Module 6: Aromatic Compounds (6.1.1)

Question 1

Kekule Model & Evidence

Answer 1

1. 6 Carbon ring with alternating single and double bond.
   • Evidence to disprove:
2. Did not decolourise bromine under electrophilic addition.
3. Enthalpy change less than expected. Benzene (-208kJmol-1) expected (-360kJmol-1)
4. X-ray diffraction found bond length in between single and double bond lengths.

Question 2

Delocalised Model of Benzene

Answer 2

1. Planar, cyclic hydrocarbon containing 6 C and 6 H.
2. Each C atom has 1 e- in P orbital at right angles to the plane.
3. Adjacent P orbitals overlap to form a ring of electron density.
4. System of π bonds all across structure which are delocalised.

Question 3

Electrophilic Substitution - Nitration of Benzene

Answer 3

• 50°C catalysed by H₂SO₄

1. HNO₃ + H₂SO₄ ——-> NO₂+ HSO₄- + H₂O
2. Mechanism
3. H+ + HSO₄ ——-> H2SO₄
4. Further heating gives 1,3-dinitrobenzene.

Question 4

Electrophilic Substitution - Halogenation of Benzene

Answer 4

• Room temp + Pressure

1. Br₂ + AlBr₃ ——-> Br+ + AlBr₄-
2. Mechanism
3. H+ + AlBr₄- ——-> AlBr₃ + HBr

Question 5

Alkylation

Answer 5

1. Increases C chain length.
2. C₆H₆ + C₂H₅Cl ——-> C₆H₅C₂H₅ + HCl
3. AlCl₃ above the arrow.

Question 6

Acylation

Answer 6

1. Increases C chain length.
2. C6H6 + CH₃COCl ——-> C6H5CH₃CO + HCl
3. Phenylethanone made.

Question 7

Reactivity o Alkenes vs. Arenes

Question 8

Acidity of Phenol

Answer 8

1. Less soluble in water than alcohols due to non polar benzene ring.
2. Partially dissociates to form phenoxide ion and H+.
3. Phenols and carboxylic acids reacted with strong bases like NaOH to form Sodium Phenoxide ion.
4. Only carboxylic acids able to react with NaCO₃ to form CO₂ gas.

Question 9

Electrophilic Addition - Nitration of Phenol

Answer 9

1. Dilute HNO₃ at room temperature
2. Mixture of 2-nitrophenol and 4-nitrophenol formed.
3. Lone pair of e- from Oxygen is donated into π system.
4. Increased electron density attracts electrophiles more, making phenol more susceptible to attacks.
5. Polarise Br molecule with no catalyst.

Question 10

Electrophilic Addition - of Bromination Phenol

Answer 10

1. No halogen carrier required and room temp

2. Decolourises bromine water and forms a white precipitate of 2,4,6-tribromophenol.

Question 11

Directing Groups

Answer 11

1. Activating Groups - Direct electrophile to 2, 4 or 6.
2. Activates ring as aromatic ring reacts more readily with electrophiles.
3. Deactivating Groups - Direct electrophile to 3 and 5.
4. • Charge attracts electrons towards it and reduces electron density.