C27 aromatic chemistry

• arenes are hydrocarbons based on benzene
• benzene is the simplest arene, with the formula C6H6
• the skeletal formula of benzene is a hexagon with a circle inside it
• benzene is an unsaturated molecule with a hexagonal ring structure, but it does not readily undergo addition reactions, and all carbon atoms are equivalent
• in benzene, each carbon-carbon bond is a double bond, and each carbon is attached to 1 hydrogen

  however, each carbon-carbon double bond is intermediate in length between what you would expect for a carbon-carbon double bond, and a carbon-carbon single bond
• the intermediate bond length in benzene is because the electrons are delocalised, and are spread equally across all 6 carbons in the ring, as each carbon has a p-orbital which overlaps with the others, forming a region of electron density above and below the ring
• delocalisation of electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene
• arenes have comparable boiling points to alkanes but noticeably higher melting points, because the flat hexagonal molecules pack very closely together when solid so are harder to separate
• like other non-polar hydrocarbons, arenes do not mix with water, but do mix with other hydrocarbons, and non-polar solvents
• substituted arenes are named as derivatives of benzene so the root is benzene, and if there are multiple substituents the ring is numbered
• the ring in aromatic compounds is an area of high electron density because of the delocalised bonding, so it is attacked by electrophiles
• the benzene ring is very stable, a lot of energy needs to be put in before it will be destroyed

  this is called the delocalisation energy, it means the ring almost always remains intact in reactions
• most of the reactions of aromatic compounds are electrophilic substitution because of the high delocalisation energy of the benzene ring
• the delocalised system of the ring in arenes has high electron density so is subject to attack by electrophiles
• typically, arenes undergo electrophilic substitution rather than electrophilic addition, as addition would require the high delocalisation energy to disrupt the delocalised system
• the mechanism of electrophilic substitution is:

  • the electrons are attracted towards the electrophile
  • a bond forms between one of the carbon atoms and the electrophile
  • this uses electrons from the delocalised system so destroys it
  • to fix it, the carbon loses a H+ ion so the electron in the C-H bond can return to the delocalised system
• nitration is the substitution of an NO2 group for one of the hydrogen atoms on an arene ring
• the electrophile NO2(+) is generated by reacting concentrated nitric acid and concentrated sulfuric acid together
• the overall equation for the generation of the NO2(+) ion is:

  H2SO4 + HNO3 → NO2(+) + H2O + HSO4(-)
• the two equations for the generation of the NO2(+) ion are:

  H2SO4 + HNO3 → H2NO3(+) + HSO4(-)

  H2NO3(+) → NO2(+) + H2O
• the mechanism of nitration is:

  • the electrons are attracted towards the NO2(+) ion
  • a bond forms between the carbon and the NO2(+) ions creating a gap in the delocalised system, shown as positive
  • a hydrogen leaves, the electron from the C-H bond replaces the missing one in the delocalised system
• friedel-crafts acylation is the delocalised system acting as a nucleophile and attacking an acyl chloride
• friedel-crafts acylation requires an intermediate and an aluminium chloride catalyst
• in friedel-crafts acylation, a reactive intermediate is produced from the acyl chloride and the alumnium chloride catalyst, the equation is:

  RCOCl + AlCl3 → RCO(+) + AlCl4(-)
• the aluminium chloride catalyst is reformed in fridel-crafts acylation by reacting with a hydrogen ion from the benzene ring, the equation is:

  AlCl4(-) + H(+) -> AlCl3 + HCl
• in friedel-crafts acylation, the delocalised system acts as a nucleophile and attacks the positive intermediate
• the mechanism of friedel-crafts acylation is:

  • benzene ring attacks the positive carbon of the intermediate
  • a bond forms between the carbon and the intermediate, creating a gap in the delocalised system, shown as positive
  • a hydrogen leaves, the electron from the C-H bond replaces the missing one in the delocalised system