Benzene

Question 1

What is the kekule model?

Answer 1

P orbitals overlap sideways in one direction to form 3 localised Pi bonds
Alternate long c-c bonds and c=c bonds

Question 2

Explain the delocalised pi electron system

Answer 2

6 p orbitals overlap sideways in both directions to form a delocalised pi system of 6 shared electrons spread out between the 6 C atoms above and below the plane of atoms

Question 3

What is the evidence for the delocalised pi system?

Answer 3

1. All c-c bonds in benzene are of equal length
2. The enthalpy of hydrogenation value for benzene is less exothermic that if it had 3 C=C bonds
3. Benzene is unreactive compared to alkenes and reacts through electrophilic substitution rather than addition

Question 4

What is the reactivity of benzene compared to alkenes and why?

Answer 4

Relatively unreactive

• high stability of delocalised pi system and lower electron density of the pi bonds
• therefore only electrophiles with a very strong attraction for a pair or electrons can react
• undergoes electrophilic substitution (replaces a H)

Question 5

Why are alkenes more reactive with bromine than benzene?

Answer 5

• higher electron density and therefore stronger ability to polarise Br2 to form an induced dipole
• reacts rapidly with Br2 at room temp where as benzene needs a halogen carrier catalyst to create the electrophile