1What thermochemical evidence supports the structure of benzene?
Hydrogenation enthalpies show benzene is more stable than expected.
Data:
Cyclohexene ΔH°ₕ = –120 kJ/mol
Benzene ΔH°ₕ (expected for 3 double bonds) = 3 × –120 = –360 kJ/mol
Observed ΔH°ₕ = –208 kJ/mol → much less exothermic
Conclusion: Benzene is more stable due to the delocalised electrons in the benzene ring (lower electron density)
2How does X-ray diffraction provide evidence for benzene structure?
bond lengths.
Observation: All C–C bonds are equal (~0.139 nm)
Conclusion: Benzene does not have alternating single and double bonds → consistent with delocalised ring structure
3How does infrared (IR) spectroscopy support benzene structure?
IR shows C=C bond stretching vibrations.
Observation:
C–H stretching: ~3100 cm⁻¹
C=C stretching: 1500–1600 cm⁻¹
Conclusion: Shows aromatic ring with delocalised π electrons
4What is the delocalised model of benzene?
Each carbon has a p-orbital above and below to the ring plane
Sideways overlap of p-orbitals above and below the plane forms a delocalised π system
Electrons are shared evenly around the ring, not in alternating single/double bonds
5How does the delocalised model of benzene explain its stability?
Delocalised electrons in benzene means the π electrons are spread out over the whole ring, not stuck between two atoms, so the lower electron density which makes it more stable
6Why is benzene resistant to bromination compared to alkenes?
-alkenes have a high electron density due to the localised electrons in the C=C
-benzene has low electron density at a single bond due to the electrons being delocalised over a ring
7what is the combustion reaction of benzene
Combustion reaction
Reaction: C₆H₆ + 7.5 O₂ → 6 CO₂ + 3 H₂O
Observation: Smoky flame
-mostly incomplete due to large ratio of benzene: water
8what is the bromination reaction of bezene
(Electrophilic substitution)
- formation of electrophile: Br2 + AlBr3 ---> (Br+) + (AlBr4-)
-Reaction: C₆H₆ + Br₂ → C₆H₅Br + HBr
-Conditions:
- heat under reflux, halogen carrier catalyst, iron fillings catalyst
9what is the Nitration reaction of benzene
(electrophilic substitution)
-Formation of electrophile: HNO3 + H2SO4 --> (NO2+) + (HSO4-) + H2O
-Electrophilic subsitution: C6H6 + HNO3 + H2SO4 ---> C6H6(NO2) + H2O
-Conditions:
-H2SO4 Catalyst
-Warm with HNO3 (50-60 Celsius)
10what is the sulfonation reaction of benzene
(electrophilic substitution)
-formation of electrophile: SO3 + H2SO4 <---> HSO3+ + HSO4-
-electrophilic subsitution: C6H6 + H2SO4 ---> C6H6(SO3H) +H2O
Conditions:
- warm with fuming sulfuric acid(40 Celsius)
11what is the friedel crafts alkylation
Friedel–Crafts alkylation (electrophilic substitution)
(form the electrophile) CH3Cl + AlCl3 ---> (CH3+) + (AlCl4-) catalyst: AlCl3, conditions: anyhdrous
(electrophilic subsitiution)C6H6 + CH3Cl ---> C6H6(CH3) + HCl
-conditions:
- halogen carrier, anyhydrous(dry)
12what is the friedel crafts acylation
(formation of electrophile )-CH3COCl + AlCl3 ---> (CH3CO+) + (AlCl4-)
(electrophilic subsituution)- C6H6 + CH3COCl ---> C6H6(COCH3) + HCl
-Conditions:
-halogen carrier, anyhydrous
13How does phenol react with bromine water?
Electrophilic substitution reaction
Reaction: C₆H₅OH + 3 Br₂ → C₆H₂Br₃OH + 3 HBr
Observation: Bromine water decolourises
14Why does phenol react more readily than benzene?
– lone pair of electrons on Oxygen delocalises into the π system
-Increases electron density of the bezene ring
-which attracts electrophiles more strongly