A-Level Chemistry AQA 7405

Bonding

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.

Students should be able to:

• 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

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.

Students should be able to outline the electrophilic substitution mechanisms of:

• nitration, including the generation of the nitronium ion
• acylation using AlCl₃ as a catalyst.