Aromatic compounds

• What is the molecular formula of benzene?

  C6H6
• What is the empirical formula of benzene?

  CH
• What is the state of benzene at room temperature?

  Liquid
• What is the bond angle of benzene?

  120°
• What are the three features of benzene that do not support Kekulé's model?

  • Benzene is resistant to addition reactions
  • Enthalpy change of hydrogenation of benzene is more stable than predicted
  • All the carbon bonds are the same length
• What technique was used to find the bond lengths of benzene?

  1. ray diffraction
• What happens to the 4th electron in the p-orbital of each carbon atom in benzene?

  It delocalises to form rings of electron density above and below the hexagon.
• How do the rings of electron density affect the stability of benzene?

  They make benzene very stable, even though it is unsaturated.
• What is the skeletal structure of cyclohexa-1,3,5-triene?

  Answer not provided in the material.

  
• Why does benzene have a relatively high melting point?

  Due to close packing of flat hexagonal molecules when solid.
• Is benzene soluble in water? Explain why.

  No, because it is non-polar.
• Why is benzene not used in schools?

  It is a carcinogen.
• How do you name compounds containing a benzene ring?

  • Use "-benzene" or "phenyl-"
  • Designate position on the ring using numbers if there is more than one substituent
• Why is benzene attacked by electrophiles?

  Due to high electron density above/below the ring from delocalised electrons.
• What type of reaction is nitration of benzene?

  Electrophilic substitution reaction.

  
• Which ion is used to nitrate benzene?

  NO2+
• What is the catalyst in the nitration of benzene?

  Sulfuric acid.

  
• How is the NO2+ ion generated? (conditions and equations)

  • Use concentrated H2SO4 and concentrated HNO3.
  • Reaction: H2SO4 + HNO3 → H2NO3+ + HSO4-
  • Overall: H2SO4 + HNO3 → HSO4- + NO2 + + H2O
• How is the H2SO4 catalyst regenerated in the nitration of benzene?

  HSO4- + H+ → H2SO4 (H+ from benzene ring).
• Draw a mechanism and write an overall equation for the nitration of benzene.

  • Overall equation: C6H6 + HNO3 → C6H5NO2 + H2O

  
• What type of catalyst is used for a Friedel-Crafts reaction?

  A halogen carrier (e.g., AlCl).
• Why does benzene not react directly with halogens?

  The aromatic ring is too stable.
• What is happening when AlCl4- is formed in terms of electrons?

  The lone pair of electrons on the chlorine atom is forming a coordinate bond to Al.
• How is the AlCl3 catalyst reformed?

  AlCl4- + H+ → HCl + AlCl3 (H+ from benzene).
• How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

  Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene.
• Draw the mechanism for the acylation of benzene from RCO+.

  Answer not provided in the material.
• Draw the structure of phenol?

  Answer not provided in the material.

  
• What reactions can you carry out to show the weak acidity of phenol?

  • Neutralisation reaction with NaOH occurs
  • No reaction with carbonates
• Write the equation of the reaction between phenol with bromine to form 2,4,6-tribromophenol.


  Answer not provided in the material.

  
• Write the equation for the reaction between phenol with dilute nitric acid.


  Answer not provided in the material.

  
• What is the relative ease of electrophilic substitution of phenol compared to benzene and why?

  • Easier for electrophilic substitution to occur with phenol
  • Oxygen lone pair of electrons from the -OH group are partially delocalised into the ring, increasing electron density
• What is the directing effect of electron donating groups OH and NH2?

  • They direct groups to the 2 and 4 positions of the ring in electrophilic substitution of aromatic compounds.
• What is the directing effect of the electron withdrawing group NO2?

  • NO2 directs atoms to the 3 position of the ring in electrophilic substitution of aromatic compounds.