6.1.1 Aromatic Compounds

Question 1

Evidence for Kekulé vs Delocalised Model of Benzene

Answer 1

• Benzene is resistant to electrophilic attack (only reacts with Br2 at a high temp / halogen carrier)
• Bond length in between short C=C and long C–C
• Enthalpy change of Hydrogenation less exothermic than expected (compared to cyclohexene)

Question 2

Delocalised Model of Benzene

Answer 2

• planar cyclic hydrocarbon with 6 carbons and 6 hydrogens in a ring
• sideways overlap of adjacent p-orbitals to form π-bonds
• π-bonds are delocalised into the ring

Question 3

Benzene → Nitrobenzene

Answer 3

Reaction: Nitration
Mechanism: Electrophilic substitution
Reagents: HNO3, acid catalyst e.g. H2SO4, heat (50°C)
C6H6 + HNO3 → C6H5NO2 + H2O

Question 4

Benzene → Bromobenzene

Answer 4

Reaction: Halogenation
Mechanism: Electrophilic substitution
Step 1: Br2 + FeBr3 → FeBr4- + Br+
Step 2: C6H6 → C6H6Br+ → C6H5Br + H+
Step 3: FeBr4- + H+ → FeBr3 + HBr

Question 5

Benzene → Alkylbenzene

Answer 5

Reaction: Alkylation
Mechanism: Electrophilic substitution (Friedel-Crafts reaction)
Reagents: haloalkane, halogen carrier e.g. AlCl3
C6H6 + RCl → C6H5R + HCl

Question 6

Benzene → Ketone

Answer 6

Reaction: Acylation
Mechanism: electrophilic substitution
Reagents: Acyl chloride, halogen carrier e.g. AlCl3
C6H6 + RCOCl → C6H5C(O)R + HCl

Question 7

Reactivity of alkenes vs arenes vs phenol

Answer 7

• Benzene π-bond electrons delocalised
• Phenol a lone pair of electrons on the oxygen of the OH group is partially delocalised into the ring
• Cyclohexene electrons localised between 2 carbons in the π-bond
• benzene has a lower electron density/phenol has a higher electron density/cyclohexene has a higher electron density
• benzene cannot polarise Br2/phenol can polarise the Br2/cyclohexene can polarise the Br–Br bond

Question 8

Acidity of phenol

Answer 8

Alcohol < Phenol < Carboxylic acid

• Ethanol does not react with NaOH (strong) / Na2CO3 (weak)
• Phenol reacts with NaOh (strong) but not Na2CO3 (weak)
• Carboxylic acids react with NaOH (strong) / Na2CO3 (weak) → CO2 gas produced

Question 9

Phenol → 2,4,6-tribromophenol

Answer 9

Reaction: Bromination
Reagents: Br2 water decolourised → white ppt
C6H5OH + 3Br2 → C6H2Br3OH + 3HBr

Question 10

Phenol → 2/4-Nitrophenol

Answer 10

Reaction: Nitration
Reagents: dilute HNO3
C6H5OH + HNO3 → C6H4NO2OH + H2O

Question 11

2 and 4- directing groups (activating)

Answer 11

–NH2 or –NHR
–OH or –OR
–C6H5 or –R
–F, –Cl, –Br, –I

Question 12

3- directing groups (deactivating)

Answer 12

–COOR or –COOH
–CHO
–RC(O)R
–CN
–NO2 or –NR3+
–SO3H