6.1.1 AROMATIC COMPOUNDS

• What are aromatic compounds that contain a benzene ring called?

  Arenes
• What is the molecular formula of benzene?


  C$_6$H$_6$
• How many carbon atoms are in a benzene ring?

  Six
• What type of electrons are present in the benzene ring structure?

  Delocalised electrons
• What results from the overlap of p-orbitals in benzene?

  The formation of π-bonds
• Why is benzene considered very stable compared to other molecules of similar size?

  Due to its delocalised ring structure
• What was the predicted structure of benzene based on empirical measurements?

  A structure similar to cyclohexatriene
• What was the predicted enthalpy change of hydrogenation for benzene?

  • 360 kJ mol$^{-1}$
• What was the actual enthalpy change of hydrogenation for benzene?

  • 208 kJ mol$^{-1}$
• What does a less negative enthalpy change of hydrogenation indicate about benzene's stability?

  Benzene is more stable than cyclohexatriene
• What did X-ray diffraction experiments reveal about the bond lengths in benzene?

  All bond lengths between carbon atoms in benzene are the same
• How does the bond length in benzene compare to that of single and double carbon bonds?

  Each bond has an intermediate length between that of a double and single bond
• Why is benzene resistant to electrophilic addition reactions?

  Because it would break up the stable delocalised ring of electrons
• What type of reactions does benzene undergo instead of electrophilic addition?

  Electrophilic substitution reactions
• What makes the delocalised ring in benzene susceptible to attack from electrophiles?


  It is an area of high electron density
• What is the role of the catalyst in the halogenation of benzene?

  To generate the electrophile
• What is the electrophile in the nitration of benzene?

  NO$_2^+$ ion
• What is produced when concentrated sulfuric acid reacts with concentrated nitric acid?

  A reactive intermediate that is the NO$_2^+$ ion
• What happens to the hydrogen ion released during the nitration of benzene?

  It reacts with HSO$_4^{-}$ to reproduce sulfuric acid
• What is the temperature at which mono-substitution occurs during nitration of benzene?

  55°C
• What is the product of Friedel-Crafts acylation?

  A phenylketone
• What must be produced for Friedel-Crafts acylation to take place?

  A reactive intermediate from the reaction of acyl chloride and aluminium chloride
• What does the H$^+$ ion removed from the benzene ring react with in Friedel-Crafts acylation?


  AlCl$_4^{-}$ ion
• Why does benzene not decolourise bromine water?

  Because of the delocalised electron density of the π-system in benzene
• What are phenols?

  Organic compounds containing a benzene ring with an OH group
• What is the formula of phenol?

  C$_6$H$_5$OH
• How do phenols behave in reactions with NaOH?

  They can be neutralised
• What happens when phenol reacts with bromine water?

  It produces 2,4,6-tribromophenol, which forms a white precipitate
• Why does phenol react more readily with bromine water than benzene?

  Due to the lone pair of electrons on the oxygen atom being delocalised into the benzene ring
• What is the result of nitration with phenol?

  A mixture of 2-nitrophenol and 4-nitrophenol
• What is the effect of the NO$_2$ group in nitration with phenol?


  It is electron withdrawing and decreases the electron density of the ring
• What do electron donating groups like OH and NH$_2$ do in electrophilic substitution?


  They direct electrophiles to substitute at the 2- and 4- positions
• What do electron-withdrawing groups like NO$_2$ do in electrophilic substitution?


  They are 3-directing in electrophilic substitution of aromatic compounds
• What are the key features of electrophilic substitution reactions in benzene?

  • Benzene is resistant to electrophilic addition.
  • Electrophilic substitution involves the attack of electrophiles on the delocalised electron ring.
  • The mechanism partially destroys the ring before restoring it.
  • Products include aromatic amines and nitrobenzene.

  
• What are the steps involved in the nitration of benzene?

  1. Formation of the NO$_2^+$ electrophile.
  2. Electrophilic substitution occurs at 55°C.
  3. Hydrogen ion released reacts with HSO$_4^{-}$ to regenerate sulfuric acid.
  4. Mono-substitution is preferred at lower temperatures.

  
• What are the steps involved in Friedel-Crafts acylation?

  1. Formation of a reactive intermediate from acyl chloride and aluminium chloride.
  2. Nucleophilic attack by the benzene ring.
  3. H$^+$ ion reacts with AlCl$_4^{-}$ to regenerate aluminium chloride.
  4. Production of a phenylketone.

  
• What are the differences in reactivity between benzene and phenol?

  • Benzene is resistant to bromination; phenol reacts readily.
  • Phenol can undergo multiple substitutions with bromine.
  • The lone pair on oxygen in phenol increases electron density, making it more reactive.

  
• What are the directing effects of substituents in electrophilic substitution reactions?

  • Electron donating groups (e.g., OH, NH$_2$) direct to 2- and 4- positions.
  • Electron-withdrawing groups (e.g., NO$_2$) are 3-directing.
  • Directing effects favor the most stable charged intermediate.