Aromatic Chemistry (A-Level)

• What is structure of benzene?

  — a cyclic planar molecule with formula. C6H6

  — each carbon is bonded to 2 other carbons and 1 hydrogen atom, and the final lone electron is in a p-orbital which sticks up above the planar ring

  — lone electrons in p-orbitals combine to form a delocalised ring of electrons, this means all c-c bonds are same length
• How can we measure the stability of benzene?

  By comparing the enthalpy change of hydrogenation in benzene and cyclohexa-1,3,5-triene (the theoretical model or kekules model0
• Describe how to prove the stability of benzene

  — cyclohexene (has 1 double bond) enthalpy change of hydrogenation is -120 kjmol-1, so we expect cyclohexa-1,3,5-triene (3 double bonds) to have an enthalpy change of hydrogenation of -360kjmol-1

  if you multiply it by 3

  — however it actually is -208kj mol-1 which is 152 kj mol-1 lower than what we expected
• what does this lower value of hydrogenation for benzene tell us?

  — the enthalpy change of hydrogenation is less exothermic than expected

  — this shows an increase in stability called the delocalisation energy

  — so benzene is more stable due to delocalised electrons

  — more energy is needed to break them bonds
• what is an arene?

  aromatic compounds are molecules that contain a benzene ring - also known as arenes
• Why do benzenes undergo electrophillic substitution reactions?

  — benzene has a high electron density as it has a delocalised ring of electrons - electrophiles will attract to it

  — not addition because it would disrupt the stability of benzene’s by disrupting the stable ring of electrons

  — a hydrogen is substituted for the electrophile
• which 2 types of electrophillic substitution reactions does benzene undergo?

  — Friedel-Crafts Acylation

  — nitration reaction
• Compare benzene and the theoretical molecule cyclohexa-1,3,5-triene in terms of stability, shape, C-C bond lengths, and suggest any reasons for differences (5)

  — benzene is more stable than cyclohexatriene

  — the enthalpy of hydrogenation of benzene is 152kj mol-1 less exothermic

  — due to delocalisation energy of electrons in benzene

  — both are planar/hexagonal

  — benzene has equal C-C bond lengths, but cyclohexatriene have different C-C bond lengths