Aromatic chemistry

• What is the structure of benzene?

  Benzene consists of a ring of six carbon atoms with six hydrogen atoms and a ring of delocalised electrons.

  
• What is the bond length characteristic of the benzene ring?

  Each bond in the benzene ring has an intermediate length between that of a double and single bond.

  
• Why is benzene considered very stable compared to other molecules of similar size?

  The delocalised electrons in the benzene ring contribute to its high stability.
• What was the initial prediction about the structure of benzene?

  It was predicted to have a structure similar to cyclohexatriene, with three double bonds and three single bonds.

  
• 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 conclusion can be drawn from the difference in enthalpy change of hydrogenation between benzene and cyclohexene?

  Benzene has a different, unusual structure compared to what was initially predicted.

  
• What are compounds that contain benzene as part of their structure called?

  They are called arenes or aromatic compounds.
• Why do arenes have high melting points?

  They have high melting points due to the high stability of the delocalised ring.
• Why do arenes have low boiling points?

  They have low boiling points because they are non-polar molecules and often cannot be dissolved in water.
• What makes the delocalised ring in benzene susceptible to electrophiles?

  The delocalised ring is an area of high electron density.
• What is the process called when electrophiles attack the benzene ring?

  The process is called electrophilic substitution.

  
• What can be produced from benzene through electrophilic substitution?

  Aromatic amines and nitrobenzene can be produced.

  
• What is the electrophile in the formation of nitrobenzene?

  The electrophile is the NO₂⁺ ion.
• How is the NO₂⁺ ion produced for the reaction with benzene?

  It is produced in the reaction of concentrated sulfuric acid with concentrated nitric acid.
• What happens when benzene is heated with concentrated sulfuric acid and nitric acid?

  The NO₂⁺ electrophile substitutes onto the benzene ring, removing a hydrogen ion.

  
• What type of substitution occurs when the reaction temperature is 55°C?

  A mono-substitution of a single NO₂⁺ electrophile occurs.
• What happens at temperatures greater than 55°C during the electrophilic substitution reaction?

  Multiple substitutions can occur at temperatures greater than 55°C.
• What is Friedel-Crafts acylation?

  It is a reaction where the delocalised electron ring in benzene acts as a nucleophile and attacks acyl chlorides.

  
• What is required for Friedel-Crafts acylation to take place?

  A reactive intermediate must be produced from the acyl chloride and an aluminium chloride catalyst.
• What happens to the H⁺ ion removed from the benzene ring during Friedel-Crafts acylation?

  It reacts with the AlCl₄^- ion to reform the aluminium chloride, showing it to be a catalyst.

  
• What is the product of Friedel-Crafts acylation?

  The product is a phenylketone.
• What is the benzene group in the product of Friedel-Crafts acylation called?

  The benzene group is called a phenyl group.
• In what industries are phenylketones commonly used?

  They are commonly used in the industrial production of dyes, pharmaceuticals, and explosives.