Arene Chemistry

• What are the two major classes of organic chemicals?

  Aliphatic and aromatic (arenes)
• What defines aromatic hydrocarbons or arenes?

  Includes one or more rings of six carbons
• What is the molecular formula of benzene?

  C6H6
• Describe the basic structure of benzene.

  Six C atoms in a hexagonal ring
• How are the carbon atoms in benzene bonded?

  Each C atom is bonded by σ-bonds
• What happens to the unused electrons on each carbon atom in benzene?

  They are in p orbitals, perpendicular to the ring
• What is the nature of the six p electrons in benzene?

  They are delocalised in a ring structure
• What is the bond angle in benzene?

  120 degrees
• How do the C-C bonds in benzene compare to single and double bonds?

  They have a length and energy between single and double bonds
• What does delocalised mean in the context of benzene?

  Electrons are not attached to a particular atom
• What are the enthalpy changes associated with hydrogenation of benzene?

  • Cyclohexene: ΔH = -120 kJ/mol
  • Cyclohexane: ΔH = -360 kJ/mol
  • Benzene: ΔH = -208 kJ/mol
• Why is the actual energy of benzene's hydrogenation less than theoretical expectations?

  Due to delocalisation of the 6 pi electrons
• What is the term for the stability increase due to delocalisation in benzene?

  Delocalisation energy
• What happens in cyclohexa-1,4-diene regarding delocalisation?

  No delocalisation occurs due to distance
• How does cyclohexa-1,3-diene compare to cyclohexa-1,4-diene?

  It has some delocalisation and extra stability
• What are some examples of aromatic compounds derived from benzene?

  • Methylbenzene
  • Ethylbenzene
  • Chlorobenzene
  • Bromobenzene
  • Nitrobenzene
  • Benzenecarboxylic acid
  • Benzaldehyde
• How are substituents on a benzene ring numbered?

  To give the lowest possible numbers
• What is the phenyl group?

  The C6H5- group acting as a substituent
• Why does benzene undergo electrophilic substitution reactions?

  It has high electron density attracting electrophiles
• What is benzene classified as in terms of toxicity?

  A carcinogen
• How does methylbenzene's reactivity compare to benzene?

  Methylbenzene is less toxic and more reactive
• What is the importance of nitration of benzene?

  It synthesizes useful compounds like explosives
• What reagents are used in the nitration of benzene?

  Concentrated nitric acid and sulfuric acid
• What is the mechanism of nitration of benzene?

  Electrophilic substitution
• What is the overall equation for the formation of the electrophile in nitration?

  HNO3 + 2H2SO4 → +NO2 + 2HSO4- + H3O+
• What happens to the H+ ion in the nitration mechanism?

  It reforms H2SO4 catalyst
• What is the change in functional group during Friedel Crafts acylation?

  Benzene to phenyl ketone
• What reagents are used in Friedel Crafts acylation?

  Acyl chloride and anhydrous aluminium chloride
• What is the mechanism of Friedel Crafts acylation?

  Electrophilic substitution
• What is the significance of the H+ ion in Friedel Crafts acylation?

  It reacts to reform the catalyst
• What is the process to reduce a nitroarene to an aromatic amine?

  Using Sn and HCl or Fe and HCl
• What happens to the ionic salt formed during the reduction of nitroarene?

  It reacts with NaOH to form phenylamine
• How does delocalisation affect the properties of side groups attached to benzene?

  It changes their properties and reactions
• What effect does delocalisation have on the C-Cl bond in chlorobenzene?

  It makes the C-Cl bond stronger
• How does phenol differ from alcohols due to delocalisation?

  It is more acidic and does not oxidise
• How does delocalisation affect the basicity of phenylamine?

  It makes it less basic than aliphatic amines
• What are the key reactions involving benzene and its derivatives?

  • Electrophilic substitution
  • Nitration
  • Friedel Crafts acylation
  • Reduction of nitroarene
• What should students remember about aromatic compounds in exams?

  • Focus on aliphatic reactions of side chains
  • Understand electrophilic substitution mechanisms
  • Recognize the importance of delocalisation