Chapter 25 AROMATIC CHEMISTRY

• Who determined the structure of benzene?

  Kekule
• What is the structure of benzene composed of?

  6 carbon atoms in a hexagonal ring with alternating single and double carbon-carbon bonds
• Why does benzene not react like an unsaturated alkene despite its structure?

  Because of the delocalization of electrons in the π system
• How does each carbon atom in the benzene ring bond?

  Each carbon atom forms 3 sigma bonds using sp² orbitals
• How is a π system formed?

  
  

  Remaining p-orbitals overlap laterally with neibouring orbitals
• What is the bond angle in benzene?

  120°
• What does the delocalization of electrons in benzene imply about its carbon-carbon bonds?

  All carbon-carbon bonds are identical in length
• What evidence supports the delocalization of electrons in benzene?

  Enthalpy changes of hydrogenation and C-C bond lengths
• What is the enthalpy change for the hydrogenation of cyclohexene?

  • -120 kJ mol^-1
• What is the expected enthalpy change for the hydrogenation of benzene based on Kekule's structure?

  -360 kJ mol^-1
• What is the actual enthalpy change obtained for the hydrogenation of benzene?

  • -208 kJ mol^-1
• What are aromatic compounds characterized by?


  • Containing one or more benzene rings
  • Conjugated π systems
  • Found in dyes, pharmaceuticals, pesticides, and polymers
• Name some compounds that contain benzene.

  Aspirin, paracetamol, ibuprofen, morphine
• Why do alkenes undergo bromination easily?

  They have high electron density
• Why does benzene require a halogen carrier for halogenation?

  Because there are no localized areas of high electron density to polarize the halogen molecule
• What are examples of halogen carriers used in benzene halogenation?

  AlCl₃ and FeBr₃
• What are the steps in the general mechanism of electrophilic substitution in benzene?

  1. Electrophile attracted to the delocalized π ring
  2. Intermediate formed with electrophile bonded to the ring
  3. Intermediate loses a proton to restore the delocalized π system
• What does nitration of benzene involve?

  Replacing a hydrogen atom with a nitro (-NO₂) group
• What is the reaction to generate the electrophile in the nitration of benzene?

  H₂SO₄ + HNO₃ → NO₂⁺ + HSO₄⁻ + H_2O
• What conditions are required for the nitration of benzene?

  Concentrated acids and a temperature of 50.0°C
• What is the role of metal halide carriers in benzene halogenation?

  They act as catalysts to create the electrophile
• What is an acyl group?

  An alkyl group containing a carbonyl (C=O)
• What is the first step in Friedel-Crafts acylation?

  Generating the alkyl electrophile using a metal halide catalyst
• What happens when phenol is treated with sodium hydroxide?

  It forms soluble salts and water
• Why is phenol considered a weak acid?

  
  

  It behaves as a very weak acid with a pH of 5/6 and because the oxygen is the most electronegative so it tends to attract the hydrogen ion back again
• Why is the phenoxide ion stable?

  Because the negative charge from the oxygen (2-) is spread over the entire ion
• How does the OH group in phenols affect the reactivity of the benzene ring?

  It increases the electron density of the ring, making it more reactive
• What is the product of bromination of phenol?

  2,4,6-tribromophenol
• What happens when phenol reacts with dilute nitric acid?

  It produces a mixture of 2-nitrophenol and 4-nitrophenol
• What are the directing effects in electrophilic substitution for aromatic compounds?

  • Electron-donating groups (-CH₃, -OH, -NH₂) activate the ring and direct substitutions to 2 and/or 4 positions
  • Electron-withdrawing groups (-NO₂) deactivate the ring and direct substitutions to the 3 position