Aromatic Chemistry
Bonding
Content:
• The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate
between single and double.
• Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-
1,3,5-triene.
Skills
• Use thermochemical evidence from enthalpies of hydrogenation to account for this extra stability.
• Explain why substitution reactions occur in preference to addition reactions.
Electrophilic Substitution
Content:
• Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.
• Nitration is an important step in synthesis, including the manufacture of explosives and formation of
amines.
• Friedel–Crafts acylation reactions are also important steps in synthesis.
Skills:
• Outline the electrophilic substitution mechanisms of:
o Nitration, including the generation of the nitronium ion.
o Acylation using AlCl3 as a catalyst.
(Also covers one section of amines topic – Preparation of aromatic amines: Aromatic amines, prepared by
the reduction of nitro compounds, are used in the manufacture of dyes.)
, Benzene
Structure of Benzene
• Molecular Formula: C6H6.
• 6 carbon atoms in a hexagonal ring, each bonded to 1 hydrogen atom.
• One delocalised electron on each carbon atom in p-orbital.
• P-orbitals overlap to form a ring of delocalised electrons.
• Flat molecule with all atoms in the same plane (bond angle of 120°).
The Kekule Structure of Benzene
• The Kekule’s structure of benzene involves 6 carbon atoms in a hexagon with alternating single and
double carbon-carbon bonds.
• Each carbon is bonded to one hydrogen atom.
• Evidence that Kekule’s structure is consistent:
o Benzene has an Mr of 78.
o Benzene is 92% carbon and 8% hydrogen by mass.
o Benzene burns with a smoky flame (unsaturated molecules tend to undergo incomplete
combustion).
• Evidence that Kekule’s structure is inconsistent:
o All carbon-carbon bonds in benzene are equal in length, but C-C bonds are longer than C=C
bonds.
o The enthalpy of hydrogenation of benzene is lower than expected (see stability).
o Only 3 isomers of dibromobenzene exist, as opposed to the theoretical 4.
Stability of Benzene
• Cyclohexene has an enthalpy of hydrogenation of -120 kJ mol-1. Hence cyclohexa-1,3,5-triene
should have an enthalpy of hydrogenation of -360 kJ mol-1.
• However the enthalpy of hydrogenation of benzene is -208 kJ mol-1 – 152 kJ mol-1 less than
expected.
• This means more energy is required to break the bonds in benzene than the theoretical cyclohexa-
1,3,5-triene, which indicates benzene’s stability.
• The 6 delocalised electrons are spread out across the entire molecule, which reduces repulsion
between them, making benzene stable.
Reactions of Benzene
• If benzene were a triene, we would expect it to undergo addition reactions.
• However, addition reactions would involve breaking the ring of delocalised electrons, which is
unfavourable.
• The high electron density of the ring of delocalised electrons attracts electrophiles.
• Electrophilic substitution is therefore more favourable as the ring is not broken.
Bonding
Content:
• The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate
between single and double.
• Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-
1,3,5-triene.
Skills
• Use thermochemical evidence from enthalpies of hydrogenation to account for this extra stability.
• Explain why substitution reactions occur in preference to addition reactions.
Electrophilic Substitution
Content:
• Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.
• Nitration is an important step in synthesis, including the manufacture of explosives and formation of
amines.
• Friedel–Crafts acylation reactions are also important steps in synthesis.
Skills:
• Outline the electrophilic substitution mechanisms of:
o Nitration, including the generation of the nitronium ion.
o Acylation using AlCl3 as a catalyst.
(Also covers one section of amines topic – Preparation of aromatic amines: Aromatic amines, prepared by
the reduction of nitro compounds, are used in the manufacture of dyes.)
, Benzene
Structure of Benzene
• Molecular Formula: C6H6.
• 6 carbon atoms in a hexagonal ring, each bonded to 1 hydrogen atom.
• One delocalised electron on each carbon atom in p-orbital.
• P-orbitals overlap to form a ring of delocalised electrons.
• Flat molecule with all atoms in the same plane (bond angle of 120°).
The Kekule Structure of Benzene
• The Kekule’s structure of benzene involves 6 carbon atoms in a hexagon with alternating single and
double carbon-carbon bonds.
• Each carbon is bonded to one hydrogen atom.
• Evidence that Kekule’s structure is consistent:
o Benzene has an Mr of 78.
o Benzene is 92% carbon and 8% hydrogen by mass.
o Benzene burns with a smoky flame (unsaturated molecules tend to undergo incomplete
combustion).
• Evidence that Kekule’s structure is inconsistent:
o All carbon-carbon bonds in benzene are equal in length, but C-C bonds are longer than C=C
bonds.
o The enthalpy of hydrogenation of benzene is lower than expected (see stability).
o Only 3 isomers of dibromobenzene exist, as opposed to the theoretical 4.
Stability of Benzene
• Cyclohexene has an enthalpy of hydrogenation of -120 kJ mol-1. Hence cyclohexa-1,3,5-triene
should have an enthalpy of hydrogenation of -360 kJ mol-1.
• However the enthalpy of hydrogenation of benzene is -208 kJ mol-1 – 152 kJ mol-1 less than
expected.
• This means more energy is required to break the bonds in benzene than the theoretical cyclohexa-
1,3,5-triene, which indicates benzene’s stability.
• The 6 delocalised electrons are spread out across the entire molecule, which reduces repulsion
between them, making benzene stable.
Reactions of Benzene
• If benzene were a triene, we would expect it to undergo addition reactions.
• However, addition reactions would involve breaking the ring of delocalised electrons, which is
unfavourable.
• The high electron density of the ring of delocalised electrons attracts electrophiles.
• Electrophilic substitution is therefore more favourable as the ring is not broken.
