organ 2

• Aromatic chemistry

  Chemistry involving aromatic compounds like benzene
• Benzene

  • Unlike alkenes, addition reaction of the benzene ring is very difficult and requires harsh conditions
  • Benzene and other aromatic compounds prefer to undergo substitution reactions instead of addition, as it allows them to maintain their stable conjugated structure
• Benzene ring

  Initially regarded as a cyclic alkene with three C=C bonds, but this didn't account for its unusual reactivity
• Physicochemical properties of benzene suggest its bond angles and bond lengths are identical
• The alternating structure of benzene is not consistent with the chemical reactivity or the structural characteristics of a hexatriene
• The cyclic arrangement of benzene is obviously influencing its chemistry somehow
• It would be expected that a cyclic triene, such as 1,3,5-cyclohexatriene (benzene) would behave in a similar fashion to 1,3,5-hexatriene, but it does not
• Delocalisation


  The movement of π-type electrons about the benzene ring, resulting in resonance structures
• Aromatic compounds


  • Flat, conjugated cyclic hydrocarbons
  • Only aromatic if they follow Huckel's Rule: 4n + 2 pi electrons where n is a whole number
• Each double bond contains 2 pi electrons
• Aromatic hydrocarbons


  • Insoluble in aqueous environments, and soluble in non-aqueous environments
  • More stable than alkenes
  • Resistant to oxidation, reduction, and addition (since all would result in a loss of aromaticity)
  • Undergo substitution reactions, since aromaticity can be maintained
• Benzene undergoes substitution, not addition reactions
• Aromatic substitution reactions


  • Chlorobenzene
  • Benzene sulphonic acid
• Benzene is not readily affected by powerful oxidising agents such as the permanganate or dichromate ions even when boiled with them
• Aromatic substitution reactions with oxidising agents

  • Nitrobenzene
• Electrophilic aromatic substitution (SEAr)

  Benzene has a high electron density at its surface that is capable of attracting electrophiles such as NO2+, SO3, Br+. It doesn't want to lose its stabilising delocalised electron cloud permanently but will allow it to be 'compromised' temporarily.
• Nitronium ion

  The electrophile NO2+ in electrophilic aromatic substitution
• Sulphur trioxide

  The electrophile SO3 in electrophilic aromatic substitution, its resonance structure helps explain why it is electrophilic despite having no formal (+) charge
• Bromonium ion

  The electrophile Br+ in electrophilic aromatic substitution, generated in situ from iron (III) tribromide
• Friedel-Crafts acylation and alkylation

  Aromatic substitution reactions similar to the electrophilic aromatic substitutions already seen
• Friedel-Crafts mechanism

  For the second substituent in electrophilic aromatic substitution
• Nucleophilic aromatic substitution (SNAr)

  Requires more extreme conditions than simple alkyl halides due to the lack of reactivity of chlorobenzene, as the electron density is increased on the carbon atom discouraging the approach of the nucleophile, and the C-Cl bond is stronger due to resonance effects discouraging an SN1 type process
• Nucleophilic aromatic substitution (SNAr)

  Rate = k[PhCl][OH-]
• A student would be expected to be able to: Understand the basic structural/electronic reasons why aromatic compounds such as benzene are extremely stable, predict products in reactions involving benzene and electrophilic reagents, self-study on Friedel-Crafts reactions and understand their outcomes, name simple substituted benzenes and derive structures from IUPAC names