Aromatic chemistry

• Arenes = hydrocarbons based on benzene


  Benzene = C6H6

  unsaturated but very stable - does not decolourise bromine water

  average bond length: 0.139 nm
• Aromatic Stability

  3 electrons used for bonding & 4th in p-orbital

  p-orbitals overlap - electrons become delocalised

  forms a ring of delocalised electrons

  region of electron density formed above & below ring
• Thermodynamic evidence for stability

  A) -120kJ/mol

  B) should be -360 kJ/mol

  C) actually -208kJ/mol

  D) -152kJ/mol delocalisation energy

  
• Properties of arenes

  • benzene is a colourless liquid
  • BP: 353K (similar to hexane)
  • Freezing point: 279K (different to hexane as it's planar and packs together well)
  • naming: 'benzene' stem or 'phenyl-/phen' prefix
• reactivity of arenes

  ring of delocalised electrons attacked by electrophiles

  reactivity depends on electron density & stability

  energy needed to break system as you can't break the ring of delocalised electrons - called delocalised energy

  ring almost always stays intact in reactions with arenes
• Nitration = produces nitrobenzene


  • uses: makes TNT, forms amines
  • amines make dyes & pharmaceuticals
  • conditions: warmed with concentrated HNO3 & concentrated H2SO4
  • sulfuric acid is a stronger acid than nitric acid -> dontes a proton to HNO3 to make it a base (H2SO4 acts as catalyst & HNO3 acts as a base)

  step 1: H2SO4 + HNO3 -> H2NO3+ + HSO4-

  step 2: H2NO3+ breaks up to form nitronium ion/nitryl cation

  H2NO3+ -> NO2+ + H2O

  step 3: electrophilic substitution with NO3+ as an electrophile

  overall: HNO3 + C6H6 -(H2SO4) -> C6H6NO2 + H2O
• Electrophilic substitution with NO3+

  A) NO2+

  B) H

  C) H+ & H2O reacts to reform H2SO4

  
• Friedel-Crafts acylation - benzene reacting with acyl groups to form useful products

  needs string electrophile -> halogen carrier catalyst e.g. AlCl3 & done under reflux

  A) AlCl3 accepts electron pair from acyl chloride

  B) lone e- pair moves & acyl chloride forms carbocation

  C) strong enough electrophile

  D) -ve AlCl4- attracted to +ve ring & Cl- bonds with H+