Arenes

Question 1

Describe the Kekulé model.

Answer 1

• 3 localised π bonds
• Formed by sideways overlap of 2 neighbouring p-orbitals above and below the plane of the ring
• Each localised π bond contains 2 shared e⁻ between 2 carbon atoms
• Higher electron density

Question 2

Describe the delocalised model.

Answer 2

• 1 delocalised π bond
• Formed by sideways overlap of 6 p-orbitals above and below the plane of the ring
• The delocalised π bond consists of 6 shared e⁻ between 6 carbon atoms
• Lower electron density

Question 3

Evidence for a delocalised model

Answer 3

Kekulé structure is not correct because:
• Enthalpy change of hydrogenation of benzene is less exothermic than expected
• Benzene is less reactive than alkenes, it will only react with bromine at high temperatures or in the presence of a halogen carrier catalyst
• X-ray diffraction experiments show that all six C-C bonds in benzene have the same length, they are intermediate between short C=C and long C-C

Question 4

Benzene vs Alkenes

Answer 4

• Benzene is less reactive than alkenes
• Only reacts with Br₂ at high temperatures or in the presence of a halogen carrier.
• Alkenes have 1 localised π bond over 2C
• Benzene has 1 π bond delocalised over 6C
• Lower electron density than cyclohexene
• Unable to polarise Br₂

Question 5

Benzene vs Phenol

Answer 5

Phenol:
• Phenol is more reactive than benzene
• Lone pair of electrons from O is partially delocalised into the ring
• Higher electron density
• More able to polarise Br₂/attract electrophiles

Question 6

Uses of Phenols

Answer 6

• Antiseptics
• Disinfectants
• Resins for paints
• Production of plastics

Question 7

Nitration of Benzene

Answer 7

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

conc. HNO₃/conc. H₂SO₄/55⁰

Question 8

Generation of NO₂⁺ electrophile

Answer 8

H₂SO₄ + HNO₃ → HSO₄⁻ + NO₂⁺ + H₂O

Question 9

Regeneration of H₂SO₄ catalyst

Answer 9

HSO₄⁻ + H⁺ → H₂SO₄

Question 10

Reduction of Nitrobenzene

Answer 10

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

Sn catalyst/conc. HCl/Reflux

Question 11

Chlorination of benzene

Answer 11

C₆H₆ + Cl₂ → C₆H₅Cl + HCl

Cl₂/AlCl₃/Anhydrous

Question 12

Bromination of benzene - Reagents, Conditions, Observations

Answer 12

C₆H₆ + Br₂ → C₆H₅Br + HBr
Reagents: Br₂/FeBr₃(or Fe)
Conditions: Anhydrous
Observations: Orange to colourless
White precipitate formed

Question 13

Generation of Cl⁺ electrophile

Answer 13

AlCl₃ + Cl₂ → AlCl₄⁻ + Cl⁺

Question 14

Regeneration of AlCl₃ catalyst

Answer 14

AlCl₄⁻ + H⁺ → AlCl₃ + HCl

Question 15

Generation of Br⁺ electrophle

Answer 15

2Fe + 3Br₂ → 2FeBr₃

FeBr₃ + Br₂ → FeBr₄⁻ + Br⁺

Question 16

Regeneration of FeBr₃ catalyst

Answer 16

FeBr₄⁻ + H⁺ → FeBr₃ + HBr

Question 17

Phenol + Metal

Answer 17

C₆H₅OH + Na → C₆H₅ONa + ½H₂

Na dissolves/effervescence/phenol dissolves

Question 18

Phenol + Base

Answer 18

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

Phenol dissolves

Question 19

Phenol + Sodium Carbonate

Answer 19

No Reaction