Benzene

Kekule’s proposed structure

• cyclohexatriene

• High ratio of C:H suggests highly unsaturated molecule, but benzene is not reactive

3 pieces of evidence to discount Kekules structure

• X-ray Crystallography - Shows benzene has 6 equal length C-C bonds

• Mechanism - With 3 double bonds it should react by electrophilic addition but it reacts via subsitution

• Thermochemical evidence from ΔΗhydrogenation - When hydrogenating cyclohexene into cyclohexane, ΔΗ is 120kJ/mol, so hydrogenation of benzene should be 3x higher, but it is a lot lower showing that it is more stable than expected

Nitration of benzene

• Generation of electrophile

• Electrophilic substitution

• Regeneration of the catalyst

• C6H6 + HNO3 + H2SO4 → C6H5NO2

• Concentrated HNO3 and concentrated H2SO4 (catalyst)

Generation of electrophile in nitration

HNO3 + 2H2SO4 → +NO2 + H3O+ + 2HSO4-

Electrophilic substitution in nitration

Makes nitrobenzene which can be reduced into phenylamine

Regeneration of catalyst in nitration

• H3O+ + 2HSO4- + H+ → 2H2SO4 + H2O

Friedel-Craft Acylation

• Acyl group is substituted for a ‘H’ on benzene ring

• Generation of electrophile

• Electrophilic substituion

• Regeneration of catalyst

Generation of electrophile in Friedel-Craft

• Acid chloride + AlCl3 → RCO+ + AlCl4-

• AlCl3 = catalyst, halogen carner

Electrophilic substitution

Makes ketone structure

Regeneration of catalyst in Friedel-Craft

AlCl4- + H+ → AlCl3 + HCl