depicted to have 3 double bonds, it should have the hydrogenation enthalpy of 3x that of cyclohexene.
  • However, actual hydrogenation enthalpy is less exothermic than expected.
  • therefore actual structure of benzene is more stable than Kekulé model.

Question 5

Describe the bonding within the delocalised model of benzene:

Answer 5

• Each carbon has 1e^- in p-orbital at right angle to plane
• Adacent p-orbitals overlap above + below the plane
  
  • Creates system of pi bonds spread over 6 carbons
• forms ring of delocalised electrons
  
  • consists of 6e^-