6.1 - Aromatic compounds, carbonyls and acids

Question 1

Explain the relative resistance to chlorination of compound C (benzene) compared with compound
B (cyclic alkene). (3)

Answer 1

Reactivity of B
- in B electrons are localised / in B π-bond is localised

Reactivity of C
- in C electrons are delocalised / In C π-system / ring is delocalised

• In B, electron density is higher / B is more susceptible to electrophilic attack
• B attracts/accepts the electrophile/Cl2 more / B polarises the electrophile/Cl2 more

Question 2

Describe, in terms of orbital overlap, the similarities and differences between the bonding
in the Kekulé model and the delocalised model of benzene. (3)

Answer 2

Similarities:
- Orbital overlap (sideways) overlap of p orbitals ✓
- π bonds above and below (bonding (C) atoms

Difference

• Kekule has: alternating π bonds / 3 π bonds / localised (π electrons) / overlap in one direction / 2 electrons in π bond
  Delocalised has: π ring (system) / all p orbitals overlap OR (π electrons) spread around ring / overlap in both directions / 6 electrons in π bond /

Question 3

Experimental evidence led to the general acceptance of the delocalised model over the Kekulé model. Describe two pieces of evidence to support the delocalised model of benzene. (2)

Answer 3

Bond length:
- (C–C) bond length is between single (C–C) and double bond (C=C)
OR all (C–C) bond lengths are the same

ΔH hydrogenation:
- ΔH hydrogenation less (exothermic) than expected

Resistance to reaction:
- Benzene is less reactive than alkenes/ bromination of benzene requires a catalyst/halogen carrier
- Benzene does not react with/decolourise bromine (at room
temperature) / benzene reacts by substitution / benzene does not (readily) react by addition

Question 4

Explain why phenol is nitrated more readily than benzene. (2)

Answer 4

• (In phenol) a (lone) pair of electrons on O is(partially) delocalised/donated into the π-system / ring
• Electron density increases/is higher (than benzene)
• (phenol) is more susceptible to electrophilic attack / (phenol) attracts/accepts electrophile/HNO3 more / (phenol) polarises electrophile/HNO3 more