arene chemistry

• What are the two major classes of organic chemicals?

  Aliphatic and aromatic (or arenes)
• What distinguishes aromatic hydrocarbons from aliphatic hydrocarbons?

  Aromatic hydrocarbons include one or more rings of six carbon atoms with delocalised bonding
• What is the molecular formula of benzene?

  C6H6
• Describe the basic structure of benzene.

  Benzene has six carbon atoms in a hexagonal ring, with one hydrogen atom bonded to each carbon atom
• How are the carbon atoms in benzene bonded?

  Each carbon atom is bonded to two other carbon atoms and one hydrogen atom by single covalent σ-bonds
• What is the significance of the p orbitals in benzene's structure?

  Each carbon atom has one unused electron in a p orbital, which is perpendicular to the plane of the ring
• What does it mean for the six p electrons in benzene to be delocalised?


  It means the electrons are spread out over the whole ring and not attached to a particular atom
• What is the bond angle in benzene?

  120 degrees
• How does the bond length in benzene compare to single and double bonds?

  All C-C bonds in benzene are the same length and have a bond energy between a C-C single bond and a C=C double bond
• What is the difference between the theoretical structure of cyclohexa-1,3,5-triene and actual benzene?

  • Theoretical cyclohexa-1,3,5-triene has 3 double bonds.
  • Actual benzene has delocalised electrons, resulting in lower energy than expected.
  • Delocalisation leads to greater thermodynamic stability.
• What is the enthalpy change for the hydrogenation of benzene?

  ΔH = -208 kJ/mol
• What is delocalisation energy in the context of benzene?


  It is the increase in stability connected to the delocalisation of electrons in benzene
• Why does cyclohexa-1,4-diene not exhibit delocalisation?

  Because the pi electrons are too far apart and do not overlap
• How does the presence of substituents affect the naming of aromatic compounds?

  Substituents must be indicated by numbers for the lowest possible positions and listed in alphabetical order
• What are some examples of aromatic compounds derived from benzene?

  • Methylbenzene
  • Ethylbenzene
  • Chlorobenzene
  • Nitrobenzene
  • Benzenecarboxylic acid
  • Benzaldehyde
• What type of reactions does benzene typically undergo?

  Benzene typically undergoes electrophilic substitution reactions
• Why is benzene considered a carcinogen?

  Because it is a cancer-causing molecule
• How does methylbenzene differ from benzene in terms of toxicity and reactivity?


  Methylbenzene is less toxic and reacts more readily than benzene
• What is the importance of nitration of benzene?

  • It is crucial for synthesising useful compounds like explosives (e.g., TNT).
  • It is also important for forming amines used in dyestuffs.
• What reagents are used in the nitration of benzene?

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

  The mechanism is electrophilic substitution
• What is the overall equation for the formation of the electrophile in the nitration of benzene?

  HNO3 + 2H2SO4 → +NO2 + 2HSO4- + H3O+
• What happens to the H+ ion during the nitration of benzene?

  The H+ ion rejoins with HSO4- to reform H2SO4 catalyst
• At what temperature is the nitration of benzene typically carried out?

  60 degrees Celsius
• What is Friedel Crafts acylation and its significance?

  • It changes the functional group from benzene to phenyl ketone.
  • It introduces a reactive functional group onto the benzene ring.
• What reagents are used in Friedel Crafts acylation?

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

  The mechanism is electrophilic substitution
• What is the overall equation for the formation of the electrophile in Friedel Crafts acylation?

  AlCl3 + CH3COCl → [CH3CO]+ + [AlCl4]-
• What happens to the H+ ion during Friedel Crafts acylation?

  The H+ ion reacts with AlCl4- to reform AlCl3 catalyst and HCl
• What is the process of reducing a nitroarene to aromatic amines?

  • Nitro group is reduced to an amine group.
  • Reagents: Sn and HCl or Fe and HCl.
  • Conditions: Heating.
• What is formed when nitrobenzene is reduced in HCl?

  C6H5NH3+Cl- (soluble in water)
• What happens when the ionic salt C6H5NH3+Cl- reacts with NaOH?

  It gives the insoluble phenylamine
• How does delocalisation affect side groups with lone pairs attached to a benzene ring?

  • Delocalisation extends to include lone pairs on N, O, and Cl.
  • Changes properties and reactions of the side group.
• What effect does delocalisation have on the C-Cl bond in chlorobenzene?


  It makes the C-Cl bond stronger and prevents typical substitution reactions
• How does delocalisation affect the properties of phenol compared to alcohols?

  Phenol is more acidic and does not oxidise like alcohols
• How does delocalisation affect the basicity of phenylamine compared to aliphatic amines?


  Phenylamine is less basic than aliphatic amines because the lone pair is delocalised
• What should students remember about questions regarding aromatic compounds?

  • Many questions will focus on normal aliphatic reactions of the side chains.
  • Not all questions will be about the benzene ring itself.