3.3.10 Aromatic chemistry - Alevel Chemistry

Bond length - Benzene

All bond lengths in the carbon ring are the same and intermediate in length between single and double bonds.

Bond length - Kekulé

Bond lengths should alternate between single and double.

Reaction types - Benzene

Can undergo substitution reactions but not addition reactions (easily)

Reaction types - Kekulé

Should undergo addition reactions (e.g alkenes and bromine water)

Enthalpy of hydrogenation - Benzene

- 208 kJ mol⁻¹

Enthalpy of hydrogenation - Kekulé

Cylcohexene has an EoH of -120 kJ mol⁻¹ ∴ should have a EoH of -360 kJ mol⁻¹ (x 3)

Description of electrons - benzene

There is one unused electron on each C atom perpendicular to the plane of the ring and so there is a ring of delocalised electrons above and below the plane of the ring.

Delocalised electrons effect - benzene

This delocalisation covers stability to the molecule

Benzene molecule structure

It's a planar molecule.

Why does benzene react the way it does?

Benzene is unsaturated with a delocalised ring of electrons and so repels -vely/attracts +vely charged atoms undergoing electrophilic substitution.

Nitration of benzene - mechanism name

Electrophilic substitution

Nitration of benzene - reagents and conditions

Concentrated HNO₃

Concentrated H₂SO₄ (catalyst)

Warm (50°C)

Nitration of benzene - formation of electrophile

HNO₃ + 2H₂SO₄ → ⁺NO₂ (nitronium ion) + 2HSO₄⁻ + H₃O⁺

Friedel-crafts acylation - mechanism name

Electrophilic substitution

Friedel-crafts acylation - formation of electrophile

AlCl₃ + RCOCl → AlCl₄⁻ + R-C⁺=O (acylium ion)

Friedel-crafts acylation - regeneration of catalyst

H⁺ + AlCl₄⁻ → HCl + AlCl₃

Friedel-crafts acylation - reagents and conditions

Acyl chloride (-anoyl chloride)

AlCl₃ as a catalyst

Friedel-crafts acylation - importance

Leads to C-C formation

It is an important step in synthesis of other products

Nitration of benzene - importance

Manufacture of explosives

-NO₂ can be reduced to -NH₂, aromatic amines formed can be made into dyestuffs