25.1 - Benzene

Question 1

What is the Kekule model and evidence disproving it?

Answer 1

Suggested benze was cyclical, alternating C-C and C=C

Evidence Disproving

1. Benzene only reacts with Br₂ at high temperatures or in the presence of a halogen carrier
   
   • Is resistant to electrophilic attack
   • Does not decolourise bromine water​
2. Bond length of benzene is an intermediate between C=C (long) and C-C (short)
3. Enthalpy change of hydrogenation of Kekule is LESS EXOTHERMIC than bezene
   
   • Benzene is more stable​

Question 2

What are the charateristics of the delocalised model?

Answer 2

• Benzen is planar, cyclical, hexagonal
• Each carbon uses 3 electrons to bond to -H and 2 -C, the other electron is in the p orbital at right angles to the C-C bonds
• adjacent p-orbitals overlap above and below the molecular plane forming a ring of electron density
• System of π-bonds creates spread of delocalised electrons over the 6 carbond

Question 3

What are the similarities and differences between the bonding in the Kekule and Delocalised model?

Answer 3

Similarities

• Sideways overlap of p-orbitals (show diagram, label the p-orbital)
• π-bond/system/ring above and below the bonding carbons in Kekule, and the plane in delovalised

Differences

• Kekule has alternating π-bonds
  
  • ​Localised π electrons
  • Has 2 electrons per π-bond
• Delocalised has a π-ring
  
  • All p-orbitals overlap
  • 6 electrons per π-bond

Question 4

How are aromatic compounds with 1 substituent group named?

Answer 4

• Benzene is the parent change
• EXCEPTION: if the alkyl chain is >6 carbnon atoms, benzene is substituent, prefix is phenyl