PRPM112: AROMATIC COMPOUNDS

• C6H6

  (highly unsaturated)


  formula of benzene
• substitution, but only 1 product


  chemical reactivity of benzene
• cyclic, planar, sp2 hybridized


  structure of benzene
• kekule structure


  a model that represents the bonds between atoms as lines

  
• robinson structure

  
• 1.39 Å

  C-C bond length of benzene
• electrophilic substitution


  chemical reactivity of benzene
• aromaticity


  the extra stability associated with aromatic compounds
• aromatic compounds

  contain 4n + 2 p electrons (n=1,2,3...)
• aromatic compounds

  these compounds are cyclic, planar, fully conjugated
• Huckel's rule


  equivalent to an odd number of p electrons pairs in the ring system
• fused rings


  rings that share set of carbons
• heterocyclic compounds

  rings that contain atoms other than carbon as part of the ring such as N, S, O, P
• omsim


  Question: Can aromatic compounds have elements other than carbon in the ring?
• carbocycles


  cyclic compounds that contain only carbon
• pyridine


  six-membered heterocycle with a nitrogen atom in its ring
• pyridine


  relatively weak base compared to normal amines but protonation does not affect aromaticity.
• pyrrole


  five-membered heterocycle with one nitrogen
• pyrrole


  lone pair electrons are in the aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base.
• EAS: Halogenation

  Cl2 and Br2


  are weak electrophiles on their own so need to be "activated" by using a Lewis acid catalyst.
• FeCl3 or FeBr3


  where commonly the corresponding iron trihalide is used
• EAS: Nitration


  sulfuric acid is used to generate a more reactivity electrophile, a nitronium ion.
• nitronium ion


  electrophile in the nitration of benzene
• EAS: Sulfonation


  it generates a benzenesulfonic acid
• sulfur trioxide

  strong electrophile used in sulfonation
• EAS: Alkylation

  this will add an alkane group to benzene. In this case we need a carbocation as the electrophile
• EAS: Alkylation

  It has two ways:

  - Friedel-Crafts alkylation

  - Alkylation using an alkene and acid
• Friedel-Crafts Alkylation

  process uses an alkyl halide (Cl or Br usually) and a Lewis acid catalyst similar to a halogenation reaction
• aluminum trihalide

  the lewis acid catalyst used in friedel crafts alkylation
• Alkylation from Alkenes

  alkylation can also be achieved by using an alkene and an acid
• an alkene and an acid


  Alkylation can also be achieved by using ________
• Friedel-Crafts Alkylation

  the product is an alkylbenzene, i.e.
• Alkylation from Alkenes

  the product is a phenyl ketone, i.e.