Aromatic Compounds Summary- OCR A-Level Chemistry Module 6
What is Kekule’s model?
A compound with 6 carbon atoms each attached to 2 other carbon atoms and 1 hydrogen atom,
where there are alternating double bonds that switch between its isomers. This is because its
p-orbitals overlap laterally with 1 of its adjacent p-orbitals.
What is the benzene model?
A compound with 6 carbon atoms each attached to 2 other carbon atoms and 1 hydrogen
atoms, where each p-orbital overlaps laterally with both of its adjacent p-orbitals. This forms ring
shaped cloud of delocalised electrons above and below the structure, with a high electron
density.
What is the experimental evidence that Kekule’ model is not benzene?
1. Enthalpy change of hydrogenation value for benzene is less exothermic than expected,
as more energy is needed to break the stable delocalised pi system structure.
2. All the bond lengths of benzene are the same, telling us that there are no double bonds
3. For benzene, electrophilic substitution occurs, not electrophilic addition, due to the stable
delocalised pi system with a high electron density
, Nitration of benzene:
Requires concentrated sulfuric acid and concentrated nitric acid.
What is Kekule’s model?
A compound with 6 carbon atoms each attached to 2 other carbon atoms and 1 hydrogen atom,
where there are alternating double bonds that switch between its isomers. This is because its
p-orbitals overlap laterally with 1 of its adjacent p-orbitals.
What is the benzene model?
A compound with 6 carbon atoms each attached to 2 other carbon atoms and 1 hydrogen
atoms, where each p-orbital overlaps laterally with both of its adjacent p-orbitals. This forms ring
shaped cloud of delocalised electrons above and below the structure, with a high electron
density.
What is the experimental evidence that Kekule’ model is not benzene?
1. Enthalpy change of hydrogenation value for benzene is less exothermic than expected,
as more energy is needed to break the stable delocalised pi system structure.
2. All the bond lengths of benzene are the same, telling us that there are no double bonds
3. For benzene, electrophilic substitution occurs, not electrophilic addition, due to the stable
delocalised pi system with a high electron density
, Nitration of benzene:
Requires concentrated sulfuric acid and concentrated nitric acid.
