Arenes

Question 1

Physical properties of benzene

Answer 1

Structure: Trigonal planar, non-polar
1. low MP and BP
2. Soluble in organic solvents, insoluble in water
3. poor electrical conductivity
( electrons in benzene delocalises within benzene molecule. electrons need to delocalise across benzene molecules)

Question 2

Explain the resonance stability of benzene

Answer 2

• Each carbon atom is sp2 hybridised, hence each C atom has an unhybridised p orbital
• The p orbital will all form a continuous sideway overlap
• The pi electrons delocalise across all 6 C atoms
• Delocalised pi electron cloud formed, making benzene very stable

Question 3

Why does benzene undergo substitution and not addition?

Answer 3

• Substitution preserves the stability of benzene
• Addition disrupts the stability of benzene, which is energetically unfavourable

Question 4

Electrophilic substitution with NO2+ to form nitrobenzene

Answer 4

• concentrated HNO3, concentrated H2SO4 catalyst, 55 degrees

Question 5

Electrophilic substitution with halogen

Answer 5

Cl2(g), anhydrous AlCl3/FeCl3(s), warm OR
Br2(g), anhydrous AlBr3/FeBr3(s), warm

Question 6

Alkylation (Step-up reaction)

Answer 6

RX, anhydrous AlX3/FeX3, warm

Question 8

Electrophilic substitution (Nitration) for methylbenzenes

Answer 8

concentrated HNO3, concentrated H2SO4 catalyst, 30 degrees

Question 9

Electrophilic Substitution (Halogenation) for methylbenzenes

Answer 9

Cl2(g), anhydrous AlCl3/FeCl3(s), room temperature OR
Br2(g), anhydrous AlBr3/FeBr3(s), room temperature

Question 10

FRS for methylbenzenes

Answer 10

Monosub: Limited Cl2/Br2, UV light
Multisub: Excess Cl2/Br2, UV light (remove all H in alkyl)

Question 11

Side-chain oxidation for methylbenzenes

Answer 11

Acidic condition:
- KMnO4, dilute H2SO4, heat under reflux
Basic condition:
- KMnO4, dilute NaOH, heat under reflux
- methylbenzene: benzoic acid (white solid) + H2O
- for ethylbenzene and longer, benzoic acid (white solid) + CO2, H2O

Question 12

Why electrophilic substitution for methylbenzene requires milder conditions?

Answer 12

• presence of e-donating alkyl group increases electron density of benzene ring
• methyl benzene more susceptible to electrophilic attack than benzene and is more reactive

Question 13

electron-donating groups are () groups
electron-withdrawing are () groups

Answer 13

electron-donating are (activating) groups
electron-withdrawing are (deactivating groups)