PMT flashcards Chapter 25

• What is the molecular formula of benzene?

  C$_6$H$_6$
• 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?

  Bond angle = 120°
• What are the three features of benzene that don’t support Kekule’s model?

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

  X-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
• How do the rings of electron density affect the stability of benzene?

  Makes benzene very stable due to aromatic stability
• What is the skeletal structure of cyclohexa-1,3,5-triene?

  Answer not provided
• Why does benzene have a relatively high melting point?

  Close packing of flat hexagonal molecules
• 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 ring using numbers if multiple substituents
• Why is benzene attacked by electrophiles?

  High electron density due to delocalised electrons
• What type of reaction is nitration of benzene?

  Electrophilic substitution reaction
• Which ion is used to nitrate benzene?

  (positive)NO$_2$
• What is the catalyst in the nitration of benzene?

  Sulfuric acid
• How is the H$_2$SO$_4$ catalyst regenerated in the nitration of benzene?


  HSO$_4$⁻ + H⁺ → H$_2$SO$_4$
• Draw a mechanism and write an overall equation for the nitration of benzene.

  • Overall equation: C$_6$H$_6$ + HNO$_3$ → C$_6$H$_5$NO$_2$ + H$_2$O
• What type of catalyst is used for a Friedel-Crafts reaction?

  A halogen carrier (e.g., AlCl$_3$)
• Why does benzene not react directly with halogens?

  The aromatic ring is too stable
• What happens when AlCl$_4$⁻ is formed in terms of electrons?


  Lone pair on chlorine forms a coordinate bond to Al
• How is the AlCl$_3$ catalyst reformed?


  AlCl$_4$⁻ + H⁺ → HCl + AlCl$_3$
• How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

  Use a halogenoalkane and AlCl$_3$ to create an electrophile
• What is the structure of phenol?

  Answer not provided
• What reactions can you carry out to show the weak acidity of phenol?

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


  Answer not provided

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


  Answer not provided

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

  Easier due to oxygen lone pair increasing electron density
• What is the directing effect of electron donating groups OH and NH₂?

  • Direct groups to the 2 and 4 positions
  • In electrophilic substitution of aromatic compounds
• What is the directing effect of the electron withdrawing group NO₂?

  • Directs atoms to the 3 position
  • In electrophilic substitution of aromatic compounds